HUMB1001 — Module 2 — How is the Body Supported?

HUMB1001 · Integrated Systems Anatomy and Physiology

Module 2:
How is the Body Supported?

Bone, the skeleton and joints

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In this lecture

The skeletal system
Bone as a tissue
Building and growing bone
The skeleton
Joints
Movement

FRAG 0: Welcome back to Integrated Systems Anatomy and Physiology. At the end of Module 1, we said we'd look next at how the body is supported, and that's where we're going now. In Module 1, you saw how the nervous and endocrine systems control the body. In this module, we turn to the framework they control: the bones, the cartilage that shapes and cushions them, and the joints that let them move. A quick reminder about how this lecture works. The narration you're hearing is an AI-generated voice reading a script written by your teaching team. Everything moves at your pace. Use the controls at the bottom of the screen to pause, or to speed the narration up. Some slides will stop and wait for you: a question to answer, something to sort, a diagram to explore. They aren't marked, so have a genuine go before you check your answer. At the end, there's a short reflection, and it becomes a page of notes you can print or save. Let's begin.

Acknowledgement of Country (Boodja). I would like to acknowledge the Whadjuk Noongar people whose land we are standing on and recognise the strength, resilience and capacity of the Noongar people where Curtin University is situated. I pay my respect to their vibrant and endless culture and the leadership of the Elders past, present and future. This country (boodja) where Curtin University is situated has belonged to the Whadjuk Noongar people for thousands of years and is a place of learning for all people now and Curtin University is very proud to continue on this very long tradition.

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What you'll be able to do

The skeleton's jobs

Five functions, four tissues

Bone is alive

Forming, growing, keeping calcium, repairing

Reading the skeleton

Bones by region, shape and landmark

Joints

Classified by structure and by movement

Movement

The right term for each action

FRAG 0: Here's where we're headed. This module has five learning outcomes, and by the end of the lecture you should be able to do each of them. Unlike Module 1, most of this is new, rather than a step on from Human Structure and Function, so give yourself time to work through it. FRAG 1: First, describe the major functions of the skeletal system. You'll see that the skeleton does far more than hold you up. It protects your organs, gives your muscles something to pull on, stores minerals and fat, and makes your blood cells. We'll also meet the four tissues it's built from. FRAG 2: Second, describe how bone forms, grows and is maintained. That covers the cells and the matrix that make bone so strong, how a bone develops before birth and lengthens through childhood, how hormones draw on bone to keep your blood calcium steady, and how bone remodels itself and repairs a fracture. FRAG 3: Third, identify, name and classify the bones of the skeleton, and use the correct terms for their features. We'll name the bones region by region and sort them by shape. Then you'll learn to read the bumps, ridges and holes on a bone's surface, using your thigh bone, the femur, as the worked example. You'll identify the bones themselves on real specimens in the lab. FRAG 4: Fourth, name and describe the major classes of joints. You'll classify joints in two ways: by what holds the bones together, and by how much they can move. Then we'll look closely at the freely movable joints, the synovial joints, and compare two of them, the shoulder and the hip. FRAG 5: And finally, describe the movements that can happen at particular joints, like the shoulder, elbow, hip, knee and ankle. Questions on movement come up every year, and these terms are the shared language you'll use to describe how people move in almost every health profession.

A question to start

Is today's skeleton the one you'll have in ten years?

Remodelling: roughly a tenth of it, every year.

FRAG 0: When you picture a skeleton, you probably imagine something dry and lifeless, hanging in the corner of a lab. So here's a question to keep in mind: once your skeleton has finished growing, is it the skeleton you'll have for the rest of your life? Your own skeleton is nothing like the one in the lab. It's living tissue. Most of it started out as a model made of cartilage, it repairs itself when it breaks, and it works as your body's calcium bank. FRAG 1: And it's being taken apart and rebuilt all the time. Each year, about ten per cent of an adult's skeleton is broken down and replaced, so over roughly ten years, you end up with what is, in effect, a new skeleton. By the end of this lecture, you'll know how that's possible, which cells do the work, and why it matters for growth, for injury and for ageing.

Words that build words

Word partMeansExample

osteo-, os

Bone

Osteocyte · osteon

chondro-

Cartilage

Chondrocyte · synchondrosis

peri- · endo-

Around · within

Periosteum · endosteum

-blast · -cyte · -clast

Builds · cell · breaks down

Osteoblast · osteocyte · osteoclast

syn- · arthr-

Together · joint

Synchondrosis · arthritis

FRAG 0: Before we start, here's a tool that'll save you a lot of memorising. Most anatomical terms are built from Greek and Latin parts, and each part carries meaning. Learn a handful of them, and you can decode words you've never seen before. FRAG 1: Osteo, and its short form os, mean bone. So an osteocyte is a bone cell, and an osteon, as you'll see shortly, is the basic building block of compact bone. FRAG 2: Chondro means cartilage, so a chondrocyte is a cartilage cell. Look out for chondro hiding in the middle of longer words, too, like synchondrosis, a kind of joint you'll meet later on. FRAG 3: Peri means around, and endo means within. The periosteum is the membrane wrapped around the outside of a bone, and the endosteum lines the spaces inside it. So you can probably guess what a perichondrium is. FRAG 4: Three endings tell you what a cell is doing. A blast is a builder, cyte simply means cell, and a clast breaks things down. So osteoblasts lay down new bone, osteocytes look after it, and osteoclasts dissolve it. Cartilage cells use the same endings. FRAG 5: And finally, syn means together, and arthr means joint. In a synchondrosis, two bones are held together by cartilage, and arthritis is inflammation of a joint. As each new term comes up, try taking it apart before I explain it.

Using only its word parts, what is the perichondrium?

Right. Peri means around and chondro means cartilage, so the perichondrium is the membrane wrapped around cartilage, just as the periosteum wraps around bone. You'll meet it again shortly.

Not quite. Split the word into two parts, peri and chondr. One tells you where something is, and the other tells you what it surrounds. Which option fits both?

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Section 1

The skeletal system

What it does, and what it's made of

Five jobs of the skeleton

Support

A rigid framework for the soft tissues

Protection

Skull, vertebrae, rib cage

Movement

Bones as levers, turning at joints

Storage

Calcium and phosphate · fat in yellow marrow

Blood cell production

Red bone marrow

FRAG 0: Ask most people what the skeleton is for, and they'll say it holds us up. That's true, but it's only the first of five jobs, and the other four matter just as much to how the whole body works. FRAG 1: First, support. Bones are rigid, and together they form a framework that carries the body's weight and gives the soft tissues something to hang from. Without it, you'd collapse like a tent without its poles. Cartilage adds firm but flexible support in places like the nose, the ear and the front of the rib cage. FRAG 2: Second, protection. Bone surrounds the organs that would be most easily damaged. The skull encloses the brain, the vertebrae wrap around the spinal cord, and the rib cage shields the heart and lungs, while still letting your chest move as you breathe. FRAG 3: Third, movement. Skeletal muscles pull on bones through their tendons, and the bones act as levers, turning at the joints between them. The shape of each joint decides which way that part of the body can move, and that's where this lecture finishes. FRAG 4: Fourth, storage. Bone is the body's mineral store, mostly calcium and phosphate. When the level of calcium in your blood starts to fall, your body can draw on that store, under the control of hormones you met in Module 1. Bone also stores fat, in the yellow marrow inside many long bones. FRAG 5: And fifth, blood cell production. Red bone marrow, in the spaces inside spongy bone, makes red blood cells, white blood cells and platelets. In an adult, most of it is in the skull, the vertebrae, the ribs and sternum, the pelvis, and the upper ends of the femur and humerus.

Four tissues, one system

Bone

Collagen hardened with mineral: rigid

Cartilage

Firm but flexible; smooth joint surfaces

Ligaments

Bone to bone

Tendons

Muscle to bone

FRAG 0: The skeletal system isn't just bones. It's built from four tissues, and all four are connective tissues. You met connective tissue in Human Structure and Function: cells scattered through a large amount of extracellular matrix, a mix of protein fibres and ground substance that the cells make and secrete around themselves. What makes each of these tissues different is mostly its matrix. FRAG 1: Bone has a matrix of collagen fibres hardened with mineral crystals. That makes it the rigid part of the system, strong enough to carry your weight. Its cells live in tiny spaces inside that hard matrix. We'll look at bone tissue in detail in the next section. FRAG 2: Cartilage has a firm but flexible matrix, rich in collagen and in molecules that hold water, so it can be squashed and then spring back. Here, it covers the ends of the bones where they meet, giving them a smooth, slippery surface. It also forms most of the skeleton before birth. FRAG 3: Ligaments are bands of dense connective tissue, packed with parallel collagen fibres. They join bone to bone, holding a joint together and stopping it from moving too far in the wrong direction. FRAG 4: Tendons are built the same way, from tightly packed parallel collagen fibres, but they join muscle to bone. When a muscle contracts, its tendon passes the pull on to the bone. So if it joins bone to bone, it's a ligament. If it joins muscle to bone, it's a tendon.

