Ch 6. Bones and Skeletal Tissues: Structure, Function, and Remodeling

Ch. 6: Bones & Skeletal Tissue

6.1 Types and Structure of Cartilage

Cartilage forms the initial framework of the human skeleton and is gradually replaced by bone, except in areas requiring flexibility. Cartilage is a resilient, water-rich tissue lacking blood vessels and nerves, surrounded by a perichondrium for support and nutrient delivery.

• Skeletal Cartilage: Composed of chondrocytes in lacunae within a jelly-like extracellular matrix.

• Perichondrium: Dense connective tissue layer surrounding cartilage, containing blood vessels for nutrient delivery.

• Types of Cartilage:

  • Hyaline Cartilage: Most abundant; provides support, flexibility, and resilience. Contains only collagen fibers. Locations: articular (joints), costal (ribs to sternum), respiratory (larynx, airways), nasal (external nose).

  • Elastic Cartilage: Similar to hyaline but with elastic fibers. Found in external ear and epiglottis.

  • Fibrocartilage: Thick collagen fibers; great tensile strength. Locations: menisci of knee, vertebral discs, pubic symphysis.

Example: The meniscus in the knee is fibrocartilage, providing shock absorption and strength.

6.2 Functions of Bones

Bones perform several essential functions for the body, ranging from structural support to metabolic regulation.

• Support: Framework for the body and soft organs.

• Protection: Skull protects brain; vertebrae protect spinal cord; rib cage protects thoracic organs.

• Movement/Anchorage: Muscles attach to bones via tendons, using bones as levers.

• Mineral Storage: Continuous deposits and withdrawals of calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis in red marrow cavities.

• Triglyceride Storage: Fat stored in yellow marrow of long bones.

• Hormone Production: Osteocalcin regulates insulin, glucose, and metabolism.

Example: The femur stores minerals and produces blood cells in its marrow.

6.3 Classification of Bones by Location and Shape

The adult skeleton contains 206 named bones, grouped by location and shape.

• Axial Skeleton: Skull, vertebral column, rib cage; provides protection and support.

• Appendicular Skeleton: Limbs and girdles; enables movement and environmental manipulation.

• Bone Shapes:

  • Long Bones: Longer than wide; shaft and two ends (e.g., humerus, femur).

  • Short Bones: Cube-shaped; carpals, tarsals, sesamoid bones (e.g., patella).

  • Flat Bones: Thin, flat, slightly curved; sternum, scapulae, ribs, cranial bones.

  • Irregular Bones: Complex shapes; vertebrae, hip bones.

Example: The scapula is a flat bone, providing muscle attachment sites.

6.4 Gross Structure of Bones: Compact and Spongy Bone

Bones are organs containing multiple tissue types. Their structure is organized at gross, microscopic, and chemical levels.

• Compact Bone: Dense, smooth outer layer.

• Spongy Bone: Honeycomb of trabeculae; spaces filled with red or yellow marrow.

• Short, Irregular, and Flat Bones: Thin plates of spongy bone (diploë) covered by compact bone; periosteum (outer membrane) and endosteum (inner membrane).

• Long Bones: Shaft (diaphysis), ends (epiphyses), and membranes. Diaphysis contains medullary cavity with yellow marrow; epiphyses have compact and spongy bone, articular cartilage, and epiphyseal line.

• Membranes:

  • Periosteum: Double-layered; fibrous (outer) and osteogenic (inner) layers; contains nerves, blood vessels, and anchoring points.

  • Endosteum: Delicate membrane covering internal surfaces; contains osteogenic cells.

• Blood Vessels & Nerves: Nutrient artery and vein serve diaphysis; nerves accompany vessels.

• Hematopoietic Tissue: Red marrow in spongy bone and diploë; yellow marrow can convert to red if needed.

• Bone Markings: Sites for muscle, ligament, tendon attachment, joint formation, and passageways for vessels and nerves.

Example: The periosteum is essential for bone growth and repair.

Table: Types of Bone Markings

Type	Description	Function
Projection	Outward bulge	Muscle/ligament attachment
Surface	Joint-forming area	Articulation
Depression/Opening	Bowl/groove/hole	Passage for vessels/nerves

Microscopic Anatomy of Bone

Bone tissue is dynamic, composed of specialized cells and organized structures.

• Cell Types:

  • Osteoprogenitor Cells: Stem cells in periosteum/endosteum; differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells; secrete osteoid (collagen and calcium-binding proteins).

