Time To Read
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The Skeletal System · Chapter 1 of 7
1985, age 10: her legs ache at night — shins and feet, worst after a hard day, sometimes still there in the morning. “Growing pains. She’ll outgrow it.” This chapter opens the thing that hurt: bone is not a scaffold but a living connective tissue with six jobs, a matrix of rebar and concrete, four kinds of cellsThe basic structural and functional units of life., and its own blood and nerves — which is the only reason it can ache at all.
The question you will answer
A dead scaffold cannot hurt. Before you can judge whether a ten-year-old’s night pain is growthAn increase in size and number of cells. or something else, you need to know what bone is made of, who lives in it, what it does for the body, and why a tissue most people picture as dry and white is wet, wired and busy.
Opener · The Legs That Ached at Night · about 1¾ minutes
The opener is the same narrated animation you will watch in Scene 1.
Watch the opener first. It sets the question; the chapter answers it.
1
Read the chart
Patient chart · Stina
1985 · age 10 · “growing pains”
1985 · 10
Night pain in both shins and feet, worst after active days; sometimes a limp in the morning. A new complaint on a chart that already held the fevers (Module 1) and the spots (Module 5).
The exam
Legs straight, joints not swollen that day, no limp in the office. No bloods.
Mom
Her own legs ached as a girl. “It runs in the family.”
Told
“Growing pains. Massage, a warm bath, she’ll outgrow it.”
Chart clues this module: 1 · The first fever · 5 · The spots · 6 · opens here
Before you go on · your prediction
A ten-year-old whose shins ache at night, worst after a hard day. “Growing pains” is the chart’s reading. What does that reading assume about bone?
Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.
That bone is inert mineral — the pain must be in the muscles pulling on it
Ask what fraction of bone is cells, whether bone has its own nerves, and whether an inert scaffold can report anything.
That growing bone hurts — the plates are stretching and the pain will stop when growth does
Ask whether “growing pains” are actually linked to growth spurts, and which features of benign growing pains hers do and do not match.
That bone is a living, innervated, vascular tissue that can be inflamed like any other
Ask what inflames bone, which cells answer the cytokines, and what the night-pain red flags are.
From Stina’s diary
2
The story
Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.
I · Growing pains
In the autumn of 1985 Stina was ten and her legs hurt at night. Shins mostly, and the feet, worst after a day of running, sometimes still there when she woke so that she came down the stairs one step at a time. Mom knew the complaint; her own legs had ached as a girl. The doctor at the Tuesday clinic straightened each leg, pressed the knees, found nothing swollen, and wrote two words that would follow her for forty years: growing pains. Warm baths, a rub, patience. Nobody drew blood. Nobody asked whether a pain that was there in the morning, that came with the fevers nobody had connected, that sat on the shins rather than behind the knees, was the benign thing the words described. The words were a diagnosis of exclusion that had excluded nothing. And underneath them sat an assumption nobody said aloud: that bone is a dry white scaffold, and a scaffold cannot hurt.
See it move: Scene 1 · The Legs That Ached at Night · Scene 2 · Six Jobs for a Bone
II · Wet, wired and busy
Bone is a connective tissue, and like every connective tissue it is mostly matrix with cells scattered through it — the cells are about two percent of its weight. The matrix has two parts. The organic part, osteoid, is collagenA structural protein in the dermis that provides strength and elasticity. fibers laid down in a deliberate arrangement plus the ground substance that glues them; it gives bone flexibility and tensile strength, the rebar. The inorganic part is mineral salts, mostly calcium phosphate crystallized as hydroxyapatiteA mineral form of calcium phosphate giving bone its hardness., about twice the weight of the collagen; it gives hardness and compressive strength, the concrete. Take the mineral away and a bone bends like a green stick — that is rickets in a child and osteomalacia in an adult, both a failure of mineral, usually from too little vitamin D. Bake the collagen out and the bone shatters. Bone also does six jobs the packet lists: support (the osteon is the unit of strength), protection (the skull), movementA fundamental property of life involving motion of the body or its parts. (muscles attach to it), mineral storage (ninety-nine percent of the body’s calcium), blood-cell formation in red marrowBone marrow that produces blood cells., and fat storage in yellow marrow.