A rolled ankle overstretches the tissue that holds the joint's bones together. Which tissue?

Correct. Ligaments join bone to bone, so they hold a joint together and limit how far it moves. A rolled ankle usually means a sprain: ankle ligaments that have been overstretched or torn.

Not quite. Ask what each tissue connects. One joins muscle to bone, one covers the ends of bones, and one wraps around the outside of a bone. You're looking for the one that joins bone to bone.

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Three kinds of cartilage

Hyaline

Smooth, glassy · joint surfaces, rib cartilages, growth plates

Fibrocartilage

Thick collagen bundles · discs, pubic symphysis, menisci

Elastic

Bends and springs back · external ear, epiglottis

FRAG 0: Cartilage comes in three kinds. All three have the same kind of cell, the chondrocyte, sitting in a small space in the matrix. What sets them apart is the fibres in that matrix, and that decides where each kind is found. FRAG 1: Hyaline cartilage is the most common, and the one most closely tied to bone. Its collagen fibres are so fine that the matrix looks smooth and glassy, which is what hyaline means. It covers the ends of bones in joints as articular cartilage, joins the ribs to the sternum, supports the nose and the airways, and forms the growth plates. It also makes up most of the skeleton of an embryo, the model that bone later replaces. FRAG 2: Fibrocartilage has far more collagen, packed into thick bundles, and that makes it tough. It resists both squashing and pulling, so you find it where the forces are greatest: in the intervertebral discs between your vertebrae, in the pubic symphysis at the front of the pelvis, and in the menisci, the C-shaped pads inside your knee. FRAG 3: Elastic cartilage has elastic fibres woven through its matrix, so it bends easily and springs back into shape. Fold the top of your ear over and let it go, and you'll feel it. It's also found in the epiglottis, the flap that covers your airway when you swallow, and in the auditory tube.

How cartilage lives and grows

No blood vessels. No nerves.

Two ways to grow: on the surface, and from within.

FRAG 0: Cartilage is built by cells called chondroblasts. They secrete matrix around themselves until they're trapped inside it, in small spaces called lacunae. From then on, they're called chondrocytes, and their job is to maintain the matrix. The surprising thing about cartilage is that it has no blood vessels and no nerves. Oxygen and nutrients have to diffuse through the matrix to reach each chondrocyte, helped by the water the matrix holds. FRAG 1: So where do new cartilage cells come from? Most cartilage is wrapped in a membrane called the perichondrium. Decode it, the way you did at the start: peri, around, and chondro, cartilage. Its outer layer carries blood vessels, and its inner layer holds cells that become chondroblasts. When they lay down new matrix on the surface, that's appositional growth. Chondrocytes inside the cartilage can also divide and add matrix from within, and that's interstitial growth. Keep both terms in mind, because they'll matter when we look at how bones grow.

A sports injury damages the articular cartilage on the end of the femur. How well will it heal, and why?

How confident are you?

Right. Cartilage has no blood vessels, so nutrients and repair cells can only arrive by slow diffusion through the matrix. Articular cartilage also has no perichondrium, so there's no layer of new chondroblasts on its surface. Damage here heals slowly, and often incompletely.

Not quite. Healing needs nutrients and new cells delivered to the injury. Think about what cartilage lacks that most other tissues have, and how that affects delivery. Then check whether articular cartilage has a perichondrium at all.

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Section 2

Bone as a tissue

Matrix, cells and structure

Reinforced concrete

FeatureOrganic (about 35%)Inorganic (about 65%)

What it is

Collagen fibres, proteoglycans

Calcium phosphate crystals (hydroxyapatite)

What it gives

Flexibility: resists stretching and twisting

Hardness: resists compression

Remove it and…

Hard but brittle: shatters

Bends like rubber

FRAG 0: Bone gets its strength from its matrix, which has two parts working together. A good way to picture it is reinforced concrete, the material used in bridges and buildings. Concrete is hard, but it cracks if it's bent. Steel rods set inside it let it flex a little without breaking. Bone's matrix works the same way. FRAG 1: About a third of the matrix, by weight, is organic, made by the bone cells themselves. It's mostly collagen, the tough protein fibre you met in cartilage, held in a ground substance of proteoglycans. The other two thirds are inorganic: tiny crystals of calcium phosphate, called hydroxyapatite, packed around the collagen fibres. FRAG 2: Each part brings something different. The mineral crystals are the concrete. They make bone hard, so it can carry heavy loads without being crushed. The collagen fibres are the steel rods. They give bone a little flexibility, so it can absorb a sudden impact or a twisting force without snapping. FRAG 3: You can see this by taking one part away. Dissolve the mineral, and the collagen is left: the bone keeps its shape, but it bends. Burn away the collagen, and the mineral is left: the bone is still hard, but it's brittle, and it crumbles. In a genetic condition called osteogenesis imperfecta, the collagen is faulty, and bones break very easily.

A chicken bone soaked in vinegar can be bent, even tied in a knot. What has the vinegar removed?

How confident are you?

Right. Vinegar is an acid. It dissolves the calcium phosphate crystals but leaves the collagen behind, so the bone keeps its shape but loses its hardness. Burn a bone instead and the collagen goes: what's left is hard, but brittle enough to crumble.

Not quite. Think about which part of the matrix is a mineral and which is a protein, and which one an acid would dissolve. Then ask which part, left behind on its own, would let a bone bend.

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Three bone cells

Osteoblasts

Build: lay down new matrix

Osteocytes

Maintain: trapped in lacunae

Osteoclasts

Break down: dissolve matrix

FRAG 0: Bone matrix doesn't build itself. Three types of cell make it, look after it and break it down. This is where the word endings from the start of the lecture pay off. FRAG 1: Osteoblasts are the builders. They make the collagen and proteoglycans of new matrix and release them onto the bone surface, and then they help the mineral crystals form within it. New osteoblasts come from stem cells in the membranes lining the bone: the periosteum on the outside and the endosteum on the inside. FRAG 2: As an osteoblast surrounds itself with matrix, it becomes trapped in a small space, a lacuna, just like a chondrocyte in cartilage. Now it's an osteocyte, a mature bone cell. Osteocytes maintain the matrix around them and stay in touch with their neighbours through long, thin extensions. They also sense the loads on the bone, which helps decide where bone is added or removed. FRAG 3: Osteoclasts break bone down. They're large cells with several nuclei, and they come from the same stem cells in red bone marrow that make some of your white blood cells. An osteoclast seals itself onto the bone surface and releases acid, which dissolves the mineral, and enzymes, which digest the collagen. That frees calcium into the blood, and it's how bone is reshaped and repaired.

Compact and spongy

Compact bone

Dense outer shell; built from osteons

Spongy bone

Trabeculae along stress lines; marrow between

Woven, then lamellar

Random collagen first, then layered sheets

FRAG 0: Cut a bone open, like this femur, and you'll see that bone tissue comes in two forms. Both have exactly the same matrix and cells. What differs is how much of the space is filled with matrix, and how it's arranged. FRAG 1: Compact bone is dense, with very few spaces. It forms the hard outer shell of every bone, and it's thickest in the shaft of a long bone like this, where the bending forces are greatest. As you'll see next, it's built from tightly packed cylinders called osteons. FRAG 2: Spongy bone, also called cancellous bone, is a lattice of thin rods and plates called trabeculae. It fills the ends of long bones and the inside of short, flat and irregular bones. The trabeculae line up along the lines of stress, so spongy bone is strong where it needs to be, but much lighter than solid bone. The spaces between them hold bone marrow, and in the upper end of the femur, that's red marrow, making blood cells. FRAG 3: There's one more distinction, and it's about how the collagen is arranged. When bone first forms, in the embryo or when a fracture heals, its collagen fibres are laid down at random. This is woven bone. It's quick to make, but not very strong, so it's gradually replaced by lamellar bone, where the matrix is laid down in thin sheets called lamellae. Within each sheet the collagen fibres run in parallel, but their direction changes from one sheet to the next, like the layers in plywood. That makes lamellar bone much stronger, and in an adult, both compact and spongy bone are lamellar.

