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Week 7 · Chapter

Chapter 7 — The Skeletal System: Bone Tissue & Structure

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 4 (describe bone as a living tissue — its functions, the gross and microscopic anatomy of a long bone, the four bone cells, ossification, growth, remodeling, and the calcium-homeostasis loop — and sort the skeleton into its axial and appendicular divisions by region while classifying joints by structure, function, and movement), Objective 3 (identify the four primary tissue types and their subtypes from their structural features, name an epithelium by its layers and cell shape, and describe the layers, accessory structures, and homeostatic functions of the integumentary system, including thermoregulation as a negative-feedback loop) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Build a Bone lab all build on it.

In plain language

Your bones are not dry sticks; they are a living tissue that is torn down and rebuilt all your life. The skeletal system (your bones and their cartilage) holds you up, lets you move, guards your organs, makes your blood cells, and stores calcium and fat. This week you read one long bone the way an anatomist does, from its shaft to its two ends and into its hollow center. You zoom in to the tiny rings and struts inside bone and meet the four cells that build it, keep it, and break it down. And you run Week 1's feedback loop again, this time on the calcium in your blood.

Why you already care about this

Picture three made-up scenes. An astronaut comes home after many months on a space station, where weightless days have cost bone. A seven-year-old falls off a playground climber, and the image of the arm shows a crack near one end of the bone, the zone where that bone is still growing. A runner adds 2 short hill runs a week, and over the next 3 months the leg bones carry that new load. You have heard versions of all three stories, and each one is bone acting like the living tissue it is.

None of those stories would happen to a stick of chalk. Chalk does not thin out in space, does not grow, and does not answer to a heavier load. Bone does all three, because inside its hard matrix are blood vessels, nerves, and cells that never stop working. This chapter gives you the map of that tissue, from a shaft you can hold in your hand down to the cells inside it.

By the end of this chapter you will be able to:

list the skeletal system's six jobs and say where blood cells and fat are stored;
sort a bone into one of the five shape classes;
label a long bone end to end and in cross-section, and say what each part does;
tell compact bone from spongy bone by the osteon and the trabeculae;
match each of the four bone cells to its job, without swapping osteoblast and osteoclast;
say how a bone forms, where it lengthens, and why it keeps rebuilding;
run the calcium loop in both directions with its receptor, control center, and effector.

Carry this line with you: blast builds; clast resorbs — not the reverse.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, gave you the feedback loop and the proximal/distal pair. Week 2, The Chemistry of Life, named calcium and phosphorus as ions the bone week would need. Week 3, The Cell & Membrane Transport, gave you the cell and its organelles. Week 4, Cellular Energy & Protein Synthesis, showed how cells build proteins such as collagen. Week 5, Tissues, put bone and cartilage in the supportive connective tissue family. Week 6, The Integumentary System, showed your skin starting the vitamin D you need to absorb calcium.
This week: the skeletal system's six jobs, bones by shape, the parts of a long bone, compact and spongy bone, the four bone cells, ossification and growth, remodeling, the calcium loop, and a structure-to-consequence read of two injuries.
How they connect: Week 5's connective-tissue family gets its hardest member, and Week 1's loop gets a new variable, blood calcium, with bone cells as its effectors.
Where we're going next: Week 8, the midterm review, gathers Weeks 1 to 7 and adds one new reading habit; after it, Week 9 sorts every bone of the skeleton and its joints.

Here is what you lean on this week. In Week 1 you learned that a feedback loop has a receptor that senses, a control center that compares with the set point, and an effector that reverses the change. You also learned that proximal means closer to where a limb attaches and distal means farther from it. In Week 5 you met connective tissue: cells scattered in a matrix that the cells themselves make. You also learned that cartilage has no blood vessels and is slow to heal. Bone is the connective tissue that breaks that pattern, because it is hard and it is full of vessels.

1. Meet a living tissue: bone is not dead mineral

Pick up a clean bone at a museum display, or picture one, and it feels like a rock. It is pale, hard, and cool to the touch. That is why so many people think of bone as dead mineral, a frame of stone that the living body hangs on.

Bone is the opposite. Bone tissue is a living supportive connective tissue with blood vessels, nerves, and cells inside a hard, calcified matrix; it rebuilds itself throughout life. It belongs to the same family as cartilage, the supportive connective tissue from Week 5. The difference is the matrix. Cartilage stays flexible and has no blood vessels of its own; bone's matrix is hardened with minerals, and vessels run all through it.

