Bones & Bone Tissue: Structure, Function, and Physiology

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Week 5 – Bones & Bone Tissue

Introduction to the Skeletal System

The skeletal system forms the structural framework of the human body, providing support, protection, and enabling movement. It is composed of bones, cartilage, ligaments, and joints, and is essential for mineral storage and blood cell production.

Axial skeleton: Includes the skull, vertebral column, and thoracic cage.
Appendicular skeleton: Comprises the limbs and girdles (shoulder and pelvic).
Joints: Articulations where two or more bones meet, allowing for movement and flexibility.

Functions of the Skeletal System

Support: Provides a framework for the attachment of soft tissues and organs.
Storage of minerals and lipids: Stores calcium, phosphate, and lipids in yellow bone marrow.
Blood cell production: Occurs in red bone marrow (hematopoiesis).
Protection: Shields vital organs (e.g., skull protects the brain, ribs protect the heart and lungs).
Leverage: Bones act as levers to facilitate movement by muscles.

Classification of Bones

Bone Shapes

Bones are classified by their shapes, which relate to their functions and locations in the body.

Long bones: Longer than they are wide (e.g., femur, humerus).
Short bones: Small and boxy (e.g., carpals, tarsals).
Flat bones: Thin with parallel surfaces (e.g., skull, sternum, ribs).
Irregular bones: Complex shapes (e.g., vertebrae, pelvic bones).
Sesamoid bones: Small, round, and flat; develop inside tendons (e.g., patella).
Sutural (Wormian) bones: Small, irregular bones found between the flat bones of the skull.

Bone Markings

Bone markings are surface features that serve as sites for muscle, ligament, and tendon attachment, or as pathways for nerves and blood vessels.

Depressions or grooves: Allow passage of blood vessels and nerves.
Elevations or projections: Sites where tendons and ligaments attach.
Tunnels: Openings or passageways for nerves and blood vessels.
Examples: Crest (prominent ridge), tubercle (small rounded projection), fossa (shallow depression), foramen (hole).

Structure of Long and Flat Bones

Long bone structure:
- Diaphysis: Shaft, composed of compact bone surrounding the medullary cavity.
- Epiphysis: Expanded ends, mostly spongy bone covered by compact bone.
- Metaphysis: Region where diaphysis and epiphysis meet.
Flat bone structure: "Sandwich" of compact bone (outer layers) with spongy bone in the middle (e.g., parietal bone of the skull).

Bone Tissue

Components of Bone Tissue

Bone tissue is a specialized connective tissue composed of cells, ground substance, and extracellular fibers.

Specialized cells: Osteogenic cells, osteoblasts, osteocytes, osteoclasts.
Ground substance: Mineralized matrix, primarily calcium phosphate.
Extracellular fibers: Mainly collagen, providing flexibility and tensile strength.

Bone Matrix

Inorganic component: Two-thirds of bone matrix is calcium phosphate, forming hydroxyapatite crystals (), which provide hardness and resistance to compression.
Organic component: One-third is collagen fibers, which provide flexibility and resistance to tension and twisting.

Bone Cells

Osteogenic cells: Stem cells located in the endosteum; divide to produce osteoblasts.
Osteoblasts: Immature bone cells that secrete osteoid (unmineralized bone matrix); responsible for bone formation (osteogenesis).
Osteocytes: Mature bone cells that maintain the bone matrix; reside in lacunae and communicate via canaliculi.
Osteoclasts: Large, multinucleate cells that resorb bone by secreting acids and enzymes; derived from macrophage lineage.

Compact vs. Spongy Bone

Compact Bone

Compact bone is dense and forms the outer layer of bones. Its basic functional unit is the osteon (Haversian system).

Osteon structure: Concentric lamellae of bone matrix surrounding a central canal containing blood vessels and nerves.
Perforating (Volkmann's) canals: Perpendicular to central canals, connecting blood supply of osteons deeper in bone.
Interstitial lamellae: Fill spaces between osteons; remnants of old osteons.
Circumferential lamellae: Found at outer and inner surfaces of bone, covered by periosteum and endosteum.