Inside compact bone: the osteon

1Osteon2Central canal3Concentric lamellae4Osteocyte in lacuna5Canaliculi6Perforating canal7Circumferential lamellae + Interstitial lamellae

FRAG 0: Let's zoom into a wedge of compact bone from the shaft. It looks solid, but it's highly organised, and the organisation has one main purpose: every osteocyte, sealed inside hard mineral, still has to be kept alive. FRAG 1: The basic unit is the osteon, also called a Haversian system. Each osteon is a long cylinder running parallel to the length of the bone, and compact bone is made of thousands of them packed side by side. Watch the terms: the osteon is the whole cylinder, not the canal in its middle. FRAG 2: Down the centre of each osteon runs the central canal, also called the Haversian canal. It carries blood vessels and nerves along the length of the bone. FRAG 3: Around the central canal, the matrix is laid down in rings, like the growth rings of a tree trunk. These are the concentric lamellae. They're the lamellar bone you've just met, with the collagen in each ring running in a different direction from the next. FRAG 4: Between the rings are the lacunae, the small spaces that each hold an osteocyte. So the osteocytes sit in circles around the central canal. FRAG 5: Running out from each lacuna are tiny channels called canaliculi, which means little canals. They link the lacunae to one another and, eventually, to the central canal. The osteocytes' long, thin extensions run through them. FRAG 6: The central canals are connected to each other, and to the surface of the bone, by perforating canals. These run at right angles to the central canals, across the bone, bringing vessels in from the outside. FRAG 7: Not all lamellae belong to osteons. Circumferential lamellae wrap around the whole bone, just under the periosteum, and around the inner surface next to the medullary cavity. Between the osteons are interstitial lamellae: fragments of older osteons that were partly removed as the bone was remodelled.

How bone gets its blood supply

Nutrient foramen + Nutrient arteryMedullary cavity (branches)Periosteal arteriesEpiphyseal arteriesPerforating canal + Central canalOsteocyte in lacuna + Canaliculi

FRAG 0: Unlike cartilage, bone has a rich blood supply. That's a big part of why a broken bone can heal well, while damaged cartilage often can't. Blood reaches bone along several routes, from the outside of the bone right down to each osteocyte. FRAG 1: The main supply to a long bone is the nutrient artery. It enters the shaft through a small hole called the nutrient foramen. A foramen is simply a hole in a bone, and you'll meet the word again when we look at bone markings. Most long bones, including the femur, have one or two of them on the shaft. FRAG 2: Inside the medullary cavity, the nutrient artery branches, sending one branch up the shaft and one down. These supply the bone marrow, the spongy bone, and the inner part of the compact bone. FRAG 3: The outer part of the compact bone is supplied from the outside, by small arteries in the periosteum, the membrane wrapped around the bone. That's one reason the periosteum matters so much when a bone heals. FRAG 4: The ends of the bone, the epiphyses, have their own supply: epiphyseal arteries, which enter through small holes near the joint. FRAG 5: Zoom into the compact bone, and you can see how blood gets inside it. Vessels from the periosteum and from the medullary cavity run in through the perforating canals and join the vessels running lengthwise in the central canals. So every osteon has a supply line running down its middle. FRAG 6: That leaves the final step. Oxygen and nutrients leave the vessels in the central canal and pass through the fluid in the canaliculi, and from one osteocyte to the next through gap junctions, where their extensions touch. Wastes travel the opposite way, and veins carry the blood out along the same routes. Now you can see why compact bone is built from osteons: arranged in rings around a central canal, no osteocyte is ever far from a blood supply. Spongy bone doesn't need osteons, because its thin trabeculae keep every osteocyte close to the vessels in the marrow spaces.

Drag each structure onto the job it does.

Drag each item, or click it and then click where it goes

Lacuna

Canaliculi

Nutrient foramen

Central canal

Concentric lamellae

Perforating canal

Carries vessels lengthwise down an osteon

Carries vessels in from the surface, at right angles

Hole where the main artery enters the shaft

Small space that holds one osteocyte

Tiny channels linking osteocytes

Rings of matrix around a central canal

All six in place. The canals and the foramen deliver blood, the lacunae and canaliculi house and connect the osteocytes, and the lamellae are the matrix itself.

Not quite yet. The ones outlined in red are in the wrong place. Hole, channel, space or layer? Then ask what it holds or carries.

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Section 3

Building and growing bone

From embryo to adult, and for the rest of your life

Where bone comes from: the germ layers

Ectoderm

Outer layer: epidermis, nervous system, neural crest

Mesoderm

Middle layer → mesenchyme → bone, cartilage, muscle, blood

Endoderm

Inner layer: lining of the gut and airways

FRAG 0: To see where bone comes from, we need to go right back to the start. In the third week of development, the embryo is a flat disc of cells, and it reorganises itself into three layers, called germ layers. Every tissue in your body comes from one of them. FRAG 1: The outer layer is the ectoderm. It forms the epidermis of your skin and your entire nervous system. One group of ectoderm cells, the neural crest cells, also breaks away and migrates into the head, where it forms much of the face, including many of the facial bones. FRAG 2: The middle layer is the mesoderm, and it's where most of the skeleton comes from. Its cells spread out as mesenchyme: loosely packed, star-shaped cells in a watery matrix. Mesenchyme is embryonic connective tissue, and it can become bone, cartilage, muscle or blood. Decode the word, the way you've been doing: meso means middle. FRAG 3: The inner layer is the endoderm. It forms the lining of the digestive tract and the airways, and glands that grow from them. It doesn't contribute to the skeleton, but it completes the picture: three layers, and bone comes almost entirely from the middle one.

Two ways to make a bone

FeatureIntramembranousEndochondral

Starts from

A sheet of mesenchyme (a membrane)

A model made of hyaline cartilage

Which bones

Flat skull bones, part of mandible, clavicle shaft

Most others: limbs, vertebrae, pelvis, skull base

How

Mesenchyme cells become osteoblasts directly

Cartilage is replaced by bone

First bone made

Woven, later remodelled to lamellar

FRAG 0: From around the eighth week of development, mesenchyme starts turning into bone. It does this in two different ways, and they're named after what the bone forms inside: a membrane, or cartilage. FRAG 1: In intramembranous ossification, intra meaning within, bone forms inside a sheet of mesenchyme that looks like a membrane. In endochondral ossification, endo also meaning within, and chondro meaning cartilage, bone forms inside a model of the bone made of hyaline cartilage, the kind you met in Section 1. FRAG 2: Intramembranous ossification makes the flat bones of the roof and sides of the skull, some of the facial bones, part of the mandible, and the shaft of the clavicle. Endochondral ossification makes almost everything else: the bones of the limbs, the vertebrae, the pelvis, and the bones at the base of the skull. FRAG 3: Intramembranous ossification is the more direct route. Mesenchyme cells gather in the membrane and turn into osteoblasts, which start laying down bone in small patches, called ossification centres, that spread and join up. A newborn's skull isn't finished yet: the soft spots, called fontanels, are membrane that hasn't ossified. Endochondral ossification takes a detour through cartilage, and we'll follow it step by step next. FRAG 4: Either way, the first bone laid down is woven bone, with its collagen running at random. Later it's remodelled into stronger lamellar bone, just as you saw in the last section.

Endochondral ossification

1Cartilage model

2Bone collar; cartilage calcifies

3Primary ossification centre

4Medullary cavity forms

5Secondary ossification centres

6Plate and joint surfaces remain

FRAG 0: Here's endochondral ossification, the way most of your bones were made. Watch how the cartilage model is replaced, from the middle outwards, while the bone keeps growing. FRAG 1: It starts with a model. Chondroblasts make a small version of the future bone out of hyaline cartilage, wrapped in perichondrium, the membrane you decoded earlier. It's left uncovered only at the ends, where the joints will form. The model already has roughly the right shape, and it grows by both appositional and interstitial growth. FRAG 2: Next, around the middle of the shaft, the perichondrium changes. Its cells start making osteoblasts instead of chondroblasts, so from now on it's called periosteum, and those osteoblasts lay down a thin collar of bone around the outside. Inside, the chondrocytes in the centre of the model swell up, and the matrix around them calcifies. Cut off from nutrients, those chondrocytes die. FRAG 3: Now blood vessels grow in through the bone collar and into the calcified cartilage, bringing osteoblasts and osteoclasts with them. The osteoblasts lay down bone on the remains of the calcified cartilage, forming spongy bone. This is the primary ossification centre, in the middle of the shaft. FRAG 4: As ossification spreads towards the ends, osteoclasts break down the spongy bone in the centre of the shaft, hollowing out the medullary cavity. Meanwhile, osteoblasts under the periosteum keep adding bone to the outside of the collar. That's appositional growth, and it's how a bone grows wider, now and throughout childhood. FRAG 5: Around the time of birth, and for some bones years later, blood vessels grow into the ends of the bone too, and the same process begins there. These are the secondary ossification centres, and they turn the epiphyses into spongy bone. FRAG 6: In the end, almost all the cartilage has been replaced, except in two places. A smooth layer stays on the joint surfaces as articular cartilage, and it stays there for life. And a thin plate of cartilage stays between each epiphysis and the shaft: the epiphyseal plate. As long as that plate is there, the bone can keep growing longer.