The proof that bone is alive is that it answers to the stress placed on it. On long space missions, astronauts can lose roughly 1 to 1.5 percent of their bone density per month. Treat that as an estimate you know in words, not a figure to memorize; it is taught in words because it varies from one mission and one person to the next. Without the daily pull of gravity, the skeleton is carrying less load, and living bone responds by thinning.

Worked example — living or not? You test the "dead mineral" picture against three observations.

A broken living bone bleeds, because vessels run through it. Dead mineral has no vessels; living bone tissue does.
A child's bones get longer every year. Dead mineral cannot grow; living bone can.
An astronaut's bones thin in space. Dead mineral would not care about gravity; living bone answers to load.
In one line: all three observations need living cells and vessels, so bone is a living tissue.

Misconception check. Students often say "bone is dead mineral," because a museum bone looks like stone and feels like it. It is tempting because the bone you can see is the mineral left behind after the living parts are gone. The cure is to remember what a living bone holds: blood vessels, nerves, and working cells inside the hard matrix. A tissue that bleeds, grows, and thins without load is alive.

2. List the jobs: six things your skeleton does

Think about one ordinary afternoon. You stand up from a chair, carry a full backpack up a staircase, and bump your head lightly on a cupboard door. Through all of it, your bones were holding you up, moving you, and guarding your brain, and they were also doing three quieter jobs you never felt.

The skeletal system is the organ system made of the bones and their cartilage that supports the body, makes movement possible, protects organs, produces blood cells, stores minerals, and stores fat. Its six functions, in the order this course uses, are:

Support. Bones hold the body up and give it its frame.
Movement. Bones are levers and joints are the pivots; muscles pull on the bones to move you.
Protection. Bones guard internal organs, as the skull guards the brain.
Blood cell production. Hematopoiesis is the production of blood cells, which takes place in the red marrow of bones. Red marrow is the soft tissue in the spaces of spongy bone where blood cells are made.
Mineral storage and release. Bone stores minerals such as calcium and phosphorus and releases them into the blood when the body needs them.
Fat storage. Yellow marrow is the fatty tissue in the medullary cavity of a long bone that stores energy as triglycerides.

Notice the Week 2 words in jobs 5 and 6. Calcium and phosphorus were two of the ions Week 2 said the bone week would need, and triglycerides were Week 2's lipid, glycerol plus fatty acids.

Worked example — match the moment to the job. You name the skeletal job behind each moment.

You stand up straight from a chair: support.
You lift your arm to wave: movement, with the bone as the lever and the shoulder as the pivot.
A bike helmet and the skull under it both shield the brain: protection.
Your body replaces worn-out blood cells: blood cell production, in red marrow.
Your blood calcium drops below its set point: mineral release from bone.
Your body tucks away extra energy: fat storage, as triglycerides in yellow marrow.

Misconception check. Students often list only the first three jobs, support, movement, and protection, because those are the ones you can feel. The cure is to count to six. Bones also make blood cells, store and release calcium and phosphorus, and store fat. A second slip is swapping the two marrows: red marrow makes blood cells; yellow marrow stores fat.

3. Sort by shape: five classes of bone

Lay out five items from a kitchen drawer: a rolling pin, a sugar cube, a cutting board, a twisted pastry, and a dried bean. You would sort them by shape without thinking. Anatomists sort bones the same way, and they use five classes.

A long bone is a bone whose shape is a shaft with two wider ends, such as the femur, the humerus, or a phalanx; the class names a shape, not a size.
A short bone belongs to the bone class that holds the carpals and the tarsals, sorted apart from the long, flat, irregular, and sesamoid classes.
A flat bone is a bone shaped like a thin plate, such as the sternum, a rib, a scapula, or a cranial bone.
An irregular bone is a bone whose shape fits none of the other classes, such as a vertebra or a facial bone.
A sesamoid bone belongs to the bone class that holds the patellae, sorted apart from the long, short, flat, and irregular classes.

Worked example — the bone key, five classes. You sort five bones, one per class, and count the classes.

