Anatomy and Physiology - Skeletal System

Overview

Bones, joints, and cartilage form a dynamic system for support, protection, movement, mineral homeostasis, and hematopoiesis.

Macroscopic anatomy

• Classification: long (femur), short (carpals), flat (sternum), irregular (vertebrae), sesamoid (patella).
• Long bone structure: diaphysis, metaphysis, epiphysis, epiphyseal plate/line, periosteum, endosteum, medullary cavity.
• Joint types: fibrous (suture, syndesmosis), cartilaginous (synchondrosis, symphysis), synovial (plane, hinge, pivot, condylar, saddle, ball‑and‑socket).

Microscopic anatomy

• Compact bone: osteons (concentric lamellae), central (Haversian) canals, perforating (Volkmann) canals; osteocytes in lacunae connected via canaliculi.
• Spongy bone: trabeculae aligned with stress lines; houses marrow and reduces mass.
• Cartilage: hyaline (articular), fibrocartilage (menisci, intervertebral discs), elastic (epiglottis); avascular nutrition via diffusion.

Cells and remodeling

• Osteoblasts: bone formation; secrete osteoid (type I collagen + ground substance), initiate mineralization.
• Osteoclasts: multinucleated, resorb bone via acidification and proteases; RANKL/OPG system regulates activation.
• Osteocytes: mechanosensors; regulate remodeling via sclerostin and canalicular fluid flow.
• Remodeling balance: coupled resorption–formation maintains strength; Wolff’s law adapts architecture to load.

Ossification and growth

• Intramembranous: mesenchyme → osteoblasts (flat bones of skull, mandible, clavicle parts).
• Endochondral: cartilage model → hypertrophy → calcification → vascular invasion → primary/secondary ossification centers; growth plate zones (reserve, proliferative, hypertrophic, calcified, ossification).
• Growth plate closure: estrogen/testosterone accelerate maturation; fractures through plates (Salter–Harris classification) impact growth.

Biomechanics and mineral homeostasis

• Stress/strain: cortical vs trabecular behavior; anisotropy along long axes.
• Mineral reservoir: PTH increases osteoclast activity indirectly (via osteoblast RANKL), raising serum Ca²⁺; calcitonin counteracts in select contexts; vitamin D enhances intestinal Ca/PO₄ absorption.

Fracture healing

• Stages: hematoma → soft callus (fibrocartilage) → hard callus (woven bone) → remodeling (lamellar bone). Adequate perfusion, immobilization, and proper reduction are key.
• Complications: malunion, nonunion, avascular necrosis (e.g., femoral head), infection (open fractures).

Pathology highlights

• Osteoporosis: low bone mass and microarchitectural deterioration; vertebral compression fractures; risk factors (age, sex hormones, glucocorticoids, immobility).
• Osteomalacia/rickets: defective mineralization (vitamin D deficiency/CKD); bone pain, deformities.
• Osteogenesis imperfecta: collagen Type I defect; brittle bones, blue sclerae (clinical correlates; qualitative scope).

Worked micro‑examples

1. Growth plate lesion

• A 12‑year‑old with physeal fracture near the distal radius: describe zones traversed and predict growth disturbance risk (higher if across proliferative/hypertrophic zones).

2. Load alignment

• Explain why trabeculae in the femoral neck align along principal compressive/tensile lines; relate to fracture patterns in osteoporosis.

3. Calcium regulation scenario

• Low dietary Ca²⁺ → ↑PTH → ↑osteoclast activity and renal Ca²⁺ reabsorption; long‑term risk for bone loss without vitamin D/calcium correction.

Pitfalls

• Confusing cartilage types at joints (articular cartilage is hyaline, not fibrocartilage).
• Mixing osteoblast vs osteoclast roles; forgetting osteocytes’ mechanosensory function.
• Overlooking periosteum/endosteum roles in appositional growth and remodeling.

Practice prompts

• Label an osteon and trace nutrient diffusion path to an osteocyte.
• Order endochondral ossification zones from epiphysis to diaphysis and list cellular events.
• Compare compact vs spongy bone functions and mechanical behavior.

References

• SciOly Wiki – Anatomy & Physiology (Skeletal system)
• OpenStax Anatomy & Physiology (Bone tissue and skeletal system)