Chapter 7: Skeletal System — Key Concepts

7.1 Introduction to the Skeletal System

• Structural components
• Bones – primary organs; rigid framework; multi-functional; dynamic living tissues
• Cartilage – semirigid CT; hyaline & fibrocartilage varieties
• Ligaments – dense regular CT (bone↔bone)
• Tendons – dense regular CT (muscle↔bone)
• Types of bone by tissue arrangement
• Compact (cortical) bone – $80\%$ of total mass; dense; outer shell, organized in osteons
• Spongy (cancellous/trabecular) bone – $20\%$ of mass; porous; internal, composed of trabeculae
• Cartilage specifics
• Hyaline
• Rib–sternum connections, articular surfaces, epiphyseal plates, embryonic bone models
• Smooth, low-friction, shock-absorbing
• Fibrocartilage
• Intervertebral discs, pubic symphysis, menisci
• Weight-bearing, resists compression

7.2 Bone Functions, Classification & Gross Anatomy

General functions

• Support & protection of organs – e.g., skull protects brain, rib cage protects heart/lungs
• Levers for skeletal muscle movement
• Hematopoiesis (in red marrow)
• Mineral & energy storage – critical reservoir for maintaining blood ion homeostasis
• Calcium – muscle, nerve, clotting
• Phosphate – ATP, nucleotides, plasma membrane

Shape classification

• Long – length > width (femur, humerus)
• Short – length≈width (carpals, tarsals, patella = sesamoid subclass)
• Flat – thin plates (cranial bones, sternum, ribs, scapulae)
• Irregular – complex (vertebrae, os coxae, sphenoid)

Long-bone regions

• Diaphysis – cylindrical shaft; thick compact bone; houses medullary cavity (red → yellow marrow transition)
• Epiphysis (proximal/distal) – knob-like ends; outer compact + inner spongy; capped with articular (hyaline) cartilage
• Metaphysis – flared zone; contains epiphyseal plate/line

Coverings & linings

• Periosteum
• Outer fibrous dense‐irregular CT – protects; tendon/ligament anchor
• Inner cellular layer – osteoprogenitor cells, osteoblasts, osteoclasts
• Anchored via perforating (Sharpey) fibres
• Endosteum – thin CT lining medullary cavity & trabeculae; same cell trio

Vascular / neural supply

• Highly vascularised; nutrient artery/vein enter via nutrient foramen
• Sensory nerves travel with vessels; monitor damage

Marrow

• Red marrow (myeloid) – hemopoietic; children = medullary cavities + spongy bone; adults = axial skeleton & proximal humerus/femur
• Yellow marrow – adipose-rich; may revert to red during severe anemia

7.2 Microscopic Anatomy of Bone & Cartilage

Bone cells

• Osteoprogenitor – mesenchymal stem; divide → osteoblast + stem; crucial for bone growth and repair throughout life
• Osteoblast – build; secrete osteoid (organic matrix)
• Osteocyte – mature; in lacunae; strain sensors
• Osteoclast – large multinucleate; resorb; ruffled border; derive from fused monocytes

Extracellular matrix

• Organic (osteoid) – collagen, proteoglycans, glycoproteins → tensile strength
• Inorganic – crystals of $Ca3(PO4)2$ + $Ca(OH)2$ → hydroxyapatite $Ca{10}(PO4)6(OH)2$ + other ions → rigidity
• Formation needs Vit D (Ca absorption), Vit C (collagen), Ca $^{2+}$ /PO $_4^{3-}$ supply
• Resorption uses osteoclastic enzymes + HCl; released ions raise blood Ca

Compact bone (osteon/Haversian system)

• Central canal w/ vessels, concentric lamellae (collagen fibers rotate $90^{\circ}$ per layer)
• Osteocytes in lacunae; canaliculi for nutrient diffusion & signalling
• Perforating canals, circumferential & interstitial lamellae supplement

Spongy bone

• Trabeculae lattice; marrow fills spaces; parallel lamellae w/ osteocytes; light but strong along stress lines

Hyaline cartilage histology

• Chondroblasts → chondrocytes in lacunae; avascular, aneural; water-rich ECM; perichondrium (dense-irregular CT) nourishes & shapes

7.3 Cartilage Growth

• Begins embryonically; two mechanisms operate throughout youth

Interstitial – growth in length; internal lacunar chondrocyte mitosis → blasts → new matrix
Appositional – growth in width; stem cells in perichondrium divide → new chondroblasts deposit matrix at periphery