The epiphyseal plate: five zones

Resting cartilage

Anchors the plate to the epiphysis

Proliferation

Chondrocytes divide, stacking in columns

Hypertrophy

Chondrocytes swell

Calcification

Matrix calcifies; chondrocytes die

Ossification

Osteoblasts lay bone on calcified cartilage

Growth stops

Plate replaced by bone → epiphyseal line

FRAG 0: So how does a bone get longer? Not by bone itself expanding. Bone matrix is too rigid for that. Instead, the growth happens in the cartilage of the epiphyseal plate, which keeps growing at one side while it's replaced by bone at the other. Under the microscope, the plate has five zones. Let's go through them from the epiphysis side towards the shaft. FRAG 1: Next to the epiphysis is the zone of resting cartilage. Its chondrocytes are small and scattered, and they don't do much. This zone anchors the plate to the epiphysis. FRAG 2: Then comes the zone of proliferation. Here the chondrocytes divide rapidly, and the new cells stack up in columns, like piles of coins, all lined up along the length of the bone. This is the interstitial growth you met in cartilage, and it's what pushes the epiphysis further away from the shaft. FRAG 3: In the zone of hypertrophy, the cells near the bottom of each column stop dividing and swell up, becoming much larger. FRAG 4: In the zone of calcification, the matrix between the columns calcifies. With nutrients no longer able to reach them, the chondrocytes die, leaving the calcified cartilage behind as a scaffold. FRAG 5: And in the zone of ossification, blood vessels and osteoblasts move in from the shaft and lay down bone on that calcified scaffold. Later, this new bone is remodelled. So cartilage is added at the top of the plate and replaced by bone at the bottom. The plate stays about the same thickness, but the bone grows longer. FRAG 6: Eventually, between about twelve and twenty-five years of age, depending on the bone, the chondrocytes stop dividing, and the whole plate is replaced by bone. What's left is a thin line of dense bone, the epiphyseal line, and the bone can't grow any longer. It can still grow wider, though, by appositional growth under the periosteum, and it keeps remodelling for life.

On a child's knee X-ray, a dark band crosses each end of the femur. Why is it dark?

Right. X-rays are blocked by mineral, so bone shows up white and soft tissue, including cartilage, looks dark. The dark band is the epiphyseal plate, so this child is still growing. In an adult, you'd see only a thin white epiphyseal line.

Not quite. On an X-ray, mineralised tissue shows up white and everything else looks dark. Which tissue is still found across the ends of a child's long bones, and is it mineralised?

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What shapes growth

FactorRole in boneWhen it goes wrong

Calcium and protein

Mineral and collagen; calcium ~1,000 mg/day (1,300: teens, women 50+, men 70+)

Smaller, weaker bones

Vitamin D

Lets the gut absorb calcium

Soft bones: rickets (children), osteomalacia (adults)

Vitamin C

Needed to make collagen

Scurvy: weak bone, slow healing

Growth and thyroid hormones

Drive growth at the plates

Too little in childhood: short stature

Sex hormones

Puberty growth spurt, then plate closure

Growth stops once the plates close

FRAG 0: Your genes set how tall you could grow and the shape of your bones. But whether you reach that potential depends on three things you've met in this module and the last: nutrition, hormones, and, as you'll see shortly, exercise. Let's take nutrition and hormones first. FRAG 1: Bone needs raw materials. Calcium and phosphate make the mineral, and protein makes the collagen. A child who doesn't get enough of them can't grow to their full height, and their bones end up smaller and weaker. Most adults need about one thousand milligrams of calcium a day, rising to one thousand three hundred for teenagers, women over fifty and men over seventy. FRAG 2: Vitamin D is needed to absorb calcium from the gut. Your skin makes it when it's exposed to sunlight, and you also get some from food; the liver and kidneys then convert it to its active form. Without enough vitamin D, the bone matrix can't mineralise properly and bones stay soft. In children that's rickets, with bowed legs, and in adults it's osteomalacia. FRAG 3: Vitamin C is needed to make collagen. Without it, the matrix is weak, wounds heal slowly, and bones fracture easily. That's scurvy, the old disease of sailors on long voyages without fresh fruit or vegetables. FRAG 4: Now hormones. You met growth hormone in Module 1. It acts mainly through growth factors released by the liver, and it drives growth at the epiphyseal plates by stimulating the cartilage there to grow, as well as appositional bone growth. The thyroid hormones, T3 and T4, are needed too, for normal growth of almost every tissue, including bone. A child with too little of either won't reach their full height. FRAG 5: At puberty, the sex hormones, oestrogen and testosterone, cause the growth spurt. But they also bring growth to an end, because they make the epiphyseal plates close. That's why girls, who usually start puberty earlier, usually stop growing earlier too.

Calcium balance: PTH and calcitonin

1Blood calcium falls

2PTH released

3Bone: PTH acts on osteoblasts → osteoclasts

4Kidneys keep calcium; make active vitamin D

5Gut absorbs more calcium

6Back in range → PTH falls (if too high: calcitonin)

FRAG 0: Remember the storage job of the skeleton? About ninety-nine per cent of the calcium in your body is in your bones. The rest is in your body fluids, and the level in your blood has to be held within a narrow range, about two point one to two point six millimoles per litre. That's because calcium is needed for nerve cells to release their neurotransmitters, for every muscle to contract, including your heart, and for blood to clot. Bone is the bank that keeps that level steady. FRAG 1: Suppose your blood calcium starts to fall. The cells of the parathyroid glands, on the back of the thyroid, detect it directly. You met this in Module 1 as the example of a humoral stimulus: the gland senses the blood itself, with no nerve or other hormone in between. FRAG 2: The parathyroid glands respond by secreting parathyroid hormone, or PTH. It's a water-soluble peptide hormone, so it travels free in the blood and acts on receptors on the surface of its target cells. It works in three places. FRAG 3: The first is bone. Here's an important detail: osteoclasts don't have receptors for PTH. Osteoblasts do. When PTH binds to them, the osteoblasts send a chemical signal to nearby osteoclasts and their precursors, so more osteoclasts form, and they become more active. They dissolve bone matrix and release calcium into the blood. So PTH's effect on osteoclasts is indirect, working through the osteoblasts. FRAG 4: The second is the kidneys. PTH makes them reabsorb more calcium from the fluid they're filtering, so less is lost in the urine. It also stimulates the kidneys to convert vitamin D to its active form. FRAG 5: That leads to the third place, the gut. Active vitamin D increases the amount of calcium absorbed from food in the small intestine. With bone, kidneys and gut all adding calcium to the blood, the level rises back into the normal range. FRAG 6: And here's the negative feedback. As calcium returns to normal, the stimulus to the parathyroid glands fades, and they release less PTH. If blood calcium rises too high, the thyroid's parafollicular cells release calcitonin, which reduces the activity of osteoclasts, so less calcium leaves the bone. That's the opposing pair you met in Module 1. In adults, though, PTH does most of the work.

PTH raises blood calcium in three ways. Which of these is NOT one of them?

How confident are you?

Right. PTH does the opposite: it makes the kidneys hold on to calcium. The other three are PTH's three routes, and remember that the bone route is indirect, with PTH acting on osteoblasts, which then switch on osteoclasts.

Not quite. Go back through the three places PTH acts: bone, kidneys and gut. For each option, ask whether it would push blood calcium up or down. Only one of them would lower it.

Press → to continue

Bone is rebuilt all the time

Out with the old

Osteoclasts remove, osteoblasts rebuild

Load builds bone

Denser bone; trabeculae along stress lines

Disuse loses bone

Bed rest, a plaster cast, spaceflight

FRAG 0: At the start of this lecture, you heard that about ten per cent of an adult skeleton is replaced every year. Here's how that happens. It's called remodelling, and it goes on throughout life. It repairs tiny cracks before they grow, lets bone change shape to suit the loads on it, and is how calcium moves in and out of the bank. FRAG 1: Remodelling always works the same way: out with the old, in with the new. In compact bone, a team of osteoclasts bores a tunnel through old bone, with a capillary following behind. Then osteoblasts line the tunnel and lay down new lamellae in rings, from the outside inwards, until only a central canal is left. The result is a brand-new osteon. The leftover pieces of older osteons around it are the interstitial lamellae you saw earlier. FRAG 2: Where remodelling adds bone depends on the loads your bones carry. Osteocytes sense the strain, and where it's greatest, more bone is laid down. Regular weight-bearing exercise, like running, jumping or lifting, makes bones thicker and denser. Even the trabeculae inside your bones line up along the lines of stress. In the top of the femur, they form arches that carry your body weight from the hip joint down into the shaft. FRAG 3: The opposite is true too. Take the load away, and osteoclasts win. A leg in a plaster cast, a long stay in bed, or months in the weightlessness of space all lead to rapid bone loss. And when breakdown outpaces building over many years, bones become thin and fragile. That's osteoporosis, which you'll meet often in practice. So the answer to the question at the start is no: the skeleton you have now isn't the one you'll have in ten years.