  • Osteocytes: Mature bone cells in lacunae; maintain matrix, act as stress sensors.

  • Bone-lining Cells: Flat cells maintaining matrix where remodeling is not occurring.

  • Osteoclasts: Multinucleate cells from hematopoietic stem cells; resorb bone.

• Microscopic Anatomy of Compact Bone:

  • Osteon (Haversian System): Structural unit; cylinders of lamellae (rings of matrix) with collagen fibers in alternating directions for strength.

  • Canals and Canaliculi: Central canal (blood vessels, nerves); perforating canals connect periosteum, medullary cavity, and central canal.

  • Lacunae: Small cavities with osteocytes at lamellar junctions.

  • Canaliculi: Hairlike channels connecting lacunae; allow communication and nutrient/waste exchange.

  • Interstitial Lamellae: Fill gaps between osteons or remnants of old osteons.

  • Circumferential Lamellae: Extend around diaphysis; resist twisting.

• Microscopic Anatomy of Spongy Bone: Trabeculae organized along stress lines; no osteons, but lamellae and osteocytes present; capillaries supply nutrients.

Example: Osteons provide strength and resist twisting forces in compact bone.

Chemical Composition of Bone

Bone is composed of organic and inorganic components, providing both flexibility and strength.

• Organic Components: Cells (osteogenic, osteoblasts, osteocytes, bone-lining, osteoclasts) and osteoid (ground substance, collagen fibers); resist tension/stretch.

• Inorganic Components: Hydroxyapatites (mineral salts, mainly calcium phosphate); 65% of bone mass; provide hardness and resistance to compression.

Example: Collagen fibers allow bone to bend slightly without breaking; hydroxyapatite makes bone hard.

6.6 Bone Remodeling: Deposition and Removal

Bone remodeling is a continuous process involving bone deposition by osteoblasts and resorption by osteoclasts, regulated by hormonal and mechanical factors.

• Bone Mass Recycling: 5–7% of bone mass recycled weekly; spongy bone replaced every 3–4 years, compact bone every 10 years.

• Remodeling Units: Adjacent osteoblasts and osteoclasts coordinate remodeling.

• Bone Resorption: Osteoclasts break down matrix, secrete enzymes and acids, phagocytize debris; regulated by PTH and immune proteins.

• Bone Deposition: Osteoblasts deposit new matrix; osteoid seam marks new matrix, calcification front is transition zone; alkaline phosphatase mineralizes bone.

• Calcium Importance: Critical for resting membrane potential, nerve transmission, muscle contraction, blood coagulation, gland/nerve secretion, cell division.

• Hormonal Controls: Negative feedback loop maintains blood Ca2+ levels; PTH stimulates osteoclasts when calcium is low.

• Mechanical Stress: Wolff’s law: bones remodel in response to stress; thicker where stressed, trabeculae align along stress lines.

Example: Weight lifters develop thicker bones at muscle attachment sites due to increased mechanical stress.

Equation: Calcium Homeostasis (Negative Feedback)

When blood calcium levels drop:

• Parathyroid glands release PTH

• PTH stimulates osteoclasts to resorb bone

• Calcium released into blood

• Blood calcium levels rise

• PTH secretion stops

LaTeX equation for calcium homeostasis:

$\text{Low } [Ca^{2+}]_{blood} \rightarrow \text{PTH release} \rightarrow \text{Osteoclast activation} \rightarrow \text{Bone resorption} \rightarrow \text{Increase } [Ca^{2+}]_{blood}$

6.7 Bone Repair: Hematoma, Callus Formation, and Remodeling

Bone repair after fracture involves several stages and classifications.

• Fracture Classification:

  • Position of Bone Ends: Nondisplaced (normal position), Displaced (misaligned).

  • Completeness: Complete (broken through), Incomplete (partial break).

  • Skin Penetration: Open (compound, skin penetrated), Closed (simple, skin intact).

• Repair Process: Hematoma formation, callus formation, and remodeling restore bone structure and function.

Example: A compound fracture of the femur requires realignment and healing through callus formation and remodeling.

Additional info: Bone remodeling is influenced by both hormonal (PTH, calcitonin) and mechanical factors (Wolff's law). Osteocalcin is a hormone produced by bone that affects metabolism. Hydroxyapatite is the primary mineral in bone, giving it strength and durability.