Anatomy and Physiology 2e · Bone Tissue and the Skeletal System
Bone Structure
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In 30 seconds
A bone is not a lifeless strut — it is a living organ built from bone tissue (osseous tissue), cartilage, blood vessels, nerves, and marrow, all organized around two jobs: mechanical support and material storage. This topic walks through the gross anatomy of a long bone, the microscopic architecture of compact and spongy bone, the four bone cell types, and how blood reaches bone. Getting this vocabulary right matters because every later topic in this chapter — growth, fracture repair, hormonal control — describes changes in the structures introduced here.
Why this matters
Bone structure explains otherwise mysterious observations: a broken bone bleeds because bones are vascular; a child's arm can heal with a slight bend while an adult's cannot, because growth-plate cartilage remains at the metaphysis; and a hip fractures more readily in osteoporosis because trabeculae thin first, weakening the spongy femoral neck. The same architecture is the substrate for orthopedic care: pins must cross cortical bone and grafts must revascularize.
The college version
Core Concepts
Gross anatomy of a long bone
Picture a typical long bone such as the femur. The shaft, or Diaphysis The shaft of a long bone, thick compact bone around a marrow cavity Full entry →, is a thick-walled tube of compact bone around a hollow medullary cavity that holds yellow marrow (fat) in adults. The expanded ends, the epiphyses, are mostly spongy bone under a thin shell of compact bone; their internal spaces house red marrow, where blood cells are made. The metaphysis is where diaphysis meets Epiphysis The expanded end of a long bone, mostly spongy bone capped with articular cartilage Full entry →; in a growing person it contains the Epiphyseal plate A disc of hyaline cartilage between diaphysis and epiphysis in growing bone Full entry →, a disc of hyaline cartilage that lets the bone lengthen, replaced after growth stops by the epiphyseal line.
The joint surfaces of each epiphysis are capped with articular cartilage — smooth hyaline cartilage that reduces friction and absorbs impact. It has no blood supply of its own and depends on diffusion from synovial fluid, which is why cartilage injuries heal slowly. Elsewhere the bone is wrapped in the Periosteum Two-layer membrane covering the outer bone surface Full entry →: an outer layer of dense irregular connective tissue and an inner (osteogenic) layer of osteogenic cells and osteoblasts. Collagen perforating (Sharpey's) fibers anchor it to the bone; it carries blood vessels and nerves and anchors tendons and ligaments. The endosteum, a thin membrane rich in osteogenic cells, lines the medullary cavity and spongy-bone spaces.
Compact bone: the osteon
Viewed under a microscope, compact bone is built from repeating cylindrical units called osteons (Haversian systems): concentric rings of mineralized matrix — the lamellae — around a central (Haversian) canal that carries blood vessels and nerves along the bone's long axis. Between lamellae sit tiny chambers, lacunae, each holding a mature bone cell, an osteocyte. Thread-like canaliculi radiate from every Lacuna A small chamber in bone matrix holding an osteocyte Full entry → and connect neighbors, letting osteocytes exchange nutrients, wastes, and signals while sealed inside mineral. Blood vessels in central canals link to the surface and marrow cavity through perforating (Volkmann's) canals, which run perpendicular to the shaft. Interstitial and circumferential lamellae — remnants of old osteons and surface layers — fill the spaces between and around osteons.
Spongy bone: trabeculae
Spongy bone (cancellous bone) is a three-dimensional lattice. Its branching struts, the trabeculae, align with the lines of stress the bone experiences, giving strength without excess weight. Trabeculae contain no true osteons: osteocytes in lacunae receive nutrients by diffusion from the red marrow filling the spaces between struts. This open structure makes spongy bone light and shock-absorbing and makes spongy-rich sites (femoral head, vertebral bodies) common locations for marrow sampling and osteoporotic fracture.
The four bone cells
Bone tissue is maintained by four cell types:
- Osteogenic cells are stem cells in the periosteum, endosteum, and vessel linings; they divide and differentiate into osteoblasts, supplying new bone-forming cells throughout life.
- Osteoblasts are the builders: they secrete osteoid (unmineralized collagen and proteins), then help deposit calcium and phosphate minerals into it. Once surrounded by matrix, they mature into osteocytes.
- Osteocytes are mature bone cells trapped in lacunae; they maintain the matrix, monitor mechanical load, and signal osteoblasts or osteoclasts when bone needs to change.
- Osteoclasts are the demolition crew: large, multinucleated cells formed by fusion of monocyte-lineage cells. Attached in shallow pits (Howship's lacunae), they secrete acid and enzymes that dissolve mineral and digest matrix, releasing stored calcium into the blood — central to Topic 7.
