Skeletal System and Bone Functions

Hyaline Cartilage: The most abundant type, found in various locations.
- Articular Cartilage: Covers the ends of most bones at moveable joints, providing a smooth surface for joint movement.
- Costal Cartilage: Connects ribs to the sternum, allowing for the expansion of the rib cage during breathing.
- Respiratory Cartilage: Forms the larynx (voice box) and reinforces other respiratory passageways, maintaining open airways.
- Nasal Cartilage: Supports the external nose, maintaining its shape.
Elastic Cartilage: Found in the ear and epiglottis, which bends to cover the larynx during swallowing, contributing to flexibility and support.
Fibrocartilage: Highly compressible and durable, found in areas subjected to heavy pressure such as knees and intervertebral discs, which provides cushioning and support.

Total Number of Bones: 206 in adults; approx. 300 in children due to the fusion of certain bones for efficient birth.

Axial Bones (80 total): Consist of the skull (28 bones), vertebrae (26 bones), and rib cage (26 bones).
Appendicular Bones (126 total): Include bones of the shoulder and arms (32x2) and bones of the hip and legs (31x2).

Long Bones: Longer than they are wide (e.g., femur).
Short Bones: Cube-like; provide stability and some movement (e.g., carpal bones).
Flat Bones: Thin and flat, providing protection for internal organs (e.g., ribs, cranial bones).
Sesamoid Bones: Embedded within tendons (e.g., patella); protect tendons from wear and tear.
Irregular Bones: Complex shapes that protect internal structures (e.g., vertebrae, pelvic girdle).

Support: Provides a framework for the body and cradles organs, maintaining structural integrity.
Protection: Shields vital organs such as the brain, spine, and heart from injury.
Movement: Facilitates movement through articulation with tendons and muscles.
Mineral and Growth Factor Storage: Stores minerals like calcium and phosphate, which are essential for body functions and growth.
Blood Cell Formation (Hematopoiesis): Red blood cell, white blood cell, and platelet production occurs in bone marrow.
Triglyceride Storage: Yellow marrow stores fat, serving as an energy reservoir.

Diaphysis: The shaft of the long bone, filled with marrow; acts as a supportive structure.
Epiphysis: The ends of long bones capped with hyaline cartilage; contains epiphyseal plates during growth, which later become epiphyseal lines in adults.

Periosteum: The vascular outer covering of bone, attached by Sharpey’s fibers, vital for growth and repair.
Endosteum: Covers the inner surfaces of bones, particularly in cancellous (spongy) bone.

Compact Bone: Dense and smooth external layer providing strength.
Spongy Bone (Cancellous Bone): Contains trabecular structures that house marrow and support lightness and strength, preventing fractures.

Projections: Bulges on the bone surface such as facets or processes grow outward.
Depressions: Openings or indentations allowing for vessels or reducing weight (e.g., fossae, sinuses, foramina).

Osteogenic Cells: Mitotic cells located in periosteum and endosteum involved in creating new bone.
Osteoblasts: Bone-forming cells that synthesize and secrete the bone matrix (osteoid).
Osteocytes: Mature bone cells responsible for nutrient distribution and monitoring bone matrix health.
Osteoclasts: Cells that break down and resorb bone matrix, crucial for bone remodeling and calcium homeostasis.

Bone is composed of 35% organic parts (cells and collagen fibers) providing flexibility and tensile strength, and 65% inorganic parts (mineral salts and calcium phosphates) allowing bones to remain intact post-mortem due to mineral content.

Key Regulatory Hormones:
- Pituitary Gland: Releases growth hormones.
- Thyroid Hormones: Moderate growth processes.
- Sex Hormones: Involved in epiphyseal plate maturation.

Bone formation occurs via ossification—initially forming a bony skeleton in embryos, with continued growth postnatally.
Bones undergo continuous remodeling to maintain homeostasis, becoming stronger over time especially in response to stress and calcium filling post-injury.

Bone Deposit: The process of adding calcium and phosphates in areas of injury or where strength is needed.
Bone Resorption: Osteoclast activity breaks down bone matrix to release calcium into the bloodstream.

Hormonal Control: Controlled primarily by parathyroid hormone (PTH), which increases blood calcium levels by stimulating osteoclasts.
Mechanical Stress: Wolff’s Law dictates that bone adapts in structure in response to mechanical stress—thicker in areas experiencing more force.

Long bones continue to grow during infancy and youth; growth generally halts during adolescence.
Certain facial bones, including the nose and jaw, can continue to grow throughout life, demonstrating ongoing adaptation and change in the skeletal structure.

Day 1: Damage to blood vessels leads to hemorrhage and accumulation of a hematoma.
Day 3: Fibroblasts and osteoblasts begin to reconstruct damaged bone tissue.
Day 9: New trabecular bone forms, completing within approximately 2 months.
Ongoing remodeling occurs to reinforce the area and restore the structural integrity of the bone.

Bone structure, function, and homeostasis are crucial aspects of the skeletal system, involving complex processes that ensure growth, repair, and adaptation throughout an individual's life.