Femur: long.
Carpal: short.
Sternum: flat.
Vertebra: irregular.
Patella: sesamoid.
Count the classes: 1 + 1 + 1 + 1 + 1 = 5.

In words: every bone in this course lands in exactly one of five shape classes, and you sort it by its shape.

Misconception check. Students often call a small bone "short" and a big bone "long," because the class names sound like sizes. The cure is the shape rule. A phalanx of your hand is small, yet it is a long bone, because it is a shaft with two ends. The class names a shape, not a size.

The named bones of each region, and the joints between them, wait for Week 9. This week you need only the five classes and one example of each.

4. Read a long bone: shaft, ends, and the hollow center

Hold a drumstick upright, or picture one, with a knob at each end. A long bone stands the same way in an atlas figure: a tube in the middle and a wider end at the top and bottom. Read it top to bottom, then cut across the middle and read it from the outside in.

Start with the parts you can see from the outside.

The diaphysis is the tubular shaft of a long bone, with walls of compact bone around the medullary cavity.
An epiphysis is either wider end of a long bone, filled with spongy bone; the plural is epiphyses. In a limb bone, the proximal epiphysis is the end closer to where the limb attaches, and the distal epiphysis is the end farther from it.
The metaphysis is the region of a long bone between the diaphysis and an epiphysis that holds the epiphyseal plate or line.
The epiphyseal plate is the layer of hyaline cartilage in the metaphysis of a growing long bone where the bone lengthens; it is also called the growth plate.
The epiphyseal line is the line of bone left in the metaphysis of an adult long bone once the epiphyseal plate has been replaced by bone.
Articular cartilage is the cartilage covering the joint surfaces of a bone, which reduces friction and absorbs shock.

Then the parts you meet when you cut across the shaft.

The periosteum is the fibrous membrane that covers the outer surface of a bone.
The endosteum is the thin membrane that lines the medullary cavity of a bone.
The medullary cavity is the hollow center of a long bone's shaft, which holds yellow marrow.

Worked example — the bone key, end to end and outside in. You read a long bone two ways.

End to end: epiphysis → metaphysis (epiphyseal plate in a child, epiphyseal line in an adult) → diaphysis → metaphysis → epiphysis.
In cross-section, outside in: periosteum → compact bone → endosteum → medullary cavity with yellow marrow.
Read the cross-section with the superficial/deep pair: the periosteum is superficial to the compact bone, and the medullary cavity is deep to the endosteum.
In one line: diaphysis = shaft, epiphysis = end, and the hollow shaft holds yellow marrow.

Read it as structure to function. The shaft is a tube, so it is strong without being solid all the way through, and its hollow center has room to store fat. The ends are wide where they meet other bones, and their articular cartilage lets them glide. The plate sits between the end and the shaft, which is exactly where a growing bone adds length.

Misconception check. Students often call the epiphysis the shaft, because both words start with "epi-" or "dia-" and blur together. The cure is one short rule: diaphysis = shaft, epiphysis = end. As a memory trick, hear "dia" as in diameter, a line through the middle; the epiphyses sit at the ends.

5. Zoom into the tissue: compact bone and spongy bone

Look closely at a cut log. You see rings packed tight around a center. Now look at a kitchen sponge, a lattice of thin walls with open spaces between them. Bone comes in both textures, and you will see both on any cut long bone.

Compact bone is the dense bone tissue of a bone's outer wall, built from osteons packed side by side. Zoom in on it and you find its unit.

An osteon is the structural unit of compact bone, made of concentric lamellae around a central canal; it is also called the Haversian system.
Lamellae are the concentric rings of calcified matrix that make up an osteon.
The central canal is the channel at the center of an osteon that carries blood vessels, nerves, and lymphatic vessels.
Lacunae are the small spaces in bone matrix where osteocytes sit.
Canaliculi are the tiny channels in bone matrix that link the lacunae, and the osteocytes inside them, to one another.

Spongy bone is the bone tissue built as a lattice of trabeculae with red marrow in the spaces; it has no osteons. Trabeculae are the thin struts of bone that form the lattice of spongy bone.

Worked example — compact or spongy? You sort three described textures from a virtual microscope view.

Rings around a central opening, packed side by side, with tiny spaces in the rings: osteons, so compact bone.
A web of thin struts with soft red tissue filling the gaps: trabeculae with red marrow, so spongy bone.
A solid wall around a hollow shaft: compact bone, the wall of the diaphysis.
In one line: osteons mean compact; trabeculae mean spongy, and spongy bone has no osteons.

