Bones and Skeletal Tissues: Structure, Function, and Development

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Bones and Skeletal Tissues

Functions of Bone

Bones serve multiple essential functions in the human body, providing structure, protection, and facilitating movement. They also play a vital role in mineral storage and blood cell formation.

Support: Framework that supports the body.
Protection: Encloses organs such as the skull (brain) and rib cage (heart and lungs).
Movement: Provides anchoring attachments for muscles, enabling locomotion.
Storage: Reservoir for minerals, especially calcium and phosphate.
Cell Formation: Hematopoiesis occurs in red bone marrow, producing red blood cells (RBCs), white blood cells (WBCs), and platelets.

Classification of Bones

Bones are classified by their location and shape, which relates to their function and structure.

Axial Skeleton: Includes bones of the skull, vertebral column, and rib cage.
Appendicular Skeleton: Includes bones of the upper and lower limbs and the shoulder and hip girdles.

Bone Shape Classification

Long Bones: Longer than wide; consist of a shaft and two ends (e.g., femur, tibia, humerus).
Short Bones: Cube-shaped; found in the wrist (carpals) and ankle (tarsals).
Flat Bones: Thin, flattened, and usually curved (e.g., sternum, scapulae, ribs).
Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Structure of Long Bone

Long bones have a unique structure that supports their function in movement and strength.

Diaphysis: Shaft; composed of compact bone surrounding a medullary cavity.
Epiphysis: Ends of the bone; contains spongy bone and red marrow.
Epiphyseal Plate: Cartilage at the junction of diaphysis and epiphysis; site of bone growth in length.
Articular Cartilage: Covers joint surfaces; reduces friction and absorbs shock.
Periosteum: Double-layered membrane covering the bone; contains nerves, blood vessels, and osteogenic cells.
Endosteum: Membrane lining the medullary cavity and trabeculae of spongy bone.
Medullary Cavity: Central cavity containing yellow marrow (fat storage).
Nutrient Foramen: Opening for blood vessels and nerves.

Microscopic Structure of Bone

Bones are composed of specialized cells and a matrix arranged in specific patterns for strength and flexibility.

Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae around a central canal.
Lamellae: Layers of bone matrix.
Haversian (Central) Canal: Contains blood vessels and nerves.
Osteocytes: Mature bone cells located in lacunae.
Lacunae: Small cavities housing osteocytes.
Canaliculi: Tiny canals connecting lacunae; allow for nutrient and waste exchange.
Volkmann's (Perforating) Canals: Run perpendicular to Haversian canals; connect blood and nerve supply.
Interstitial Lamellae: Remnants of old osteons found between newer ones.
Circumferential Lamellae: Extend around the entire circumference of the diaphysis.

Structure of Short, Irregular, and Flat Bones

These bones have a thin plate of periosteum-covered compact bone on the exterior and spongy bone inside, called diploë in flat bones.

Bone marrow is located between the trabeculae.

Chemical Composition of Bone

Bones are composed of organic and inorganic components that provide strength and flexibility.

Organic Components (~1/3):
- Osteoblasts: Bone-forming cells; secrete matrix and form new bone.
- Osteoclasts: Bone-destroying cells; resorb bone matrix.
- Osteocytes: Mature bone cells trapped in matrix; maintain bone tissue.
- Matrix includes proteoglycans, glycoproteins, and collagen.
Inorganic Components (~2/3):
- Mineral salts, mainly hydroxyapatite (calcium phosphate crystals).
- Calcium carbonate and other minerals.

Bone Development and Growth

Bones form through ossification, which occurs by two main processes: intramembranous and endochondral ossification.

Ossification (Osteogenesis)

Intramembranous Ossification: Forms flat bones (e.g., skull, clavicles) from fibrous connective tissue membranes.
Endochondral Ossification: Forms most bones below the skull from hyaline cartilage templates.

Steps in Endochondral Ossification

Bone collar formation: Osteoblasts secrete bone matrix around diaphysis.
Cavitation of hyaline cartilage: Chondrocytes enlarge and die, leaving cavities.
Formation of spongy bone: Osteoblasts secrete matrix; periosteal bud invades cavity.
Diaphysis elongates and medullary cavity forms: Cartilage continues to grow at epiphyses.
Ossification of epiphyses: Secondary ossification centers form in epiphyses.

Bone Growth in Length

Occurs at the epiphyseal plate by division of chondrocytes and ossification.
Growth hormone and sex hormones regulate activity.
Epiphyseal plate closure occurs at about 18 years (females) and 21 years (males).

Bone Growth in Thickness (Appositional Growth)

Osteoblasts beneath periosteum secrete bone matrix.
Osteoclasts remove bone from endosteal surface.
Matrix is added to the outside and removed from inside, increasing diameter.

Bone Homeostasis: Remodeling and Repair

Bones are constantly remodeled and repaired throughout life to maintain strength and mineral balance.

Remodeling: Osteoblasts build new bone; osteoclasts resorb old bone.
Deposit and Resorption: Occur at bone surfaces; regulated by hormones and mechanical stress.
Osteoid seam: Unmineralized band of bone matrix.
Calcification front: Transition between osteoid seam and mineralized bone.
Enzymes: Osteoblasts secrete alkaline phosphatase (for mineralization) and lysosomal enzymes (osteoclasts).

Bone Repair

Bone repair follows a sequence of events after a fracture.

Hematoma formation: Blood vessels rupture; hematoma forms.
Fibrocartilaginous callus formation: Soft callus forms; fibroblasts and osteoblasts migrate and deposit collagen fibers.
Bony callus formation: Osteoblasts form spongy bone, replacing the callus.
Remodeling: Excess bone is removed; spongy bone is converted to compact bone.

Hormonal Mechanisms

Bone remodeling is regulated by hormones that control calcium levels.

Parathyroid hormone (PTH): Released when blood calcium declines; stimulates osteoclasts to resorb bone.
Calcitonin: Released by thyroid gland when blood calcium rises; inhibits bone resorption.

Fractures

Fractures are classified by their nature and severity.

Type	Description
Displaced vs. Nondisplaced	Bone ends out of alignment vs. in normal position
Complete vs. Incomplete	Bone broken all the way through vs. partial break
Simple (Closed)	Bone ends do not penetrate skin
Compound (Open)	Bone ends penetrate skin
Comminuted	Bone fragments into many pieces
Compression	Bone is crushed
Depressed	Broken bone portion pressed inward
Impacted	Broken bone ends forced into each other
Spiral	Ragged break due to twisting forces
Greenstick	Bone breaks incompletely (common in children)

Repair of Fractures

Reduction: Realignment of bone ends (manual or surgical).
Immobilization: Cast or traction for 8-12 weeks; longer for large bones or elderly.

Homeostatic Imbalances of Bone

Several disorders can affect bone mineralization and strength.

Osteomalacia: Inadequate mineralization; soft bones and pain due to insufficient calcium or vitamin D.
Rickets: Osteomalacia in children; results in bowed legs and deformities.
Osteoporosis: Bone resorption outpaces deposit; bones become porous and fragile.
Paget's Disease: Excessive bone deposit and resorption; abnormal bone structure.

Key Terms and Definitions

Osteoblast: Bone-forming cell.
Osteoclast: Bone-resorbing cell.
Osteocyte: Mature bone cell maintaining bone tissue.
Epiphyseal Plate: Growth plate in long bones.
Hematopoiesis: Formation of blood cells in bone marrow.
Hydroxyapatite: Mineral form of calcium phosphate in bone.

Important Equations

Bone Remodeling Rate:
Calcium Homeostasis:

Additional info: Some explanations and definitions have been expanded for clarity and completeness.