Skeletal System: Structure, Function, and Cellular Components

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The skeletal system is a complex organ system that provides structural support, protection, and several other vital functions for the human body.

Support: Provides a rigid framework that supports the body and maintains its shape.
Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).
Movement: Serves as levers for muscles to act upon, enabling movement.
Mineral Storage: Stores minerals, especially calcium and phosphate, which can be released into the bloodstream as needed.
Blood Cell Production: Houses red bone marrow, which produces blood cells (hematopoiesis).
Energy Storage: Stores lipids in yellow bone marrow.

The periosteum is a dense, fibrous membrane covering the outer surface of bones except at joints.

Function: Protects bone, provides a surface for attachment of tendons and ligaments, contains cells for bone growth and repair, and supplies blood to bone tissue.
Composition: Consists of an outer fibrous layer (dense irregular connective tissue) and an inner cellular layer (osteogenic cells).

Perforating fibers are collagen fibers that anchor the periosteum to the underlying bone.

Function: Secure the periosteum to the bone, providing stability for tendons and ligaments attached to the bone.

The endosteum is a thin membrane lining the internal surfaces of bones, including the medullary cavity.

Function: Contains osteogenic cells important for bone growth, repair, and remodeling.

Bones are composed of two main types of osseous tissue: compact and spongy bone.

Compact Bone: Dense and solid; forms the outer layer of bones. Provides strength for weight-bearing and protection.
Spongy (Cancellous) Bone: Porous and less dense; found at the ends of long bones and inside flat bones. Reduces bone weight and contains red bone marrow for hematopoiesis.

Epiphyseal Plate (Growth Plate): Hyaline cartilage plate in growing bones where lengthwise growth occurs.
Epiphyseal Line: Remnant of the epiphyseal plate in adult bones, indicating where growth has ceased.

Sinuses are air-filled cavities within certain bones (e.g., skull bones).

Function: Lighten the weight of the bone, enhance voice resonance, and help warm and humidify inhaled air.

Bones receive blood through various routes depending on their shape and structure.

Long Bones: Supplied by nutrient arteries entering through nutrient foramina, periosteal vessels, and metaphyseal/epiphyseal arteries.
Short, Flat, Irregular, and Sesamoid Bones: Receive blood from periosteal vessels and small nutrient arteries that penetrate the bone at multiple points.

Red Bone Marrow: Site of hematopoiesis (production of red blood cells, white blood cells, and platelets); composed of hematopoietic tissue.
Yellow Bone Marrow: Stores fat (adipocytes); found mainly in the medullary cavity of long bones in adults.

The ECM of bone is a composite material providing strength and flexibility.

Organic Component: Mainly collagen fibers (type I) and ground substance; provides tensile strength and flexibility.
Inorganic Component: Primarily hydroxyapatite crystals (calcium phosphate); provides hardness and resistance to compression.

Osteoblasts: Bone-forming cells; synthesize and secrete the bone matrix.
Osteocytes: Mature bone cells; maintain bone tissue and communicate via canaliculi.
Osteoclasts: Large, multinucleated cells; break down bone matrix (bone resorption).

Central canals are longitudinal channels in the center of osteons in compact bone.

Canaliculi are microscopic canals between the lacunae of ossified bone.

Function: Allow for the exchange of nutrients, waste, and signals between osteocytes and blood vessels.

Lamellae are concentric layers of bone matrix in compact bone, classified by their location and arrangement.

Type of Lamellae	Location	Function
Concentric Lamellae	Surround central canals (osteons)	Provide strength and structure to osteons
Interstitial Lamellae	Between osteons	Fill spaces, add strength, and result from bone remodeling
Circumferential Lamellae	Encircle the entire bone beneath the periosteum or lining the medullary cavity	Increase bone diameter and resist twisting forces

Bone remodeling is a continuous process involving osteoblasts and osteoclasts, maintaining bone strength and mineral homeostasis.
Hydroxyapatite chemical formula: