11.4 Skeletal System: Bone, Cartilage & Joints

Functions of the Skeleton

The AAMC outline names four functions, and each has a testable physiological consequence.

1. Structural rigidity and support — a rigid framework against which muscles generate movement (see 11.3).
2. Calcium and phosphate storage — bone holds about 99% of body calcium, making it the reservoir that buffers serum $\text{Ca}^{2+}$ under hormonal control.
3. Physical protection — cranium, vertebral column, rib cage.
4. Hematopoiesis — red marrow within spongy bone generates all formed elements of blood (see 10.2).

The skeleton divides into the axial skeleton (skull, vertebral column, rib cage) and the appendicular skeleton (limbs and their girdles). Contrast this endoskeleton — living, internal, mineralized, capable of growth and remodeling — with the arthropod exoskeleton of chitin, which is non-living, external, provides better protection and desiccation resistance, but constrains growth and must be shed periodically in molting (ecdysis).

Bone Microarchitecture

Compact (Cortical) Bone

Dense outer bone organized into repeating cylindrical osteons (Haversian systems):

• A central Haversian canal carries a blood vessel and nerve along the bone's long axis.
• Concentric rings of mineralized matrix called lamellae surround it.
• Osteocytes sit in small cavities called lacunae between lamellae, communicating through tiny channels called canaliculi that carry cytoplasmic extensions joined by gap junctions — the only way nutrients reach cells embedded in solid mineral.
• Volkmann's (perforating) canals run perpendicular, connecting adjacent Haversian canals and the periosteum.

Spongy (Cancellous, Trabecular) Bone

An open lattice of trabeculae oriented along lines of mechanical stress, found at the epiphyses and inside flat bones. It is much lighter than compact bone and its spaces house red marrow. Trabeculae have no osteons; osteocytes are nourished by diffusion from marrow.

Coverings and Cavity

Periosteum (fibrous outer layer, osteogenic inner layer) covers the outside except at articular surfaces; endosteum lines the medullary cavity, which contains yellow (fatty) marrow in adults.

Matrix Composition

• Organic (~35%): type I collagen plus ground substance — provides tensile strength and flexibility. Defects give osteogenesis imperfecta, the brittle-bone disease.
• Inorganic (~65%): hydroxyapatite, $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$ — provides compressive strength and hardness. Loss gives osteoporosis; defective mineralization from vitamin D deficiency gives rickets in children and osteomalacia in adults.

Bone Cells and Remodeling

Remodeling is continuous — roughly 10% of the adult skeleton is replaced yearly — and it obeys Wolff's law: bone is deposited along lines of mechanical loading and resorbed where load is absent, which is why weight-bearing exercise builds density and why bed rest and microgravity cause rapid loss.

A mechanism worth memorizing. Osteoclasts have no PTH receptor. Parathyroid hormone binds osteoblasts, which then express RANKL; RANKL binds RANK on osteoclast precursors and drives their maturation. Osteoblasts also secrete osteoprotegerin, a decoy receptor that sequesters RANKL and brakes the process. Passages frequently test this indirect route.

Ossification

In endochondral ossification a primary ossification center appears in the diaphysis and secondary centers appear in the epiphyses. Between them, the epiphyseal (growth) plate of proliferating hyaline cartilage continues to lengthen the bone until sex steroids at puberty drive plate closure — which is why estrogen deficiency delays closure and produces tall stature, and why growth hormone excess before closure produces gigantism but after closure produces acromegaly.

Cartilage

Cartilage is an avascular, aneural connective tissue (see 8.3) whose chondrocytes sit in lacunae within a matrix of collagen and highly hydrated proteoglycans; it is nourished by diffusion from the surrounding perichondrium, which is why it heals slowly and poorly.

Joints (Articulations)

A synovial joint has articular (hyaline) cartilage on the bone ends, a two-layer articular capsule whose inner synovial membrane secretes viscous synovial fluid for lubrication and chondrocyte nutrition, and often menisci, bursae and tendon sheaths. Subtypes include hinge, ball-and-socket, pivot, saddle, condyloid and gliding joints. Osteoarthritis is degeneration of this articular cartilage; rheumatoid arthritis is autoimmune inflammation of the synovial membrane (see 10.5).

Tendons and Ligaments

Both are dense regular connective tissue — parallel type I collagen bundles with fibroblasts squeezed between them (see 8.3).

• Tendons attach muscle to bone and transmit contractile force; they are slightly elastic and store energy during locomotion.
• Ligaments attach bone to bone across a joint, limiting the range of motion and providing stability.

Both are poorly vascularized, which is why sprains and tendon ruptures heal slowly.

Endocrine Control of Bone and Calcium

This table connects directly to Section 9.4, and it explains the two classic passage scenarios: post-menopausal osteoporosis from estrogen withdrawal, and the elevated serum calcium with low phosphate that signals primary hyperparathyroidism.