Repairing a fracture

Panel 1: a broken bone shaft with a blood clot (haematoma) filling the break under the lifted periosteum.

Panel 2: fibrocartilage and fibrous tissue bridging the break as an internal callus between the bone ends and an external callus around the outside.

Panel 3: the callus replaced by woven bone, a bony callus still bulging around the break.

Panel 4: the bone remodelled into compact bone with a continuous medullary cavity and a slight thickening.

1 · Haematoma

FRAG 0: Bone's rich blood supply and its ability to remodel mean that, unlike cartilage, it usually heals well when it breaks. Healing happens in four overlapping stages. The first is the haematoma. A fracture tears the blood vessels in the bone and the periosteum, and blood pours into the break and clots. Without their blood supply, bone cells close to the break die, and the area becomes swollen and inflamed. FRAG 1: Within days, the clot is invaded by blood vessels and new cells, many coming from the periosteum and the endosteum. They build a callus, a mass of fibrous tissue and fibrocartilage. Some of it forms between the broken ends, the internal callus, and some forms a collar around the outside, the external callus. It holds the two ends together, but it isn't strong yet. FRAG 2: Next, osteoblasts replace the callus with bone. As you'd expect from the last section, the first bone laid down is woven bone, with its collagen running at random. This bony callus joins the ends firmly, but it's bulky and not as strong as the original bone. FRAG 3: Finally, remodelling takes over. Osteoclasts remove the excess bone, and osteoblasts replace the woven bone with compact lamellar bone. Over months, the bone returns close to its original shape, often with a slight thickening that can last for years. Now think back to the question in your readings: what should someone eat after a bad fracture? Plenty of protein for collagen, calcium for the mineral, vitamin D to absorb that calcium, and vitamin C to make collagen.

Put the stages of fracture repair in order.

Drag the steps into order, first at the top, or use the arrow buttons.

⋮⋮Lamellar bone replaces the woven bone
⋮⋮Fibrocartilage and fibrous tissue bridge the gap
⋮⋮A blood clot fills the break
⋮⋮Woven bone replaces the callus

That's the sequence: haematoma, callus, bony callus, remodelling. The callus is first made of cartilage and fibrous tissue, and woven bone comes before lamellar bone, just as it does in the embryo.

Not quite. The first thing a break does is bleed. Then think about which tissue is quickest to make, and which kind of bone always comes first: woven or lamellar?

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Section 4

The skeleton

Naming bones, and reading their surfaces

Axial and appendicular

Axial: skull

Cranium and face, plus ear ossicles and hyoid

Axial: trunk

Vertebral column, ribs and sternum

Appendicular: upper

Pectoral girdle and upper limbs

Appendicular: lower

Pelvic girdle and lower limbs

FRAG 0: An adult skeleton has two hundred and six bones, and they fall into two divisions. The axial skeleton, eighty bones, runs down the centre line, or axis, of the body. Its main jobs are support and protection. The appendicular skeleton, one hundred and twenty-six bones, is made up of the limbs, or appendages, and the girdles that attach them to the axial skeleton. Its main job is movement. FRAG 1: The axial skeleton starts with the skull: the cranium, which encloses the brain, and the bones of the face. Also counted here are the tiny auditory ossicles, three in each middle ear, and the hyoid, a small U-shaped bone in the neck that anchors the tongue. FRAG 2: Below the skull are the vertebral column, which protects the spinal cord, and the rib cage, made of twelve pairs of ribs and the sternum at the front. Together they form the trunk. FRAG 3: In the appendicular skeleton, each upper limb hangs from a pectoral girdle, made of the clavicle and the scapula. This girdle is attached to the trunk only at one small joint, where the clavicle meets the sternum. That makes the shoulder very mobile. FRAG 4: Each lower limb attaches to the pelvic girdle, formed by the two hip bones. Unlike the pectoral girdle, they're locked firmly onto the sacrum at the back and joined to each other at the front. That makes the pelvic girdle stable enough to carry your body weight. Keep that contrast in mind, because it comes back when we compare the shoulder and the hip.

Tour the skeleton

Click a region to see the bones you need to know there.

Bones to know

–

Tip

–

0 of 7 explored

Four shapes of bone

Long

Longer than wide: limbs, fingers, toes

Short

About as wide as long: wrist and ankle

Flat

Thin plates: skull roof, ribs, sternum, scapula

Irregular

Complex shapes: vertebrae, some facial bones

FRAG 0: Bones are also classified by shape, into four groups. The shape often hints at the job, and each group has its own arrangement of compact and spongy bone. FRAG 1: Long bones are longer than they are wide, with a shaft and two expanded ends. Most bones of the limbs are long bones, from the femur and humerus right down to the small bones of your fingers and toes. They act as levers for movement. FRAG 2: Short bones are roughly as wide as they are long. The carpals of the wrist and the tarsals of the ankle are short bones. Inside, they're like the end of a long bone: spongy bone inside a thin shell of compact bone. They let a joint move a little in many directions while staying strong. FRAG 3: Flat bones are thin, and often curved, like the bones of the roof of the skull, the ribs, the sternum and the scapula. Each is a sandwich: two thin plates of compact bone with a layer of spongy bone between them. They protect the organs beneath them and give large surfaces for muscles to attach to. FRAG 4: Irregular bones don't fit any of the other groups. The vertebrae and some of the bones of the face are irregular. Some flat and irregular bones of the skull also contain air-filled spaces called sinuses, lined by mucous membrane, which make the skull lighter.

Anatomy of a long bone

1Diaphysis2Proximal epiphysis + Distal epiphysis3Articular cartilage4Epiphyseal line5Medullary cavity (yellow marrow)6Red marrow (in spongy bone)7Periosteum8Endosteum

FRAG 0: Here's the femur you saw cut open in Section 2, now with all its parts labelled. Its structure is typical of every long bone, so learn it once and you'll recognise it in the humerus, the tibia, or a bone of your finger. FRAG 1: The shaft is the diaphysis. Its wall is thick compact bone, which resists the bending forces on the middle of a long bone. FRAG 2: Each end is an epiphysis, the proximal one closest to the trunk and the distal one furthest away. The epiphyses are mostly spongy bone inside a thin shell of compact bone, and they're expanded to make a broad surface for the joint. FRAG 3: Where the bone forms a joint, its surface is covered by articular cartilage, the hyaline cartilage left over from the original model. It reduces friction and absorbs shock. FRAG 4: Between each epiphysis and the diaphysis is the epiphyseal line. This is an adult bone, so the plate has been replaced by bone and growth in length has stopped. In a child's femur, you'd see a band of cartilage here instead. FRAG 5: Inside the diaphysis is the medullary cavity. In an adult long bone, it's filled with yellow marrow, which is mostly fat, the fat storage you met in the skeleton's five jobs. FRAG 6: Red marrow, which makes blood cells, fills the spaces in spongy bone. In a child, almost every bone contains red marrow. In an adult, it's mostly found in the bones of the trunk and head: the skull, the vertebrae, the ribs, the sternum and the pelvis, plus the upper ends of the femur and the humerus, as you can see here. In the rest of the limb bones, it's been replaced by yellow marrow. So the skull keeps its red marrow throughout life. FRAG 7: Covering the outside of the bone, everywhere except the articular cartilage, is the periosteum. Its tough outer layer of dense connective tissue gives tendons and ligaments something to anchor to: their collagen fibres blend with it, and some, called perforating fibres, run right into the bone. Its inner layer holds the osteoblasts, osteoclasts and stem cells that remodel and repair bone. FRAG 8: Finally, the endosteum is a thin membrane lining the medullary cavity and the spaces of the spongy bone. It holds the same kinds of cells as the inner periosteum, so bone can be added or removed from the inside too.