Spongy Bone

Spongy bone (cancellous bone) is found at the ends of long bones and inside flat bones. It consists of a network of trabeculae (supporting fibers).

Trabeculae: Meshwork of bone tissue; spaces between trabeculae are filled with bone marrow.
No osteons: Lamellae are arranged irregularly; nutrients reach osteocytes by diffusion through canaliculi.

Periosteum and Endosteum

Periosteum: Outer fibrous and inner cellular layer; isolates bone, provides route for blood vessels and nerves, and participates in growth and repair.
Endosteum: Incomplete cellular layer lining medullary cavity and central canals; contains osteoblasts, osteogenic cells, and osteoclasts.

Bone Formation and Growth

Ossification and Calcification

Ossification: The process of bone formation.
Calcification: Deposition of calcium salts within tissue.

There are two main types of ossification:

Endochondral ossification: Most bones develop from hyaline cartilage models, which are gradually replaced by bone.
Intramembranous ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull, mandible, clavicle).

Bone Growth

Appositional growth: Increase in bone diameter by adding new bone to the surface.
Interstitial growth: Increase in bone length at the epiphyseal (growth) plates.

Blood and Nerve Supply

Nutrient arteries and veins: Supply diaphysis via nutrient foramina.
Metaphyseal and epiphyseal vessels: Supply the ends of long bones.
Periosteal vessels: Supply the superficial osteons and periosteum.
Nerves: Accompany blood vessels; bones are well-innervated and sensitive to injury.

Bone Remodeling and Homeostasis

Bone Remodeling

Bone is a dynamic tissue, constantly being broken down and rebuilt in response to mechanical stress and metabolic needs.

Osteoblasts: Build new bone matrix.
Osteoclasts: Resorb bone matrix.
Remodeling rate: In young adults, the skeleton is remodeled every year.

Effects of Exercise, Hormones, and Nutrition

Exercise: Increases bone density and strength; lack of use leads to bone loss.
Nutrition: Adequate intake of calcium, phosphate, vitamin D (for calcitriol synthesis), vitamin C (for collagen), vitamin A, K, and B12 is essential.
Hormones: Calcitriol (from kidneys), calcitonin (from thyroid), and parathyroid hormone (PTH) regulate bone metabolism and calcium homeostasis.

Calcium Homeostasis

Role of Calcium in the Skeletal System

Calcium storage: Bones store 99% of the body's calcium.
Importance: Calcium is vital for nerve impulse transmission, muscle contraction, and blood clotting.

Hormonal Regulation

Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and decreasing renal excretion.
Calcitonin: Decreases blood calcium by inhibiting osteoclasts and increasing renal excretion.

Table: Major Minerals in Bone and Body

Mineral	% in Bone	% in Body
Calcium	99%	33%
Phosphate	99%	17%
Potassium	4%	0.2%
Sodium	35%	0.7%
Magnesium	50%	0.4%
Carbonate	10%	4.7%

Fractures and Bone Repair

Types of Fractures

Closed (simple) fracture: Bone does not break the skin.
Open (compound) fracture: Bone breaks through the skin, increasing risk of infection.

Fracture Repair Process

Formation of a hematoma (blood clot).
Formation of an internal and external callus (fibrocartilaginous tissue).
Replacement of cartilage with new bone (endochondral ossification).
Remodeling of bone to restore original shape and structure.

Aging and the Skeletal System

Effects of Aging

Osteopenia: Inadequate ossification leading to thinner, weaker bones due to decreased osteoblast activity and continued osteoclast activity.
Osteoporosis: Severe loss of bone mass that impairs normal function and increases fracture risk.

Osteoporosis

Common in postmenopausal women and older men.
Related to hormonal changes, especially decreased estrogen.
Prevention includes adequate calcium and vitamin D intake, regular weight-bearing exercise, and hormone therapy if indicated.

Additional info:

Fibrodysplasia Ossificans Progressiva (FOP): A rare genetic disorder where soft tissues progressively ossify, forming bone in abnormal locations (heterotopic ossification).