Read it as structure to function. Each osteon's central canal brings blood to cells locked inside hard matrix, and the canaliculi let those cells reach one another through the stone around them. Spongy bone's open lattice is light, and its spaces give the red marrow room to make blood cells.

Misconception check. Students often say "spongy bone is made of osteons," because both textures are bone and the osteon is the unit everyone memorizes. The cure is the shape. Osteons are rings around a canal, and they belong to compact bone. Spongy bone is a lattice of trabeculae with red marrow in the spaces, and it has no osteons at all.

6. Name the cells: who builds, who keeps, who breaks down

Picture a crew renovating a house. One worker trains new hires, one builds new walls, one lives in the house and keeps it in good shape, and one knocks old walls down. Bone has the same four jobs, and four cells to do them.

An osteogenic cell is a bone stem cell that becomes an osteoblast.
An osteoblast is a bone cell that builds new bone; its job is bone formation.
An osteocyte is a mature bone cell, housed in a lacuna, that maintains the matrix around it.
An osteoclast is a bone cell that resorbs bone, breaking down matrix and releasing calcium into the blood.
Resorption is the breakdown of bone matrix by osteoclasts, which releases its minerals into the blood.

Worked example — the bone key, four cells. You name each cell by its job and count them.

Osteogenic cells become osteoblasts.
Osteoblasts build.
Osteocytes maintain.
Osteoclasts resorb.
Count the cells: 1 + 1 + 1 + 1 = 4.

In words: one cell type replenishes the builders, one builds, one maintains, and one breaks bone down and frees its calcium.

Misconception check. Students often write "osteoblasts break down bone." This is the pair that costs the most points in the bone unit, because the two words differ by only a few letters. The cure is a sound you can hear in your head: blast builds; clast resorbs. If a sentence gives the building job to a clast, two words have traded places.

Pause and think, then choose one. (a) make up your own memory trick for osteoblast and osteoclast that does not use the words "build" or "resorb," and test it on a friend. (b) describe the renovation crew picture in your own words, matching each worker to one of the four bone cells.

7. Watch bone form and grow: two routes and a growth plate

A newborn's skeleton is not a small copy of an adult's. Some of it is already bone, and some of it is still a cartilage model waiting to be replaced. Over the years of childhood, the bones lengthen and widen until growth ends in early adulthood.

Ossification is the formation of bone tissue, by either the intramembranous or the endochondral route.

Mesenchyme is the embryonic tissue from which bone forms directly, in sheets, in intramembranous ossification.
Intramembranous ossification is the route in which bone forms directly from mesenchyme with no cartilage model first, as in the clavicles, the flat bones of the face, and most cranial bones.
Endochondral ossification is the route in which a hyaline-cartilage model is laid down first and bone then replaces it, as in the long bones and the skull base.

Once a long bone has formed, it grows in two directions. It lengthens at its epiphyseal plate until early adulthood. Then the plate's cartilage is replaced by bone and leaves the epiphyseal line. It widens by appositional growth, the widening of a bone as new bone is laid down beneath the periosteum.

Worked example — the bone key, two routes. You sort two bones by how they formed and count the routes.

The clavicle: intramembranous, with no cartilage model.
The femur: endochondral, with a hyaline-cartilage model first.
Count the routes: 1 + 1 = 2.
Where the femur lengthens: at the epiphyseal plates near its two ends, until early adulthood.
Where the femur widens: under the periosteum, by appositional growth.

Misconception check. Students often say "long bones lengthen from the middle," because the shaft looks like the biggest part and feels like where growth would happen. The cure is the plate. Lengthening happens at the epiphyseal plates, near the ends of the bone, between each epiphysis and the shaft. That is why the plate is a line of cartilage in a child and a line of bone in an adult.

Honesty note: this course names two ossification routes and their example bones and stops there; the step-by-step stages inside each route are beyond this course.

8. Watch bone rebuild: remodeling for life

Think about a footpath across a lawn. Wherever people walk most, the path gets worn in, and where nobody walks, the grass grows back. Bone keeps changing in the same way: it follows the load.