See it move: Scene 2 · Six Jobs for a Bone · Scene 3 · Rebar and Concrete
III · Four cells and a demolition crew
The cells come in four kinds. Osteogenic cellsStem cells that develop into osteoblasts. are the stem cells, sitting in the periosteum and endosteumA thin connective tissue lining the marrow cavity., able to divide and become osteoblasts. Osteoblasts Bone-forming cells that secrete osteoid. are the builders: they secrete osteoid and then mineralize it. An osteoblast that walls itself in becomes an osteocyteMature bone cells that maintain bone tissue., the maintainer, a star-shaped cell in a little chamber called a lacunaSmall spaces in bone tissue where osteocytes reside., its arms reaching down tiny canals to neighbors — the most numerous bone cell and the one that senses strain. The fourth cell is not born in bone at all. The osteoclast is a giant, multinucleated cell from the monocyte–macrophage line — immune stock — that parks on a bone surface, seals a ring, pumps acidA substance that releases hydrogen ions (H⁺) in solution. and enzymesProteins that speed up chemical reactions in the body. under it, and dissolves matrix, releasing calcium into the blood. The site calls it the Pac-Man ghost. Bone is built, kept and dismantled by cells, and the cells answer signals: hormones, mechanical strain, and — the thing the 1985 chart never considered — the same cytokines, IL-1β and IL-6, that were already running her fevers. A tissue with blood, nerves and immune-descended cells can be inflamed. A scaffold cannot.
See it move: Scene 4 · Four Cells and a Demolition Crew · Scene 5 · Under the Microscope: Decalcified Bone
3
Watch · explore · think · check
5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.
How to read the scenes
- Animation (dark teal) — a short animated explainer, one idea each
- Slide (dark teal) — a still to study: a micrograph, a diagram or a map
- Explore (purple) — something to push on while you watch
- Game (purple) — a quick challenge
- Think (gold) — reason it out, then spot-check yourself with a few true/false questions; not graded
- Check (gold) — a quick H5P that scores itself
Scene 1 of 5
The Legs That Ached at Night
Two words that excluded nothing — and the assumption underneath them.
Animation · in production · 1 min 45 s
The Legs That Ached at Night
A child on the stairs, one step at a time. The clinic: legs straightened, knees pressed, “growing pains”. A checklist appears beside her — bilateral, evening, gone by morning, normal exam, no fever, no swelling — and three boxes fail to tick. Then the section A cut or slice of the body or an organ for study.: skinThe body’s largest organ, providing protection and regulation., fat, muscle, periosteum, bone — and the bone lights with vessels and nerves. Caption: a scaffold cannot hurt; this is not a scaffold.
Prefer the full lecture? Bone as a Connective Tissue (original video on the site)
Explore
Is it growing pains?
The benign pattern on one side, her 1985 chart on the other. Match them before the overlay, sort the red flags, set one exam against a week of nights, then name the test nobody ordered.
Text description of this explore
A clinic look. Step 1: a two-column pattern match headed Tuesday clinic, Stina, age 10, autumn 1985. Left, the seven features of benign growing pains; right, what her chart says for each: both legs (both shins and both feet); late in the day or at night (at night, worst after active days); gone by morning (sometimes still there when she woke); no limp (down the stairs one step at a time on bad mornings, no limp in the office); nothing swollen or warm (joints not swollen on the day of the exam); a well child with no fever (fevers already on her chart, never connected to her legs); a normal exam (legs straight, knees pressed, nothing found that day). For each feature, choose whether her chart matches it, contradicts it, or never checked it; after you commit an overlay column fills in: three match, three contradict, and nothing swollen matched only on the day it was checked. Step 2: a red-flag sorter. Six findings (pain on waking, pain on one side only, swelling or warmth, fever, a limp, and a raised ESR or CRP) each go into present, absent or never checked in her 1985 record. Step 3: an illustrative week, a teaching model, drawn as a grid of seven days with rows for day, night and on waking: Wednesday, Friday and Saturday are busy days and Tuesday holds the clinic visit. A classmate says the normal exam that Tuesday proves her legs were not inflamed; support or refute. After you commit, the nights after the busy days are marked ache, Thursday and Sunday are marked sore on waking, and the visit is outlined as a quiet day between bad nights. Step 4: write the one test the Tuesday clinic could have ordered and compare it with a model answer.
