Chapter 06 - Skeletal System: Bones and Bone Tissue

Overview and Functions of the Skeletal System

  • Composition: Composed of bones, cartilage, and ligaments.

  • Major Functions:

    • Support: Hard bone tissue and flexible cartilage support structure; ligaments connect bone to bone.

    • Protection: Skull encloses brain; thoracic cage protects heart and lungs.

    • Movement: Muscles pull bones via tendons; ligaments stabilize joints.

    • Storage: Stores calcium and phosphate; yellow marrow stores lipids.

    • Blood Cell Production: Red bone marrow produces blood cells (hemopoiesis).

Bone Markings

  • Projections & Bumps:

    • Process: Bump or projection.

    • Ramus: Extension forming an angle with bone structure.

  • Attachment Sites:

    • Trochanter: Large, rough projection.

    • Crest: Prominent ridge.

    • Spine: Pointed process.

    • Line: Low ridge.

    • Tubercle: Small, rounded process.

    • Tuberosity: Rough, elevated projection.

  • Articulations:

    • Head: Expanded articular end on a narrow neck.

    • Neck: Narrow connection between epiphysis and diaphysis.

    • Facet: Small, flat articular surface.

    • Condyle: Smooth, rounded articular process.

    • Trochlea: Pulley-shaped articular process.

  • Openings & Passageways:

    • Sinus: Air-filled chamber.

    • Foramen: Round hole for vessels and nerves.

    • Fissure: Slit-like gap.

    • Meatus: Tube-like channel opening.

    • Canal: Tubular duct.

  • Depressions:

    • Sulcus: Shallow groove.

    • Fossa: Shallow basin.

Cartilage Histology and Growth

  • Types: Hyaline (most abundant), fibrocartilage, and elastic cartilage.

  • Cells:

    • Chondroblasts: Produce new matrix on periphery.

    • Chondrocytes: Mature cells housed in lacunae.

  • Matrix: Collagen fibers (tensile strength) and proteoglycans (resiliency).

  • Perichondrium: Outer fibrous layer (vascular) and inner cellular layer (chondroblasts). Absent on articular cartilage.

  • Growth Mechanisms:

    • Appositional Growth: Addition of new matrix along outer boundary.

    • Interstitial Growth: Chondrocytes divide internally, expanding tissue from within.

Microscopic Structure and Composition of Bone

  • Extracellular Matrix:

    • Organic Matrix (35%35\%): Type I collagen and proteoglycans for tensile strength and flexibility.

    • Inorganic Matrix (65%65\%): Hydroxyapatite crystals Ca10(PO4)6(OH)2\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2 for hardness and compressive strength.

  • Bone Cell Types:

    • Osteoblasts: Form bone matrix (osteoid) and promote mineralization.

    • Osteocytes: Mature cells in lacunae maintaining matrix; communicate via canaliculi and gap junctions.

    • Osteoclasts: Multinucleated cells derived from monocytes; resorb bone via H+\text{H}^+ ions and proteolytic enzymes.

    • Osteochondral Progenitor Cells: Mesenchymal stem cells in periosteum and endosteum.

Anatomical Structure of Bones

  • Long Bone Structure:

    • Diaphysis: Compact bone shaft surrounding medullary cavity (red marrow in children, yellow marrow in adults).

    • Epiphysis: Expanded spongy bone ends covered by compact bone and articular cartilage.

    • Epiphyseal Plate/Line: Hyaline cartilage growth region that ossifies into a line upon full maturity.

  • Bone Membranes:

    • Periosteum: Outer fibrous layer and inner cellular layer; anchored by Sharpey's fibers.

    • Endosteum: Single cellular layer lining medullary cavity and internal spaces.

  • Other Bone Types: Flat, short, irregular, sutural, and sesamoid bones consist of a spongy core encased in compact bone.

  • Microscopic Architecture:

    • Compact Bone: Organized into osteons (central canal, concentric lamellae, lacunae, canaliculi) and perforating (Volkmann's) canals.

    • Spongy Bone: Network of trabeculae aligned along lines of mechanical stress; lacks osteons.

Bone Development and Growth

  • Ossification Types:

    • Endochondral Ossification: Replaces hyaline cartilage model; forms long bones and most of skeleton via primary and secondary ossification centers.

    • Intramembranous Ossification: Forms directly from embryonic connective tissue membrane; creates flat skull bones, mandible, and clavicles; unossified areas form fontanels.

  • Growth Mechanics:

    • Longitudinal Growth: Occurs at epiphyseal plate via 55 zones (resting, proliferation, hypertrophy, calcification, ossification); closes between ages 1212 and 2525.

    • Appositional Growth: Increases bone width; periosteal osteoblasts deposit bone while endosteal osteoclasts resorb bone.

  • Regulating Factors: Vitamins D (calcium absorption) and C (collagen synthesis); Growth Hormone, Thyroid Hormones, and Sex Hormones.

Remodeling, Calcium Homeostasis, and Repair

  • Bone Remodeling: Executed by Basic Multicellular Units (BMUs); adapts bone structure to mechanical stress ("use it or lose it").

  • Calcium Homeostasis:

    • Parathyroid Hormone (PTH): Secreted during hypocalcemia (<8.5 mg/dL< 8.5\,\text{mg/dL}); stimulates osteoclasts, increases renal calcium reabsorption, activates Calcitriol.

    • Calcitonin: Secreted during hypercalcemia (>11 mg/dL> 11\,\text{mg/dL}); inhibits osteoclasts and increases renal calcium excretion.

  • Fracture Repair Stages:

    1. Fracture hematoma formation.

    2. Fibrocartilaginous (callus) formation.

    3. Callus ossification into spongy bone.

    4. Bone remodeling into compact bone.

Pathology and Aging

  • Osteoporosis: Bone resorption outpaces deposition; reduction in mass makes bones porous and fragile; common in post-menopausal women.

  • Aging Effects: Reduced collagen synthesis makes bones brittle; loss of bone mass and articular cartilage degeneration lead to fractures, stature loss, and osteoarthritis.