Remodeling is the lifelong cycle in which osteoclasts remove bone and osteoblasts build it, shaped by the mechanical stress placed on the bone. More stress means more deposition. Less stress, as in long spaceflight, means bone is lost.

Worked example — follow the load. You predict the direction of remodeling in three made-up situations.

A climber's fingers grip holds for hours each week. More stress on those bones, so remodeling favors deposition: osteoblasts build.
A leg rests in a cast for weeks and carries little weight. Less stress on that bone, so remodeling favors removal: osteoclasts resorb more than osteoblasts build.
An astronaut floats for months on a space station. Less load on the whole skeleton, so bone is lost, which is what the 1 to 1.5 percent per month estimate from Section 1 describes in words.
In one line: more stress, more deposition; less stress, less bone.

Misconception check. Students often say "adult bone never changes," because adults stop getting taller. The cure is to separate two kinds of change. Growth in length stops when the epiphyseal plate becomes the epiphyseal line. Remodeling never stops: osteoclasts and osteoblasts keep removing and building bone all your life.

Caveat: the two everyday cases in that worked example are made up to show the direction of remodeling; they are not measurements, and this course gives no advice about exercise, casts, or injury.

9. Run the loop on calcium: parathyroid hormone and calcitonin

Your muscles and nerves need calcium in the blood, and bone is the body's calcium bank. When the blood runs low, the body makes a withdrawal from bone. When the blood runs high, the body slows the withdrawals. That is a feedback loop, and you already know its three parts from Week 1.

A typical blood calcium level is about 10 mg/dL (ten milligrams per deciliter). Treat that as a value taught in words, not a figure to memorize; laboratories report slightly different ranges. Two hormones carry the signals in this loop, and this course names them only as loop parts.

Parathyroid hormone is a hormone that raises blood calcium by stimulating osteoclasts to resorb bone and release calcium.
Calcitonin is a hormone that lowers blood calcium by inhibiting osteoclasts.

Worked example — the bone key, the calcium loop in both directions. You run the loop one way and then the other, and name the three parts.

Low blood calcium: receptor senses the drop → control center compares it with the set point → parathyroid hormone → osteoclasts resorb bone → calcium is released → blood calcium rises. The response opposes the drop.
High blood calcium: receptor senses the rise → control center compares it with the set point → calcitonin → osteoclasts are inhibited → blood calcium falls. The response opposes the rise.
Count the loop parts: receptor + control center + effector = 3, the same three parts as every Week 1 loop, so the calcium loop is negative feedback.

Say it in words: when the blood's calcium drops, parathyroid hormone sends the osteoclasts to work, they free calcium from bone, and the level climbs back toward its set point.

Misconception check. Students often say parathyroid hormone "builds bone to raise calcium," because it is the hormone that helps calcium. The cure is to follow the calcium. To raise blood calcium, calcium has to leave bone, and the cell that frees it is the osteoclast. Parathyroid hormone stimulates osteoclasts; calcitonin inhibits them. A second slip is calling the loop positive feedback because the calcium "goes up." The names mean direction against the change, not up or down: the response opposes the change, so it is negative feedback.

Honesty note: the endocrine glands that make these hormones belong to the second course in this sequence, so this chapter names the two hormones only as parts of the calcium loop.

10. Read the injury: the growth plate versus the shaft

Two children fall off the same playground climber, and each one cracks a bone in the forearm. On the first image, the crack runs through the epiphyseal plate near one end of the bone. On the second, it runs across the middle of the shaft. You can say which structure each crack reached from what you learned in Section 4.

Worked example — a structure-to-consequence read. You name the structure each crack reached.

The first crack runs through the epiphyseal plate. The plate is the layer of hyaline cartilage where a growing bone lengthens, so this crack reached the zone where the bone grows in length.
The second crack runs across the middle of the shaft. That is the diaphysis, a tube of compact bone around the medullary cavity, away from the plate.
In one line: a growth-plate fracture reaches the bone's lengthening zone; a mid-shaft crack sits in the compact bone of the diaphysis.

That is the week's clinical connection: an injury read by the structure it reaches. You name the structure and you stop there. How a fracture is classified, how it heals, and any care for it are not part of this course.

Misconception check. Students often assume a crack near the end of a child's bone is minor because it is small and far from the middle. The cure is the structure, not the size. Near the end of a growing bone sits the epiphyseal plate, the one place the bone lengthens, so a crack there reaches a structure that a mid-shaft crack does not.

