The Skeletal System: Structure and Function of Bone Tissue

The Skeletal System: Structure and Function of Bone Tissue

Functions of the Skeletal System

The skeletal system performs several essential functions that are critical for survival and homeostasis in the human body.

• Support: Provides a structural framework for the body, supporting soft tissues and providing attachment points for muscles.

• Protection: Shields vital organs such as the brain, heart, and lungs from injury (e.g., the skull protects the brain).

• Assistance in Movement: Serves as levers that muscles act upon to produce movement.

• Mineral Homeostasis: Stores minerals, especially calcium and phosphorus, and releases them into the blood as needed.

• Blood Cell Production: Houses red bone marrow, which produces red blood cells, white blood cells, and platelets (a process called hematopoiesis).

• Triglyceride Storage: Stores energy in the form of triglycerides in yellow bone marrow.

Structure of Long Bones

Long bones have a complex structure that supports their function in movement and mineral storage.

• Diaphysis: The long, cylindrical shaft of the bone.

• Epiphyses: The proximal and distal ends of the bone.

• Metaphyses: The regions between the diaphysis and epiphyses; includes the epiphyseal (growth) plate in growing bones.

• Articular Cartilage: Thin layer of hyaline cartilage covering the epiphyses where the bone forms a joint with another bone. Reduces friction and absorbs shock.

• Periosteum: Tough, dense irregular connective tissue covering the bone except at articular cartilage. Contains blood vessels, nerves, and serves as an attachment point for tendons and ligaments.

• Medullary (Marrow) Cavity: Space within the diaphysis that contains yellow bone marrow in adults.

• Endosteum: Thin membrane lining the medullary cavity, containing bone-forming cells.

Diagram: Structure of a Long Bone

Key labeled parts: Proximal epiphysis, metaphysis, diaphysis, distal epiphysis, articular cartilage, spongy bone (red marrow), compact bone, periosteum, endosteum, medullary cavity (yellow marrow), nutrient artery.

Bone Cells

Bone tissue contains several types of cells, each with specialized functions in bone formation, maintenance, and remodeling.

• Osteogenic Cells: Stem cells derived from mesenchyme. Undergo cell division and differentiate into osteoblasts. Located in the inner periosteum, endosteum, and canals containing blood vessels.

• Osteoblasts: Bone-building cells that synthesize and secrete collagen fibers and other organic components. Initiate calcification and become trapped in their secretions, differentiating into osteocytes.

• Osteocytes: Mature bone cells that maintain daily metabolism of bone tissue.

• Osteoclasts: Large cells derived from the fusion of many monocytes (a type of white blood cell). Responsible for bone resorption by releasing lysosomal enzymes and acids that digest bone matrix.

Summary Table: Bone Cell Types

Cell Type	Origin	Function
Osteogenic cell	Mesenchymal stem cell	Bone stem cell; differentiates into osteoblasts
Osteoblast	Osteogenic cell	Bone formation; secretes matrix
Osteocyte	Osteoblast	Mature bone cell; maintains bone tissue
Osteoclast	Hematopoietic stem cell (monocyte fusion)	Bone resorption; breaks down bone matrix

Bone Matrix

The bone matrix is a composite material that provides strength and flexibility to bone tissue.

• Composition: Approximately 25% water, 25% collagen fibers, and 50% crystallized mineral salts.

• Hydroxyapatite: The most abundant mineral salt in bone, formed by the combination of calcium phosphate and calcium hydroxide.

Formula for Hydroxyapatite:

$Ca_{10}(PO_4)_6(OH)_2$

This mineral, along with other salts, provides the framework for bone calcification and hardness.

Compact Bone Tissue

Compact bone is the densest form of bone tissue, providing strength and support. It forms the outer layer of all bones and the bulk of the diaphysis in long bones.

• Osteons (Haversian Systems): Structural units of compact bone, consisting of concentric lamellae arranged around a central (Haversian) canal.

• Volkmann's (Perforating) Canals: Transverse canals that carry blood vessels, nerves, and lymphatics from the periosteum to the osteons.

• Haversian (Central) Canal: Runs longitudinally through the bone, containing blood vessels and nerves.

• Concentric Lamellae: Rings of calcified matrix surrounding the Haversian canal.

• Lacunae: Small spaces between lamellae that house osteocytes.

• Canaliculi: Tiny channels radiating from lacunae, allowing communication and nutrient/waste exchange between osteocytes.

• Interstitial Lamellae: Fragments of older osteons found between current osteons.

• Circumferential Lamellae: Lamellae that encircle the bone just beneath the periosteum or endosteum.

Spongy (Cancellous) Bone Tissue

Spongy bone is lighter and less dense than compact bone. It is found mainly at the ends of long bones and inside flat, short, and irregular bones.

• Trabeculae: Irregular lattice of thin columns that form the framework of spongy bone. Spaces between trabeculae are filled with red bone marrow (site of hematopoiesis) or yellow marrow.

• Location: Found in the epiphyses of long bones, interior of short, flat, and irregular bones, and the medullary cavity of long bones.

• Function: Reduces bone weight, supports and protects bone marrow, and provides structural support for bone without the weight of compact bone.

• Blood Supply: Trabeculae are oriented along lines of stress and contain osteocytes in lacunae, nourished by blood circulating through marrow cavities.

Example: Spongy bone is prominent in the hip bones, ribs, sternum, vertebrae, and ends of long bones, where red bone marrow is stored and blood cell formation occurs in adults.