Think · not graded
“Growing pains” is a diagnosis of exclusion: it is correct only when the history fits and the exam and labs are normal. Which parts of that were done in 1985, and which single feature of her history would make a careful clinician stop using the words?
Spot-check your thinking: true or false?
Check · Multiple Choice
What makes bone a living tissue rather than an inert scaffold?
Evidence card 1
Add it to your evidence board
Night pain on the shins, sometimes there on waking, in a child with unexplained fevers — three features the benign pattern does not have, on a chart that checked none of them.
Scene 2 of 5
Six Jobs for a Bone
Support, protection, movement, a mineral bank, a blood factory and a fat store — the skeleton as an organ system.
Slide · in production
What Bone Does
Six jobs, and four of them — storage, blood, movement, protection — are shared with other modules of this course. Bone is a system, not a frame.
Prefer the full lecture? Bone as a Connective Tissue (original video on the site)
Game
Match the job to the part
Six parts, six jobs, and the later modules that lean on them. Pair them before the key, map an adult’s marrow, then decide what yellow marrow can still do.
Text description of this game
A matching board on cream paper. Step 1: six part tiles (osteon; skull; muscle attachment; calcium in the matrix; red marrow; yellow marrow). For each, choose its job (support, protection, movement, mineral bank, blood-cell formation or fat storage) and the later module that leans on it most (Module 7, Muscular; Modules 9 and 10, Spinal Cord and Brain; Module 13, Endocrine; Module 14, Blood; or no later module, the skeleton’s own job). Amy’s packet lists six jobs; OpenStax counts five, folding mineral and fat storage into one. Step 2: a marrow map on the site’s drawing of a human skeleton, front view, with ten lettered sites: A, the top of the skull; B, the head of the humerus at her right shoulder; C, the sternum; D, a rib on her left side; E, the shaft of her right humerus; F, a lumbar vertebra; G, her right hip bone; H, the upper end of her left femur; I, the shaft of her right femur; J, the shaft of her right tibia. For each, choose red or yellow marrow in an adult; after you commit the letters color in: red at A, B, C, D, F, G and H, yellow at E, I and J. Step 3: an adult loses blood slowly for months; choose what the yellow marrow in the femur’s shaft can do (turn back to red marrow and make blood; nothing; harden into bone; or release its fat to make red cells).
Think · not graded
If the skeleton were only support and protection, bone could be a dead mineral like a shell. Which of its six jobs require living, perfused tissue — and which one makes a bone bank account for the rest of the body?
Spot-check your thinking: true or false?
Check · Drag the Words
Drop each function beside the structure that performs it.
Evidence card 2
Add it to your evidence board
The calcium bank opens here — the thread that runs to Module 7’s contraction, Module 13’s wandering calcium and Module 20’s fluids.
Scene 3 of 5
Rebar and Concrete
Collagen for bend, mineral for crush — and two diseases of missing mineral.
Animation · in production · 1 min 30 s
Rebar and Concrete
A bone bent in acid: rubbery, it bends to a bow — collagen alone. A bone baked: chalk, it snaps — mineral alone. Then the two together: the collagen fibers laid in deliberate directions, hydroxyapatite crystals packed along them, twice the weight of the fibers. Rickets: a child’s bowed legs, mineral missing, vitamin D missing. Osteomalacia: the adult version, soft bone and aching. Caption: osteoid is the rebar, salts are the concrete, cells are the crew.
Prefer the full lecture? Bone as a Connective Tissue (original video on the site)
Explore
Bend it, crush it
A testing rig with two sliders: collagen, the rebar, and mineral, the concrete. Load a bar of bone two ways, put two patients on the rig, then age the collagen.
