Bones and Bone Structure

Functions of the Skeletal System

The skeletal system is a complex framework that provides structural support and facilitates movement. It consists of bones, cartilages, ligaments, and other connective tissues.

• Support: Provides the body with its shape and supports soft tissues.

• Storage of minerals and lipids: Stores calcium, phosphorus, and lipids for energy.

• Blood cell production: Houses red bone marrow, which produces blood cells.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Leverage: Acts as levers for muscles to facilitate movement.

Classification of Bones

Bones are classified by their shape and structure, each serving distinct functional roles.

• Sutural (Wormian) Bones: Small, flat, irregular bones found between skull bones. Borders resemble jigsaw puzzle pieces.

• Irregular Bones: Complex shapes with notched or ridged surfaces. Examples: vertebrae, pelvic bones.

• Short Bones: Boxlike in appearance. Examples: carpal (wrist) and tarsal (ankle) bones.

• Flat Bones: Thin, parallel surfaces. Provide protection and muscle attachment. Examples: skull roof, sternum, ribs, scapulae.

• Long Bones: Long and slender, found in limbs, fingers, toes. Example: femur.

• Sesamoid Bones: Small, round, flat bones within tendons near joints. Example: patella (kneecap).

Bone Markings (Surface Features)

Bone markings are projections, openings, and depressions that serve as attachment points or passageways for nerves and blood vessels.

• Projections: Sites for muscle, tendon, and ligament attachment; also form joints.

• Openings and Depressions: Allow passage of blood vessels and nerves.

Examples of Bone Markings

• Process: Projection or bump.

• Ramus: Forms an angle with the rest of the bone.

• Foramen: Rounded passageway for vessels/nerves.

• Fissure: Deep furrow or slit.

• Meatus/Canal: Passage or channel.

• Trochanter, Crest, Spine, Tubercle, Tuberosity: Various projections for muscle attachment.

• Head, Neck, Facet, Condyle, Trochlea: Projections for joint formation.

• Sulcus, Fossa: Depressions for vessels or muscle attachment.

Structure of Bones

Long Bones

• Diaphysis: Shaft, composed of compact bone, surrounds the medullary cavity.

• Epiphysis: Ends of the bone, mostly spongy bone.

• Metaphysis: Region where diaphysis and epiphysis meet.

Flat Bones

• Consist of spongy bone (diploë) sandwiched between two layers of compact bone.

Bone Tissue

Bone tissue is a dense, supportive connective tissue with specialized cells and a solid matrix.

• Matrix: Composed of calcium phosphate (Ca3(PO4)2), calcium hydroxide (Ca(OH)2), forming hydroxyapatite (Ca10(PO4)6(OH)2), and collagen fibers.

• Cells: Four types—osteogenic cells, osteoblasts, osteocytes, osteoclasts.

Types of Bone Cells

• Osteogenic Cells: Stem cells that produce osteoblasts; found in periosteum and endosteum.

• Osteoblasts: Immature cells that synthesize new bone matrix (osteoid).

• Osteocytes: Mature bone cells maintaining matrix; reside in lacunae, connected by canaliculi.

• Osteoclasts: Multinucleate cells that resorb bone matrix, important for homeostasis.

Compact Bone vs. Spongy Bone

Compact Bone

• Osteon: Functional unit; concentric lamellae surround a central canal.

• Perforating Canals: Carry blood vessels perpendicular to bone surface.

• Lamellae: Layers of bone matrix—concentric, interstitial, circumferential.

Spongy Bone

• Lacks osteons; matrix forms trabeculae.

• Red bone marrow fills spaces, produces blood cells.

• Yellow bone marrow stores fat.

Periosteum and Endosteum

• Periosteum: Membrane covering bone; outer fibrous and inner cellular layers. Functions in isolation, blood supply, and repair.

• Endosteum: Incomplete cellular layer lining medullary cavity; active in growth and repair.

Bone Formation and Growth

Ossification

• Endochondral Ossification: Most bones form this way; bone replaces hyaline cartilage in seven steps.

• Intramembranous Ossification: Occurs in dermis; forms flat bones like mandible and clavicle in five steps.

Bone Growth

• Interstitial Growth: Increases length; involves epiphyseal plate.

• Appositional Growth: Increases width; circumferential lamellae added.

Blood Supply to Bones

• Nutrient Artery and Vein: Enter through nutrient foramina.

• Metaphyseal Vessels: Supply epiphyseal cartilages.

• Periosteal Vessels: Supply superficial osteons and secondary ossification centers.

Bone Remodeling

Bone remodeling is a continuous process involving osteocytes, osteoblasts, and osteoclasts. It maintains bone integrity by recycling and renewing matrix.

• Balanced activity keeps bones healthy.

• Imbalance can lead to weakened or strengthened bones.

Exercise, Nutrition, and Hormones

• Exercise: Stimulates osteoblasts; bones adapt and become stronger.

• Nutrition: Requires calcium, phosphorus, magnesium, fluoride, iron, manganese, vitamins D3, C, A, K, B12.

• Hormones: Growth hormone, thyroxine, sex hormones, parathyroid hormone, and calcitonin regulate bone growth and calcium homeostasis.

Calcium Homeostasis

Bones store 99% of the body's calcium. Calcium ions are essential for physiological processes and must be tightly regulated.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and reducing kidney excretion.

• Calcitonin: Decreases blood calcium by inhibiting osteoclasts, increasing kidney excretion, and reducing intestinal absorption.

Fractures and Repair

• Fractures: Cracks or breaks in bones; classified as open (compound) or closed (simple). Types include transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, and Pott’s fractures.

• Repair Process: Four steps—fracture hematoma formation, callus formation, spongy bone formation, compact bone formation.

Effects of Aging on the Skeletal System

• Osteopenia: Reduction in bone mass, begins between ages 30–40; more pronounced in women.

• Osteoporosis: Severe bone loss, compromises function; more common in women after menopause.

• Cancer: Can cause severe osteoporosis via osteoclast-activating factor.

Summary Table: Bone Cell Types

Summary Table: Types of Bone Markings

Key Equations

• Hydroxyapatite Formation:

Additional info: Academic context was added to clarify bone cell functions, bone markings, and the effects of hormones and nutrition on bone health.