Bones & Skeletal Tissues

2Classification & functions of bones

3Gross anatomy of bone

Every bone has a dense outer layer of compact bone surrounding spongy bone — a honeycomb of flat pieces called trabeculae, whose spaces hold red or yellow marrow.

Diaphysis

The tubular shaft: a thick collar of compact bone around the central medullary cavity, which holds yellow marrow (fat) in adults.

Epiphyses

The bone ends: compact bone outside, spongy bone inside; joint surfaces covered by articular (hyaline) cartilage.

Epiphyseal line

Remnant of the epiphyseal plate — the hyaline cartilage disc that let the bone grow in length during childhood.

Metaphysis

Region where the diaphysis and epiphysis meet.

Periosteum

Double-layered membrane covering the outside (except joint surfaces): outer fibrous layer of dense irregular CT + inner osteogenic layer (osteoblasts and osteoclasts). Richly supplied with nerves and vessels; anchored by perforating (Sharpey's) fibers.

Endosteum

Delicate membrane lining internal bone surfaces (medullary cavity, trabeculae, canals); also contains osteogenic cells.

Flat bones are a sandwich: two plates of compact bone with spongy bone (diploë) between. In adults, red marrow is found in the trabecular cavities of the heads of the femur and humerus and in the diploë of flat bones (sternum) and hip bones — which is why marrow is sampled from the iliac crest.

4Microscopic anatomy & composition

Compact bone is built of osteons (Haversian systems) — long weight-bearing pillars.

Lamellae

Concentric rings of matrix; collagen fibers in adjacent rings run in opposite directions to resist twisting.

Central (Haversian) canal

Runs through the core of each osteon; carries blood vessels and nerves.

Perforating (Volkmann's) canals

Run at right angles to the central canals, connecting them to the periosteum and medullary cavity.

Lacunae & canaliculi

Osteocytes sit in lacunae; hair-like canaliculi connect them so nutrients and wastes pass cell to cell.

Interstitial & circumferential lamellae

Fill gaps between osteons / extend around the entire bone circumference.

Spongy bone has no osteons: trabeculae contain irregular lamellae and osteocytes nourished from the marrow through canaliculi.

5Bone development (ossification)

Ossification (osteogenesis) begins in the embryo. There are two pathways:

Growth in length happens at the epiphyseal plate: cartilage cells in the proliferation zone divide, then enlarge (hypertrophic zone), the matrix calcifies, cells die, and bone replaces the calcified cartilage (ossification zone). The plate stays about the same thickness while the diaphysis lengthens. Growth in width is appositional: osteoblasts under the periosteum add bone while osteoclasts on the endosteal surface remove it.

6Remodeling & calcium homeostasis

Bone is dynamic: about 5–7% of bone mass is recycled each week. Bone deposit (osteoblasts) and bone resorption (osteoclasts) occur at periosteal and endosteal surfaces. Spongy bone is replaced roughly every 3–4 years, compact bone about every 10 years.

Blood Ca²⁺ is held between about 9–11 mg/dL because calcium is essential for nerve transmission, muscle contraction and blood clotting. Two controls act on bone:

Hormonal control — PTH

When blood Ca²⁺ falls, the parathyroid glands release parathyroid hormone (PTH) → osteoclasts resorb bone → Ca²⁺ released into blood. Rising Ca²⁺ shuts off PTH (negative feedback).

Hormonal control — calcitonin

Released by thyroid C cells when blood Ca²⁺ is high; may promote deposit, but its effect in healthy adults is minor at normal levels.

Mechanical stress — Wolff's law

Bone grows or remodels in response to the forces placed on it: bones are thickest where stress is greatest; projections form where active muscles attach; unused bones lose mass.

7Fractures & repair

Fractures are classified by: position of the ends (nondisplaced vs displaced), completeness (complete vs incomplete), and whether the bone ends penetrate the skin (open/compound vs closed/simple).

Treatment involves reduction — realignment of the broken ends: closed (manipulated by hand) or open (surgery, with pins or plates) — followed by immobilization.

8Bone disorders

Osteoporosis risk factors: being female and postmenopausal (falling estrogen, which normally restrains osteoclasts), older age, insufficient exercise, diet low in calcium and protein, smoking, low body weight, some hormonal conditions and medications (long-term glucocorticoids). Prevention and treatment: calcium and vitamin D, weight-bearing exercise, and drugs that slow resorption (bisphosphonates, denosumab).