Reading the landmarks

FamilyJobTerms

Projections

Anchor muscles, tendons and ligaments

Process, tubercle, tuberosity, trochanter, crest, spine, epicondyle, line

Articular surfaces

Form a joint with another bone

Head, condyle, facet

Depressions and openings

House structures or let them pass

Fossa, groove, notch, foramen, canal, meatus, fissure

FRAG 0: Every bone is covered in bumps, ridges, smooth surfaces and holes, and each one has a name. There are dozens of them, but the terms fall into just three families. Once you know which family a term belongs to, you know roughly what that feature does, even on a bone you've never seen before. FRAG 1: The first family is projections: features that stick out from the bone, where muscles, tendons and ligaments attach. A process is any prominent projection. A tubercle is a small, rounded bump, and a tuberosity is a larger, rougher one. A trochanter is a very large projection, and the femur is the only bone that has them. A crest is a prominent ridge, a line is a lower one, and a spine is a sharp, pointed projection. An epicondyle sits just above a condyle. Epi means upon. FRAG 2: The second family is articular surfaces: smooth areas where a bone forms a joint with another bone, covered in articular cartilage in life. A head is a rounded end, often set on a narrower neck. A condyle is a smooth, rounded knob, and a facet is a small, flat surface. FRAG 3: The third family is depressions and openings. A fossa is a shallow depression, a groove is a long, narrow one, and a notch is a gap in the edge of a bone. Openings let blood vessels and nerves pass through. A foramen is a hole, like the nutrient foramen you met in Section 2. A canal, or meatus, is a tunnel through bone, and a fissure is a narrow slit.

Worked example: the femur

1Head + Fovea capitis2Neck3Greater trochanter4Lesser trochanter5Intertrochanteric line + Intertrochanteric crest6Gluteal tuberosity7Linea aspera + Nutrient foramen8Medial condyle + Lateral condyle + Patellar surface9Medial epicondyle + Lateral epicondyle + Adductor tubercle10Intercondylar fossa

FRAG 0: Let's put the three families to work on one bone: the right femur, seen from the front on the left and from behind on the right. For each feature, notice how its name tells you what kind of feature it is, and then what it does. FRAG 1: At the top is the head, a smooth, rounded articular surface that fits into the socket of the hip bone to form the hip joint. In its centre is a small pit, the fovea capitis, where a small ligament attaches the head to the socket. FRAG 2: Below the head is the neck, the narrower part that joins the head to the shaft at an angle. It's a common place for older people with thin bones to fracture, which is what's usually meant by a broken hip. FRAG 3: On the lateral side is the greater trochanter, a very large projection. Muscles of the buttock attach here and move the hip. You can feel it on the side of your own hip. FRAG 4: On the back and inner side, below the neck, is the smaller lesser trochanter, another projection. It anchors one of the strongest muscles that flex the hip. FRAG 5: Joining the two trochanters is a ridge: the intertrochanteric line at the front, and the more prominent intertrochanteric crest at the back. Inter means between, so the names tell you exactly where they are. The strong capsule and ligaments of the hip joint attach along the line. FRAG 6: Just below the greater trochanter, on the back of the shaft, is the gluteal tuberosity, a roughened bump where the largest muscle of the buttock attaches. FRAG 7: Running down the back of the shaft is the linea aspera, which means rough line. It's a long, raised ridge, a line in our terms, where several thigh muscles attach. Close to it is the nutrient foramen, where the main artery enters the shaft. FRAG 8: At the lower end are the medial and lateral condyles, two large, rounded articular surfaces. They sit on the top of the tibia to form the knee joint. Between them at the front is the patellar surface, where the patella glides as you bend and straighten your knee. FRAG 9: Just above each condyle, on the sides, is an epicondyle, a projection that sits upon the condyle. The epicondyles anchor ligaments that stop the knee bending sideways, and some muscles. On top of the medial epicondyle is a small bump, the adductor tubercle, where one of the muscles that pulls the thigh inwards attaches. FRAG 10: And between the condyles at the back is a deep depression, the intercondylar fossa. It houses the ligaments that cross inside the knee and hold the femur and tibia together.

Click the feature whose name tells you it's a depression.

Click a numbered marker.

Correct: the intercondylar fossa. A fossa is a shallow depression, and this one, between the condyles at the back, houses the ligaments that cross inside the knee.

Not quite. That one is a projection or an articular surface. Of the three families, you're looking for the depressions. Which term on the femur belongs to that family, and where between the articular surfaces would you find it?

Sort each feature into its family.

Drag each item, or click it and then click where it goes

Foramen magnum

Olecranon fossa

External acoustic meatus

Spinous process of a vertebra

Head of the humerus

Tibial tuberosity

Facet on a thoracic vertebra

Condyle of the mandible

Iliac crest

Projection

Articular surface

Depression or opening

All nine sorted. The feature word carries the clue every time: crest, tuberosity and process stick out; head, condyle and facet form joints; foramen, meatus and fossa are holes, tunnels or hollows. (The external acoustic meatus is also called the external auditory meatus.)

Not quite yet. The ones outlined in red are in the wrong place. Ignore the bone. Look at the feature word: which family does it belong to?

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Section 5

Joints

Where bones meet, and how much they move

Two ways to classify a joint

Structural classWhat joins the bonesHow much it usually moves

Fibrous

Dense fibrous connective tissue; no cavity

Immovable or slightly movablesynarthrosis or amphiarthrosis

Cartilaginous

Cartilage; no cavity

Immovable or slightly movablesynarthrosis or amphiarthrosis

Synovial

A fluid-filled cavity inside a capsule

Freely movablediarthrosis

FRAG 0: There are two ways to classify a joint. The structural classification asks what joins the bones together. The functional classification asks how much the joint can move. Structurally, there are three classes. Functionally, there are three too, and each has a plain description and a formal name. You'll see both in this section. FRAG 1: In a fibrous joint, the bones are joined by dense fibrous connective tissue, and there's no space between them. Most fibrous joints are immovable, which is called a synarthrosis, but a few allow slight movement, which is called an amphiarthrosis. FRAG 2: In a cartilaginous joint, the bones are joined by cartilage, again with no space between them. These joints are also either immovable or slightly movable, depending on the type of cartilage and how it's arranged. FRAG 3: In a synovial joint, the ends of the bones are separated by a cavity filled with fluid and enclosed in a capsule. That arrangement lets the bones move freely against each other, so synovial joints are freely movable, which is called a diarthrosis. Decode the formal names as you go. Arthr means joint, so syn-arthrosis is bones held together, amphi means on both sides, and dia means through, as in moving through a range.

Fibrous joints

Sutures

Interlocking skull bonesImmovable – synarthrosis

Syndesmoses

Bones joined by ligament or membraneSlightly movable – amphiarthrosis

Gomphoses

A tooth in its socketImmovable – synarthrosis

FRAG 0: Fibrous joints come in three types, and you can tell them apart by how the bones are joined. FRAG 1: Sutures are found only between the bones of the skull. The edges of the bones interlock like the pieces of a jigsaw puzzle, held together by a thin layer of dense connective tissue, so they're immovable. You met their beginnings in Section 3: the fontanels of a baby's skull close up as the sutures form. In older adults, some sutures ossify completely, and the bones fuse. A joint where bones have fused like this is called a synostosis, syn meaning together and ost meaning bone. FRAG 2: In a syndesmosis, the bones are further apart, joined by a ligament or by a broad sheet of fibrous tissue called an interosseous membrane, meaning between the bones. There's one between the shafts of the radius and ulna, and another between the tibia and fibula. They allow slight movement, so they're amphiarthroses. FRAG 3: A gomphosis is a peg in a socket, and the only examples are your teeth. Each tooth is held in its socket in the jaw by a periodontal ligament, peri meaning around and dont meaning tooth. It's classed as immovable, although a healthy tooth has a tiny bit of give.

Cartilaginous joints

Synchondroses

Hyaline cartilageMostly immovable – synarthrosis

Symphyses

A pad of fibrocartilageSlightly movable – amphiarthrosis

FRAG 0: Cartilaginous joints come in two types, and the difference is the kind of cartilage that joins the bones. Both are names you can decode. FRAG 1: In a synchondrosis, syn meaning together and chondro meaning cartilage, the bones are joined by hyaline cartilage. Some synchondroses are temporary. The epiphyseal plate is a synchondrosis, and when growth stops, its cartilage is replaced by bone, so it becomes a synostosis. Others are permanent: the joint between the first rib's costal cartilage and the sternum, and the costochondral joints, where each rib meets its costal cartilage. Synchondroses are mostly immovable. One thing to watch: the joints between the costal cartilages of ribs two to seven and the sternum aren't synchondroses. They're small synovial joints, which lets the rib cage move as you breathe. FRAG 2: In a symphysis, the bones are joined by a pad of fibrocartilage, the tough, compressible cartilage you met in Section 1. Symphyses allow slight movement, so they're amphiarthroses. The pubic symphysis, at the front of the pelvis, is one, and it gives a little during childbirth. The intervertebral discs are another: each one only allows a small movement, but together they let your spine bend and twist. The joint between the manubrium and the body of the sternum is also a symphysis.