Blood and nerve supply
Bones are well vascularized: the nutrient artery enters through a nutrient foramen in the diaphysis, periosteal and metaphyseal/epiphyseal arteries serve the surface and ends, and nerves travel with the vessels — the pain-sensitive periosteum is why fractures hurt acutely. Disrupted blood supply (as in certain fractures) can kill bone tissue.
Common Confusions
| Do Not Confuse | With | The Difference |
|---|---|---|
| Compact bone | Spongy bone | Compact is dense with osteons, forms shaft walls; spongy is a trabecular lattice with marrow between, at bone ends and inside flat/irregular bones |
| Osteoblasts | Osteoclasts | Blasts build (secrete + mineralize matrix); clasts destroy (dissolve bone, release calcium) — "blasts build, clasts crush" |
| Osteocyte | Osteoclast | Osteocyte is a mature bone cell in a lacuna; osteoclast is a large multinucleated resorbing cell |
| Central canal | Perforating (Volkmann's) canal | Central canals run along the long axis inside each osteon; perforating canals run across the bone, connecting central canals to the surface |
| Epiphyseal plate | Epiphyseal line | Plate = active growth cartilage in youth; line = its calcified remnant after growth stops |
| Medullary cavity | Marrow spaces of spongy bone | Medullary cavity is the diaphysis's central hollow (yellow marrow in adults); spongy spaces are smaller and hold red marrow in many bones |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A bone is like a living building with a pipe in the middle. The outside skin has builders ready to work, the walls are tiny brick columns (osteons) with tunnels so the bricks share food, and the ends are a spongy net, light but strong. Some cells build, some watch, and a few big cells tear down old walls for materials.
Worked example
A 14-year-old soccer player arrives after a hard tackle with a painful, swollen thigh. The femur X-ray shows a crack near the knee end — in the metaphyseal region, close to the still-open epiphyseal plate. If the fracture disrupts the plate, the leg could stop growing evenly, so orthopedic follow-up will track it until closure. The swelling reflects bleeding from the richly vascular periosteum and nutrient artery; and because the injury is at the joint end, the articular cartilage of the knee must also be evaluated — it has no blood supply and repairs poorly. Finally, the fracture crosses both compact bone (the epiphysis's thin shell) and spongy bone; since trabeculae align with weight-bearing stress, a break running across them is more unstable than one following them.
Key takeaways
- A long bone has a diaphysis (shaft), epiphyses (ends, spongy bone + red marrow), metaphyses, and — in youth — epiphyseal plates that become epiphyseal lines.
- Periosteum and endosteum both harbor stem cells that can become osteoblasts.
- Compact bone = osteons: lamellae around a central canal, lacunae (osteocytes) linked by canaliculi; Volkmann's canals connect to the surface.
- Spongy bone = trabeculae along stress lines; no osteons; nourished by diffusion from marrow.
- Cell lineup: osteogenic cell → osteoblast → osteocyte; osteoclast (multinucleated, monocyte-derived) resorbs.
- Articular cartilage is avascular (heals poorly); periosteum is pain-sensitive (injuries hurt).
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Name, in order from outside to inside, the layers a scalpel would pass through to reach the medullary cavity of a long bone's diaphysis.
Show answer
Periosteum (fibrous + osteogenic layers), then compact bone (osteons, lamellae, lacunae, canals), then endosteum, then the medullary cavity.
Why are osteocytes able to survive even though they are surrounded by mineralized matrix?
Show answer
Because canaliculi connect each lacuna to its neighbors and to central-canal blood vessels, allowing diffusion of nutrients and wastes.
What structural difference explains why spongy bone is lighter than compact bone yet still strong?
Show answer
Spongy bone is a lattice of trabeculae with open marrow spaces between; struts align along lines of stress, concentrating material only where needed.
Which bone cell type is responsible for raising blood calcium by dissolving bone, and where does it come from?
Show answer
The osteoclast — large, multinucleated, formed by fusion of monocyte-lineage cells — secretes acid and enzymes that dissolve bone mineral and matrix.
What happens to the epiphyseal plate when longitudinal bone growth is complete, and what is the remnant called?
Show answer
The plate's cartilage is replaced by bone, leaving the epiphyseal line — a thin imaging remnant; the bone can no longer lengthen there.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Diaphysis
- The shaft of a long bone, thick compact bone around a marrow cavity
- Epiphysis
- The expanded end of a long bone, mostly spongy bone capped with articular cartilage
- Epiphyseal plate
- A disc of hyaline cartilage between diaphysis and epiphysis in growing bone
- Periosteum
- Two-layer membrane covering the outer bone surface
- Osteon (Haversian system)
- Cylindrical unit of compact bone: lamellae around a central canal
- Lacuna
- A small chamber in bone matrix holding an osteocyte
- Trabecula
- A strut of bone in spongy bone
- Osteoblast
- Bone-building cell that secretes and mineralizes osteoid
- Osteoclast
- Large multinucleated cell that dissolves bone
Sources & references
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