Key terms

Bone tissue — A living supportive connective tissue with blood vessels, nerves, and cells inside a hard, calcified matrix; it rebuilds itself throughout life. (first used: 1. Meet a living tissue: bone is not dead mineral)
Skeletal system — The organ system made of the bones and their cartilage that supports the body, makes movement possible, protects organs, produces blood cells, stores minerals, and stores fat. (first used: 2. List the jobs: six things your skeleton does)
Hematopoiesis — The production of blood cells, which takes place in the red marrow of bones. (first used: 2. List the jobs: six things your skeleton does)
Red marrow — The soft tissue in the spaces of spongy bone where blood cells are made. (first used: 2. List the jobs: six things your skeleton does)
Yellow marrow — The fatty tissue in the medullary cavity of a long bone that stores energy as triglycerides. (first used: 2. List the jobs: six things your skeleton does)
Long bone — A bone whose shape is a shaft with two wider ends, such as the femur, the humerus, or a phalanx; the class names a shape, not a size. (first used: 3. Sort by shape: five classes of bone)
Short bone — The bone class that holds the carpals and the tarsals, sorted apart from the long, flat, irregular, and sesamoid classes. (first used: 3. Sort by shape: five classes of bone)
Flat bone — A bone shaped like a thin plate, such as the sternum, a rib, a scapula, or a cranial bone. (first used: 3. Sort by shape: five classes of bone)
Irregular bone — A bone whose shape fits none of the other classes, such as a vertebra or a facial bone. (first used: 3. Sort by shape: five classes of bone)
Sesamoid bone — The bone class that holds the patellae, sorted apart from the long, short, flat, and irregular classes. (first used: 3. Sort by shape: five classes of bone)
Diaphysis — The tubular shaft of a long bone, with walls of compact bone around the medullary cavity. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Epiphysis — Either wider end of a long bone, filled with spongy bone; the plural is epiphyses. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Metaphysis — The region of a long bone between the diaphysis and an epiphysis that holds the epiphyseal plate or line. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Epiphyseal plate — The layer of hyaline cartilage in the metaphysis of a growing long bone where the bone lengthens; it is also called the growth plate. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Epiphyseal line — The line of bone left in the metaphysis of an adult long bone once the epiphyseal plate has been replaced by bone. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Articular cartilage — The cartilage covering the joint surfaces of a bone, which reduces friction and absorbs shock. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Periosteum — The fibrous membrane that covers the outer surface of a bone. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Endosteum — The thin membrane that lines the medullary cavity of a bone. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Medullary cavity — The hollow center of a long bone's shaft, which holds yellow marrow. (first used: 4. Read a long bone: shaft, ends, and the hollow center)
Compact bone — The dense bone tissue of a bone's outer wall, built from osteons packed side by side. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Osteon — The structural unit of compact bone, made of concentric lamellae around a central canal; it is also called the Haversian system. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Lamellae — The concentric rings of calcified matrix that make up an osteon. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Central canal — The channel at the center of an osteon that carries blood vessels, nerves, and lymphatic vessels. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Lacunae — The small spaces in bone matrix where osteocytes sit. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Canaliculi — The tiny channels in bone matrix that link the lacunae, and the osteocytes inside them, to one another. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Spongy bone — The bone tissue built as a lattice of trabeculae with red marrow in the spaces; it has no osteons. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Trabeculae — The thin struts of bone that form the lattice of spongy bone. (first used: 5. Zoom into the tissue: compact bone and spongy bone)
Osteogenic cell — A bone stem cell that becomes an osteoblast. (first used: 6. Name the cells: who builds, who keeps, who breaks down)
Osteoblast — A bone cell that builds new bone; its job is bone formation. (first used: 6. Name the cells: who builds, who keeps, who breaks down)
Osteocyte — A mature bone cell, housed in a lacuna, that maintains the matrix around it. (first used: 6. Name the cells: who builds, who keeps, who breaks down)
Osteoclast — A bone cell that resorbs bone, breaking down matrix and releasing calcium into the blood. (first used: 6. Name the cells: who builds, who keeps, who breaks down)
Resorption — The breakdown of bone matrix by osteoclasts, which releases its minerals into the blood. (first used: 6. Name the cells: who builds, who keeps, who breaks down)
Ossification — The formation of bone tissue, by either the intramembranous or the endochondral route. (first used: 7. Watch bone form and grow: two routes and a growth plate)
Mesenchyme — The embryonic tissue from which bone forms directly, in sheets, in intramembranous ossification. (first used: 7. Watch bone form and grow: two routes and a growth plate)
Intramembranous ossification — The route in which bone forms directly from mesenchyme with no cartilage model first, as in the clavicles, the flat bones of the face, and most cranial bones. (first used: 7. Watch bone form and grow: two routes and a growth plate)
Endochondral ossification — The route in which a hyaline-cartilage model is laid down first and bone then replaces it, as in the long bones and the skull base. (first used: 7. Watch bone form and grow: two routes and a growth plate)
Appositional growth — The widening of a bone as new bone is laid down beneath the periosteum. (first used: 7. Watch bone form and grow: two routes and a growth plate)
Remodeling — The lifelong cycle in which osteoclasts remove bone and osteoblasts build it, shaped by the mechanical stress placed on the bone. (first used: 8. Watch bone rebuild: remodeling for life)
Parathyroid hormone — A hormone that raises blood calcium by stimulating osteoclasts to resorb bone and release calcium. (first used: 9. Run the loop on calcium: parathyroid hormone and calcitonin)
Calcitonin — A hormone that lowers blood calcium by inhibiting osteoclasts. (first used: 9. Run the loop on calcium: parathyroid hormone and calcitonin)