Text description of this explore
A materials-testing rig: a steel frame with a crosshead, a ram and a dial gauge. Step 1: a bar of bone drawn with wavy lines for collagen fibers and gray specks for mineral crystals. Two sliders set collagen and mineral from 0 to 100 % of normal, and a menu sets the load: bending (the bar across two supports, pushed down in the middle) or crushing (the bar on end, pressed from the top). Two buttons load patient presets: a toddler with bowed legs (collagen 100 %, mineral 35 %, bending) and a woman with aching soft bones (collagen 100 %, mineral 60 %, crushing). Predict whether the bar bends or squashes, snaps, or holds, then run it; the drawing shows the result (bowed, cracked in two, bulged, or holding) with a stamp, and a readout gives the settings and result. The cutoffs are a teaching model. Step 2: four people, each sorted into rickets, osteomalacia, osteoporosis or none of these, with what is missing (vitamin D; vitamin C; no vitamin, a normal recipe with less bone; or not measured in 1985): a toddler with legs bowing since walking, thick wrists and little sun; a woman of 45 with a deep ache in her hips and shins, weak thighs and straight legs, rarely outdoors; a woman of 78 with a hip broken in a stumble and a matrix normal in makeup but thinner; and Stina at ten in 1985, with night shin pain, straight legs, no fractures and no bloods. Step 3: the rig holds a bar with all its mineral and half its collagen under a bending load; predict whether it snaps, bends, holds or squashes, then the drawing shows it snapped.
Think · not graded
Reinforced concrete fails two ways: the steel rusts out, or the concrete crumbles. Match each failure to a bone disease, name the ingredient missing in rickets and osteomalacia, and say why the same deficiency gives bowed legs in a child and aching in an adult.
Spot-check your thinking: true or false?
Check · Drag the Words
Drop in the property each matrix component gives, then name the two diseases of missing mineral.
Evidence card 3
Add it to your evidence board
Nothing on her 1985 chart suggested missing mineral — no bowing, no fractureBreaks in bones due to stress or injury.; her low vitamin D would not be measured until 2013.
Scene 4 of 5
Four Cells and a Demolition Crew
Stem cell, builder, maintainer — and a giant from the immune line that dissolves bone to order.
Animation · in production · 1 min 50 s
Four Cells and a Demolition Crew
An osteogenic cell in the periosteum divides; one daughter becomes an osteoblast laying osteoid like bricks; the brick-layer walls itself in and sprouts arms — an osteocyte in a lacuna. Across the panel a myeloid stem cell makes monocytes; several fuse into a multinucleated osteoclast that seals onto bone, pumps acid under its ruffled border and dissolves a pit, calcium leaking into a capillary. Then the signals: PTH, strain, and IL-1β / IL-6 waking the demolition crew. Caption: built, kept, dismantled — by cells that take orders. Terms: mineralization (osteoblasts), osteolysis (osteoclasts).
Prefer the full lecture? Cells of Osseous Tissue (original video on the site)
Explore
Who does what
Four cells on a bone surface and one passer-by. Name each and give it a job, trace two family lines, send three work orders to the site, then see which crew wins a fever night.
Text description of this explore
A construction site in hi-vis colors. Step 1: a drawn cross-section of a bone surface. Top, the periosteum, with cell A, flat and yellow, pinching in two in its inner layer. Middle, bone matrix with cell C, a star-shaped cell in an oval chamber with arms in fine canals. The lower surface, left: a pink seam of new matrix with a row of cube-shaped orange cells, B. Right: a scooped pit holding D, a large lavender cell with four nuclei and a ruffled border. Below, in the marrow, a capillary carries red cells and E, a round cell with a kidney-shaped nucleus. For each letter choose its name (osteogenic cell, osteoblast, osteocyte, osteoclast, or none of the four yet) and its job (divide, build, maintain and sense, resorb, or ride the blood until called); after you commit a key names them: A osteogenic cell, B osteoblasts, C osteocyte, D osteoclast, E monocyte. Step 2: eight cells (osteogenic cell, osteoblast, osteocyte, osteoclast, chondrocyte, fibroblast, macrophage, neutrophil) sorted into the connective-tissue (mesenchymal) stock or the immune (myeloid) stock of the red marrow; after you commit, the site’s bone-cell lineage picture appears: osteogenic cell to osteoblast to osteocyte on one side, and a myeloid stem cell giving a monocyte, a megakaryocyte, a neutrophil and a red blood cell, with the monocyte line leading to an osteoclast. Step 3: three work orders (PTH when blood calcium has dipped; strain from a day of running and jumping; IL-1β on a fever night). For each, choose whether calcium moves from bone to blood or blood to bone, and which cell reads the order first (osteogenic cell, osteoblast, osteocyte or osteoclast). Step 4: a balance on a construction site with builders (osteoblasts, orange blocks) in the left pan and the demolition crew (osteoclasts, lavender domes) in the right, under tiles for bone and blood. Choose the day (rest or active), IL-1β (none, a fever night, or fevers week after week) and PTH (normal or raised), with three presets (a quiet day, the fitness-test day, a fever after a hard day); predict whether bone gains calcium, breaks even or loses it, then run it. The pans fill, the beam tips, an arrow shows where calcium goes, and a readout gives RANKL, OPG, demolition, building and the net (a teaching model).