What type of joint is a child's epiphyseal plate, and what does it become in an adult?

Right. The plate joins the epiphysis to the shaft with hyaline cartilage, so it's a synchondrosis. When growth stops, the cartilage is replaced by bone and the two parts fuse: a synostosis, which is the epiphyseal line you saw on the adult femur.

Not quite. First ask what kind of cartilage the plate is made of, and which joint type uses it. Then think back to what happens to the plate when growth stops. What replaces the cartilage?

Press → to continue

Anatomy of a synovial joint

Freely movablediarthrosis

1Articular cartilage2Joint cavity (synovial fluid)3Synovial membrane4Fibrous capsule5Ligament6Bursa7Tendon

FRAG 0: Here's the joint you saw at the start of the lecture, now with all its parts labelled. Every synovial joint, from your knuckles to your hip, shares these features. Most of them are in the appendicular skeleton, the part built for movement. FRAG 1: The ends of the bones are covered with articular cartilage, smooth hyaline cartilage that cuts friction and absorbs shock. Remember that it has no blood vessels and no perichondrium. So how does it get its nutrients? From the fluid in the joint. FRAG 2: Between the bones is the joint cavity, a narrow space filled with synovial fluid. This thin, slippery fluid lubricates the joint and feeds the chondrocytes of the articular cartilage. Moving the joint helps circulate it, which is one reason gentle movement is good for joints. FRAG 3: The fluid is made by the synovial membrane, which lines the inside of the capsule everywhere except over the articular cartilage. FRAG 4: Around the outside is the fibrous capsule, made of dense connective tissue. It's continuous with the periosteum of each bone, so it seals the joint and holds the bones together. The fibrous capsule and the synovial membrane together form the joint capsule. FRAG 5: Ligaments strengthen the joint. Some are thickenings of the capsule, and others, like this one, are separate bands outside it. As you saw earlier, they join bone to bone and stop the joint moving too far. Some joints, like the knee, also have pads of fibrocartilage inside, like the menisci you met in Section 1. FRAG 6: A bursa is a small, flattened sac of synovial fluid, sitting where a tendon or muscle rubs against bone or another structure. It reduces friction. When a bursa becomes inflamed, that's bursitis. FRAG 7: And tendons cross the joint to attach to the bone beyond it. When their muscles contract, they move the bones at the joint. Some long tendons are wrapped in a tendon sheath, a tube-shaped bursa that lets them glide.

Six shapes of synovial joint

Plane

Flat surfaces slide · between carpals and tarsals

Hinge

One plane, like a door · elbow, knee

Pivot

Rotation around one axis · atlas and axis, radius

Condyloid

Oval in an oval · wrist, knuckles

Saddle

Two saddles fit together · base of the thumb

Ball-and-socket

Every direction, including rotation · shoulder, hip

FRAG 0: Synovial joints share the same basic structure, but the shapes of the articulating surfaces differ, and the shape decides which way the joint can move. There are six shapes to know. FRAG 1: In a plane joint, the two surfaces are flat, or nearly flat, so the bones can only slide a little over each other. That's called gliding. You'll find plane joints between the carpals of the wrist and the tarsals of the ankle. FRAG 2: In a hinge joint, a rounded surface fits into a curved one, so movement happens in one plane only, like a door on its hinges. Your elbow is a hinge joint, and so are the joints between the bones of your fingers. The knee works mostly like a hinge too. FRAG 3: In a pivot joint, a rounded or peg-shaped part of one bone turns inside a ring formed by another bone and a ligament. The only movement is rotation. The first vertebra, the atlas, pivots around a peg on the second, the axis, when you shake your head to say no. At the elbow, the head of the radius pivots too, which lets you turn your palm up and down. FRAG 4: In a condyloid joint, also called an ellipsoid joint, an oval surface fits into an oval hollow. It can move in two planes, forwards and back, and side to side, but it can't rotate. Your wrist is a condyloid joint, and so are your knuckles. FRAG 5: In a saddle joint, each surface is shaped like a saddle, and they fit together crosswise, like a rider on a horse. It also moves in two planes, but with a wider range. The best example is at the base of your thumb, and it's what lets your thumb reach across to touch each of your fingers. FRAG 6: And in a ball-and-socket joint, a ball-shaped head fits into a cup-shaped socket. It can move in every direction, including rotation, so it's the most mobile type of joint. There are two big examples: the shoulder and the hip.

Shoulder vs hip

FeatureShoulderHip

The socket

Shallow glenoid cavity; large head

Deep acetabulum grips the head

Capsule and ligaments

Loose capsule; relatively weak ligaments

Thick capsule; very strong ligaments

Muscles

Rotator cuff tendons hold the head in place

Powerful muscles all around add support

The result

Most mobile joint; dislocates most often

Very stable; rarely dislocates

FRAG 0: The shoulder and the hip are both ball-and-socket joints, but they behave very differently. Comparing them shows a trade-off that applies to every joint: the more mobile a joint is, the less stable it tends to be. You'll explore this with real bones in the lab. FRAG 1: Start with the socket. At the shoulder, the large, rounded head of the humerus sits against the glenoid cavity of the scapula, which is small and shallow, a bit like a golf ball on its tee. At the hip, the head of the femur sits deep inside the acetabulum, a cup in the hip bone that wraps around much of the head. FRAG 2: Next, the capsule and ligaments. The shoulder's capsule is loose, which gives the arm its huge range of movement, and its ligaments are relatively weak. The hip has a thick capsule, reinforced by some of the strongest ligaments in the body. FRAG 3: So what keeps the shoulder in place? Mainly a group of four muscles called the rotator cuff. Their tendons wrap around the joint and hold the head of the humerus against the socket. The hip is surrounded by powerful muscles too, like the buttock muscles that attach to the greater trochanter, and they add to its stability. FRAG 4: The result is a classic trade-off. The shoulder is the most mobile joint in the body, but it's also the one that dislocates most often, and rotator cuff injuries are common in sport. The hip has a smaller range of movement, but it's very stable: it takes a huge force, like a car crash, to dislocate a healthy hip. Remember the girdles too: the pectoral girdle barely attaches to the trunk, while the pelvic girdle is locked onto the sacrum.

Sort each joint into its structural class.

Drag each item, or click it and then click where it goes

Knee

Shoulder

Tooth in its socket

Interosseous membrane between radius and ulna

Child's epiphyseal plate

Elbow

Intervertebral disc

Coronal suture

Pubic symphysis

Fibrous

Cartilaginous

Synovial

All nine sorted. Sutures, gomphoses and syndesmoses are fibrous; synchondroses and symphyses are cartilaginous; and every freely movable joint with a fluid-filled cavity is synovial.

Not quite yet. The ones outlined in red are in the wrong place. Ask what joins the bones: fibrous tissue, cartilage, or a fluid-filled cavity?

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Section 6

Movement

Naming what joints do

The reference point

Every movement starts from the anatomical position.

Most come in opposite pairs. Two reference planes: coronal and median.

FRAG 0: Every movement is named as if it starts from the anatomical position: standing upright, facing forward, arms by your sides with your palms facing forward, and feet together pointing forward. In this position, most of your joints are fully extended, except the ankle, which is in a neutral position. The simplest movement of all is gliding, where flat surfaces slide a little over each other, as they do at the plane joints between your carpals. FRAG 1: Most other movements come in opposite pairs: one takes a body part away from the anatomical position, and its partner brings it back. To name them, you need two reference planes, which you met in Human Structure and Function. The coronal plane divides the body into front and back. The median plane runs down the middle, dividing it into left and right. Keep those two planes in mind, because the first movements we'll look at are defined by them.