Summary

Bone is a living connective tissue, not dead mineral.
Six jobs: support, movement, protection, blood cells, minerals, fat.
Red marrow makes blood cells; yellow marrow stores fat.
Five shape classes: long, short, flat, irregular, sesamoid.
Diaphysis is the shaft; epiphysis is the end.
The epiphyseal plate becomes the epiphyseal line in adults.
Compact bone has osteons; spongy bone has trabeculae and no osteons.
Osteoblasts build; osteoclasts resorb; osteocytes maintain.
Intramembranous skips the cartilage model; endochondral replaces one.
Long bones lengthen at the plates and widen under the periosteum.
Remodeling never stops: more stress, more deposition.
Parathyroid hormone raises blood calcium; calcitonin lowers it.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A made-up flash card reads: "Osteoclasts build new bone, and osteoblasts break it down to free calcium." Say what is wrong and rewrite the card correctly.
In a made-up virtual-atlas view, you see a lattice of thin struts with soft red tissue in the gaps, inside the knob at the top of a femur standing upright. Name the bone texture, the struts, the soft tissue, and the part of the long bone you are looking at.
A made-up study guide sorts four bones this way: "rib, long; tarsal, irregular; humerus, long; vertebra, flat." Find each error and give the correct class.
A classmate says, "When blood calcium falls, calcitonin tells osteoblasts to pull calcium out of bone." Using this week's loop, explain what is right and what is wrong about that statement.

Answers (worked reasoning — read after attempting)

The card swapped the pair. Osteoblasts build new bone; osteoclasts break bone down, which is resorption, and release its calcium into the blood. The corrected card: "Osteoblasts build new bone, and osteoclasts resorb it to free calcium." Blast builds; clast resorbs.
A lattice of struts with red tissue in the gaps is spongy bone. The struts are trabeculae, and the soft red tissue is red marrow, where blood cells are made. The knob at the end of a long bone is an epiphysis, and since this one is at the end nearer where the limb attaches, it is the proximal epiphysis. No osteons appear, because spongy bone has none.
The rib is a flat bone, not long. The tarsal is a short bone, not irregular. The humerus is long, and the guide got that one right. The vertebra is an irregular bone, not flat. Count the errors: 1 + 1 + 1 = 3 wrong out of 4.
What is right: falling blood calcium does start a negative-feedback loop that pulls calcium out of bone. What is wrong: the hormone and the cell are both swapped. When blood calcium falls, the signal is parathyroid hormone, not calcitonin, and the cell that frees calcium is the osteoclast, not the osteoblast. Calcitonin is the signal when blood calcium is too high, and it inhibits osteoclasts.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz. Next week the midterm review adds the three-step read: name the structure, state its function, name the clinical connection.

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