Think · not graded
Three of the four bone cells are one family at different ages; the fourth is an immigrant from the blood. Trace the family, say where the immigrant comes from, and explain why its origin matters for a child whose fevers run on IL-1β and IL-6.
Spot-check your thinking: true or false?
Check · Multiple Choice
Which bone cell builds new matrix?
Evidence card 4
Add it to your evidence board
OsteoclastsBone-resorbing cells that break down bone matrix. descend from immune cells and answer immune signals — the first mechanical link between her fevers and her bones.
Scene 5 of 5
Under the Microscope: Decalcified Bone
Take the mineral away and the cells show: lacunae, canaliculiTiny channels that connect osteocytes, allowing communication and nutrient exchange., and the sites of the two kinds of bone.
Slide · in production
Decalcified Bone, Slide 050
Decalcified for cells, ground for architecture: two preparations of the same tissue, each blind to what the other shows.
Prefer the full lecture? Bone as a Connective Tissue (original video on the site) · histology page: Bone · Captain Concrete
Explore
Find the cells
Three slides under the scope. Decide what a field can show before the labels, find the cells by where they live, flip the stain, then picture the same bone ground instead.
Text description of this explore
A microscope look with micrographs from the site. Step 1: slide 050, compact bone decalcified and stained with H&E at 40X: an even red matrix with faint rings around several round central canals and many small white lacunae. Choose which bone cells to expect in this field (osteocytes in their lacunae, with the other three working on surfaces the field barely has; all four spread evenly; osteoclasts around every canal; or no cells at all). After you commit, ring 1 marks a central canal and ring 2 an osteocyte shrunk inside its lacuna. Step 2: slide 048, bone forming in a fetal leg, H&E, with four rings lettered A to D: A, a small chamber in the pink matrix with a dark nucleus; B, a row of flattened nuclei lining the edge of a space; C, crowded round cells filling a space at the upper right; D, a column of small red discs in a channel. For six targets (an osteocyte, a lacuna, osteoblasts on a surface, an osteoclast in its pit, blood-forming marrow, a blood vessel) choose the letter or none you can be sure of here. Answers: osteocyte and lacuna A, osteoblasts B, marrow C, vessel D, osteoclast none in this field. Step 3: a stain toggle. Choose the field (fetal bone, slide 048, or trachea cartilage, slides 126 and 127) and the stain (H&E or Masson’s trichrome) to see four site micrographs; predict how to tell bone from cartilage in Masson (by color, bone teal-green and cartilage red; not by color alone, because both matrices take the blue-green and the chambers decide; by color, bone red and cartilage blue; or not at all), then run it. A readout gives the matrix color (bone pink-red in H&E and deep teal-green in Masson; cartilage deep purple in H&E and pale blue-green in Masson) and the chambers (bone: small flattened lacunae, one cell each, rows of cells lining every space; cartilage: large round lacunae, often in groups, no vessels). Step 4: predict what slide 050 would look like if the bone had been ground instead of decalcified; after you commit a ground section from the site appears beside it: gray-brown and unstained, osteons as rings around dark canals, dark lacunae with fine canaliculi, and no cells.