7.4 Ossification (Bone Formation)

Intramembranous (dermal) ossification

• Starts $\approx$ week 8 in mesenchyme; produces flat skull bones, some facial bones, mandible, clavicle center
• Steps

Ossification centers – osteoblast clusters secrete osteoid
Calcification – osteoid crystallises; cells → osteocytes
Woven (primary) bone & periosteum form
Lamellar bone replaces woven; results in diploë (spongy) sandwiched by compact

Endochondral ossification

• Hyaline model (week 8–12) → most skeletal bones
• Sequence in long bone

Cartilage model grows
Cartilage calcifies; periosteal bone collar appears
Primary ossification center (diaphysis) – vascular invasion
Secondary centers (epiphyses) – around birth
Cartilage persists as articular cartilage + epiphyseal plates
Plates ossify (10–25 yrs) → epiphyseal lines, growth ceases

7.5 Post-natal Bone Growth & Remodeling

Interstitial (length)

• Occurs at epiphyseal plate; five histological zones

Resting cartilage
Proliferating cartilage – rapid mitosis
Hypertrophic cartilage – enlarge
Calcified cartilage – matrix mineralises, cells die
Ossification – osteoblasts deposit bone
• Plate maintains thickness until adolescence; closure → epiphyseal line

Appositional (width)

• Periosteal osteoblasts add external circumferential lamellae; endosteal osteoclasts enlarge medullary cavity → proportional thickening

Remodeling

• Continuous, $\approx20\%$ skeleton/yr; regional rates differ (distal femur 4–6 mo vs diaphysis lifetime)
• Coupled osteoblast/osteoclast activity; directed by
• Mechanical stress – weight-bearing ↑ bone mass; disuse ↓ mass
• Hormones (below)

Hormonal regulation

• Growth hormone + IGF – cartilage proliferation
• Thyroid hormone – basal metabolic rate of bone cells
• Sex steroids (estrogen, testosterone) – adolescent growth spurt, plate closure
• Glucocorticoids – high levels ↑ bone loss & ↓ growth
• Serotonin – excess hinders osteoblast differentiation; links to osteoporosis

7.6 Regulation of Blood Calcium

• Physiologic roles: muscle contraction, exocytosis, pacemaker activity, clotting
• Key hormones

Activation of vitamin D (→ calcitriol) Flowchart:

(1) Skin UV + 7-dehydrocholesterol → Vitamin D $_3$ (cholecalciferol)
(2) Liver hydroxylates → calcidiol
(3) Kidney hydroxylates (↑ by PTH) → calcitriol
Calcitriol ↑ intestinal Ca $^{2+}$ absorption

Parathyroid hormone (PTH) + calcitriol

• Released when blood Ca $^{2+}$ low
• Effects (synergistic unless noted):
• Bone – ↑ osteoclast activity → Ca release
• Kidney – ↓ urinary Ca loss; PTH also ↑ calcitriol synthesis
• Intestine – calcitriol only, ↑ absorption

Calcitonin

• Released from thyroid when Ca $^{2+}$ high / during exercise
• Inhibits osteoclasts; ↑ renal Ca $^{2+}$ excretion
• Greater impact in children (high turnover)

7.7 Aging & Bone Disorders

• Osteoblast protein synthesis ↓; inorganic fraction ↑ → brittle bones
• Osteopenia – natural, starts 35–40 yrs; vertebrae, jaw, epiphyses lose most
• Osteoporosis – pathologic density loss; post-menopausal women at higher risk due to estrogen decline; also tied to low Vit D, GH, testosterone

Clinical & Applied Perspectives

• Fracture types (selected examples) – greenstick (partial), comminuted (shattered), colles (distal radius), pott (ankle), spiral (twist), stress (microscopic), avulsion, etc.
• Repair sequence: hematoma → fibrocartilaginous callus → hard callus → remodeling
• Mechanical loading (exercise, weight training) is a non-pharmacologic strategy to build peak bone mass and mitigate age-related loss.
• Excess exogenous glucocorticoids (e.g., asthma therapy) demand monitoring of growth velocity & bone density in children.
• SSRIs may alter bone density via serotonin pathways; clinical consideration for long-term use.
• Severe anemia can reconvert yellow marrow → red, showcasing marrow plasticity and systemic integration.