Angular movements

1Flexion and extension

2Hyperextension

3The knee is the exception

4Plantar flexion and dorsiflexion

5Abduction and adduction

6Fingers and toes

FRAG 0: The first group is the angular movements, where the angle between two bones changes. They're defined by the two reference planes you've just met. FRAG 1: Flexion is movement of a body part forward of the coronal plane. Extension is movement back behind it. So raising your arm forwards at the shoulder is flexion, and bringing it back down to your side is extension. At the elbow, bending moves the forearm forwards, so bending your elbow is flexion and straightening it is extension. Try it: flex your neck to look down at your feet, then extend it. FRAG 2: Hyperextension is extension beyond the anatomical position. Swing your arm backwards behind you, or tip your head back to look at the ceiling, and you're hyperextending the shoulder or the neck. FRAG 3: The knee is the exception to the rule. When you bend your knee, your lower leg moves backwards, behind the coronal plane, but this is still called flexion. Straightening the knee is extension. It's easiest to remember that bending a joint is almost always flexion. FRAG 4: The ankle has its own terms. Pointing your foot down, as when you stand on tiptoe, is plantar flexion, because the foot moves towards its plantar surface, the sole. Pulling your toes up towards your shin is dorsiflexion, towards the dorsum, the top of the foot. FRAG 5: Abduction is movement away from the median plane, and adduction is movement towards it. Lift your arm out to the side, and you're abducting it at the shoulder. Lower it back down, and you're adducting it. The same applies at the hip. Here's a way to remember: abduction takes the limb away, just as an abduction takes a person away. FRAG 6: For fingers and toes, the reference line isn't the median plane of the body. For the fingers, it's a line through the middle finger. Spreading your fingers apart is abduction, and bringing them together is adduction.

Circular movements

1Rotation

2Pronation and supination

3Circumduction

FRAG 0: The next three movements are circular: a part turns around an axis, or moves in an arc. FRAG 1: Rotation is turning a bone around its own long axis. Bend your elbow to ninety degrees, then swing your hand in across your body. That's medial rotation at the shoulder. Swing it out to the side, and that's lateral rotation. The hip rotates in the same way. When you shake your head to say no, you're rotating your head at the pivot joint between the atlas and the axis. FRAG 2: Pronation and supination are the special rotation of the forearm: turning your palm up and down. With your elbow bent and your forearm in front of you, turn your palm down. That's pronation, and the radius crosses over the ulna to do it. Turn your palm up, and that's supination. It happens at the pivot joints between the radius and the ulna. A memory aid: you hold a bowl of soup with your palm up, in supination. FRAG 3: Circumduction is a combination of flexion, extension, abduction and adduction, one after another. Swing your straight arm in a circle: your hand traces a circle, and your whole arm sweeps out a cone. Only joints that can do all four of those movements can circumduct, like the shoulder and the hip.

Special movements

1Elevation and depression

2Protraction and retraction

3Excursion

4Opposition and reposition

5Inversion and eversion

FRAG 0: Now the special movements. Each one is found at only one or two places in the body, and like the others, most come in opposite pairs. FRAG 1: Elevation moves a part upwards, and depression moves it down. Shrug your shoulders, and you're elevating your scapulae. Open your mouth, and you're depressing your mandible. Closing it is elevation. FRAG 2: Protraction is a gliding movement forwards, and retraction brings it back. Jut your jaw forward, and that's protraction of the mandible. Pull it back, and that's retraction. Your scapulae protract when you reach forward to push something, and retract when you squeeze your shoulder blades together. FRAG 3: Excursion is moving the mandible from side to side. Moving it out to one side is lateral excursion, and bringing it back to the middle is medial excursion. You use it every time you grind your food between your molars. FRAG 4: Opposition is the movement of the thumb reaching across the palm to touch the tips of your fingers, especially the little finger. It's made possible by the saddle joint at the base of your thumb, and it's what lets you grip a pen or pick up a coin. Moving the thumb back to the anatomical position is reposition. FRAG 5: Finally, inversion and eversion happen in the foot. Turning your soles inwards, to face each other, is inversion. Turning them outwards is eversion. Most rolled ankles happen when the foot is forced into inversion, which overstretches the ligaments on the outside of the ankle.

Movements at selected joints

Click a joint to see its type and the movements it allows.

Joint type

–

Movements

–

0 of 7 explored

Drag each action onto the movement it involves.

Drag each item, or click it and then click where it goes

Bending at the hip as you sit down

Turning your palm up to receive change

Pushing your jaw forward

Touching your thumb to your little finger

Shrugging your shoulders

Standing on tiptoe

Elevation

Plantar flexion

Supination

Opposition

Protraction

Flexion (at the hip)

All six matched. Shrugging lifts the scapulae, tiptoe points the foot towards the sole, supination turns the palm up, opposition brings the thumb across the palm, protraction glides the jaw forward, and sitting bends the hip forward of the coronal plane.

Not quite yet. The ones outlined in red are in the wrong place. Do the action yourself, then ask which way the part moves and which pair that movement belongs to.

Press → to continue

Flex your shoulder to 90°, flex your elbow to 90°, then fully pronate your forearm. Which way does your palm face?

How confident are you?

Right. Flexing the shoulder points your arm forward, and flexing the elbow points your forearm up, so your palm faces back towards your face. Full pronation then turns the palm half a turn, so it faces forwards, away from you.

Not quite. Do it step by step, starting from the anatomical position with your palm facing forward. Notice which way your palm faces after each step, then remember that pronation turns the palm over, half a turn.

Press → to continue

Five big ideas

Bone is alive

Cells in a mineral-and-collagen matrix, with a blood supply

Built on a model

Cartilage first; growth at the plate until it closes

A calcium bank

PTH draws on bone; remodelling never stops

The name gives the job

Projections, articular surfaces, depressions and openings

Structure decides movement

Joint type and shape set the range

FRAG 0: We've covered a lot of ground, from the cells inside bone to the way your whole body moves. Before you reflect on it, here are five big ideas to take away. If these make sense, the details will hang together. FRAG 1: First: bone is living tissue. Its matrix combines collagen, which lets it flex, with mineral crystals, which make it hard. Osteoblasts build it, osteocytes maintain it from inside their lacunae, and osteoclasts break it down. And unlike cartilage, it has a rich blood supply, reaching every osteocyte through central canals and canaliculi. FRAG 2: Second: most bones are built on a model. In the embryo, mesenchyme becomes either a membrane or a cartilage model, and bone replaces it. Long bones then grow in length at the epiphyseal plate, through its five zones, until the plate closes and becomes a line. They grow in width under the periosteum. FRAG 3: Third: bone is the body's calcium bank. When blood calcium falls, PTH acts on osteoblasts, which switch on osteoclasts to release calcium, while the kidneys and the gut add more. And bone is never finished: it's remodelled throughout life, responding to the loads you put on it, and repairing itself when it breaks. That's the answer to the question at the start. About ten per cent of your skeleton is replaced every year. FRAG 4: Fourth: the name of a bone marking tells you its job. Projections, like trochanters and tuberosities, anchor muscles and ligaments. Articular surfaces, like heads and condyles, form joints. Depressions and openings, like fossae and foramina, hold structures or let them pass. FRAG 5: And fifth: a joint's structure decides how it moves. Fibrous and cartilaginous joints barely move, while synovial joints move freely, and their shape sets the directions. The shoulder trades stability for mobility, and the hip does the opposite. Every movement is named from the anatomical position, and most come in opposite pairs.

Reflect

1. A young adult spent their whole childhood in a refugee camp. Would you expect them to reach their potential height? Explain why or why not.

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2. Choose one idea from this lecture that will matter in your future profession, and explain how.

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Your notes

HUMB1001 · Module 2 · How is the Body Supported?

Five big ideas

Bone is alive — Cells in a mineral-and-collagen matrix, with a blood supply
Built on a model — Cartilage first; growth at the plate until it closes
A calcium bank — PTH draws on bone; remodelling never stops
The name gives the job — Projections, articular surfaces, depressions and openings
Structure decides movement — Joint type and shape set the range

My reflections

A young adult spent their whole childhood in a refugee camp. Would you expect them to reach their potential height? Explain why or why not.

Choose one idea from this lecture that will matter in your future profession, and explain how.

Before your lab this week

Seeley chapters 6–8
Pre-lab “The Skeleton”; print your lab notes
Lab coat, safety glasses, closed-in shoes; long hair tied back
Anatomy Compliance Quiz at 100% → authorisation sticker
After the lab: Module 2 revision quizzes and FeedForward questions

FRAG 0: Here's your notes page. It brings together the five big ideas and your own reflections, and you can print it or save it as a PDF. Before your lab this week, read chapters six to eight of your textbook, complete the pre-lab activity on the skeleton, and print your lab notes. Bring your lab coat, safety glasses and closed-in shoes, and tie long hair back. If you haven't already, complete the Anatomy Compliance Quiz on the unit site: you need full marks to receive the anatomy authorisation sticker. Without these, you can't work in the laboratory. Afterwards, the revision quizzes and FeedForward questions are a good way to check your understanding. They're not marked, so use them as often as you like. That's the end of Module 2. Next week, in Module 3, we'll ask how we move, building on the bones and joints you've met here. See you in the lab.