Think · not graded
A decalcified section keeps the cells and loses the architecture; a ground section keeps the architecture and loses the cells. If you wanted to prove that a child’s bone was inflamed — more osteoclasts in pits, neutrophils in the marrow — which preparation would you order, and what would a scan show instead?
Spot-check your thinking: true or false?
Check · Drag and Drop
On slide 048b (developing bone, Masson’s trichrome), drag each label into the box that matches its tag number: osteoblasts, an osteocyte in its lacuna, an osteoclast, bone matrix and a marrow space. Hint: find the chambers first, then the surfaces — a row of cells, or one giant in a pit.
Evidence card 5
Add it to your evidence board
Bone that can be seen to have builders, maintainers and a demolition crew is bone that can be inflamed — the chart’s scaffold assumption fails under 40X.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Inflammatory bone pain in a living, innervated tissue, Neither on its own, or Benign growing pains of childhood. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.
5
Your verdict
Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.
Look back at your prediction
Night pain on the shins, sometimes there on waking, in a ten-year-old with unexplained fevers — the reading that fits best is…
Find the one you chose at the top. Does the evidence support it, refute it, or revise it?
- That bone is inert mineral — the pain must be in the muscles pulling on it
- That growing bone hurts — the plates are stretching and the pain will stop when growth does
- That bone is a living, innervated, vascular tissue that can be inflamed like any other
Build your verdict · no writing
Choose the statement that holds in each set. The ones you keep become your argument.
6
The finding
Finding · growing pains
Two Words, Nothing Excluded
1985, age 10: night pain in the shins and feet, sometimes present on waking, in a child with unexplained fevers; exam normal on the day; no bloods; “growing pains, she’ll outgrow it.” Chart entry: bone is a living, vascular, innervated connective tissue whose demolition cells descend from immune stock and answer IL-1β and IL-6 — it can be inflamed, and the benign pattern was never checked against hers. Held open: what in a living bone can be felt, and what a hard run would do to it.
Next · Chapter 2
The Fitness Test
One mile on a cinder track in October, and shins that came up hot and red by evening. Before we can read that, we need the bone as an organ: its shapes, its ends and shaft, its wrapper — and the vessels and nerves that make a wrapper something that can burn.
Reliable information on rare diseases
NORD · NIH GARD · Orphanet · MedlinePlus Genetics: FMF · FMF & AID Global Association · Autoinflammatory Alliance · Global Genes · Amyloidosis Foundation
Stina has one rare disease. In the United States, about 1 in 10 people live with one of the more than 7,000 known rare diseases. These organizations are where patients, families and clinicians go for trustworthy information.
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← Familial Mediterranean Fever — case home
All pages for Module 6 — The Skeletal System
6.0 Module Overview
6.1 Growing Pains
6.2 The Fitness Test
6.3 Pillars and Trusses
6.4 She’ll Grow Out of It
6.5 The Joint That Swelled With the Fever
6.6 Five Percent a Week
6.7 Broken, Mended, Read Whole
6.8 Conclusion and Assessment
6.9 The Evidence Behind the Case
All Modules
- Anatomical Language, Membranes & Homeostasis
- Just Enough Chemistry
- The Cell & Its Transport
- Making Cells & Proteins
- The Integumentary System
- The Skeletal System
- The Muscular System
- Nervous Tissue & the Senses
- The Spinal Cord
- The Brain & the Blood–Brain Barrier
- The Autonomic Nervous System
- Special Senses (in developmentThe process of growth and differentiation.)
- The Endocrine System
- Blood
- The Heart
- Blood Vessels
- The Digestive System
- The Respiratory System
- The Urinary System
- Fluids, Electrolytes & Acid–Base Balance
- The Reproductive System
- The Immune System
List of terms
- cells
- growth
- anatomy
- physiology
- collagen
- hydroxyapatite
- movement
- red marrow
- Osteogenic cells
- endosteum
- Osteoblasts
- osteocyte
- lacuna
- acid
- enzymes
- section
- skin
- fracture
- Osteoclasts
- canaliculi
- development