The Skeletal System: Structure and Function of Bone Tissue

Functions of the Skeletal System

The skeletal system is essential for maintaining the structure and function of the human body. It provides a framework for movement, protection, and several metabolic processes.

• Support: Bones form the rigid framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs, such as the brain (skull), heart and lungs (rib cage).

• Assist in Movement: Bones act as levers for muscles, enabling movement.

• Mineral Homeostasis: Bones store minerals, especially calcium and phosphorus, and release them into the bloodstream as needed.

• Blood Cell Production: Hematopoiesis occurs in the red bone marrow, producing red and white blood cells and platelets.

• Triglyceride Storage: Yellow bone marrow stores fat as an energy reserve.

Structure of a Long Bone

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

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

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

• Metaphysis: The region where the diaphysis meets the epiphysis; contains the epiphyseal plate (growth plate) in children.

• Articular Cartilage: Hyaline cartilage covering the epiphyses at joints; reduces friction and absorbs shock.

• Periosteum: Dense, fibrous membrane covering the bone except at joint surfaces; contains nerves and blood vessels, and serves as an attachment point for tendons and ligaments.

• Medullary (Marrow) Cavity: Central cavity within the diaphysis containing yellow bone marrow in adults.

• Endosteum: Thin membrane lining the medullary cavity; contains bone-forming cells.

Bone Cell Types and Development

Bones are dynamic tissues composed of several cell types, each with specialized functions in growth, maintenance, and repair.

• Osteoprogenitor Cells: Stem cells derived from mesenchyme; found in periosteum and endosteum; undergo cell division to produce osteoblasts.

• Osteoblasts: Bone-forming cells; synthesize and secrete collagen fibers and other organic components; initiate calcification.

• Osteocytes: Mature bone cells; maintain bone tissue; reside in lacunae.

• Osteoclasts: Large, multinucleated cells derived from monocytes; responsible for bone resorption; secrete acids and enzymes to digest bone matrix.

Example: Osteoblasts build new bone during growth and repair, while osteoclasts break down bone during remodeling.

Bone Cell Lineages

• Mesenchymal Stem Cell Lineage: Mesenchymal stem cell → Osteoprogenitor cell → Osteoblast → Osteocyte

• Hematopoietic Stem Cell Lineage: Hematopoietic stem cell → Monocyte/Macrophage → Osteoclast

Bone Matrix Composition

The bone matrix provides strength and flexibility, allowing bones to withstand mechanical stress.

• Organic Matrix: Composed of collagen fibers (about 25%) and other proteins; provides tensile strength and flexibility.

• Inorganic Matrix: Composed of mineral salts (about 50%), primarily hydroxyapatite (); provides hardness and resistance to compression.

• Water: Makes up about 25% of bone matrix.

Compact Bone Structure

Compact bone is the dense, strong form of bone tissue found beneath the periosteum and forming the bulk of long bones.

• Osteon (Haversian System): The structural unit of compact bone; consists of concentric lamellae arranged around a central (Haversian) canal.

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

• Haversian Canal: Central channel containing blood vessels and nerves.

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

• Lacunae: Small spaces between lamellae containing osteocytes.

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

• Interstitial Lamellae: Remnants of old osteons found between newer osteons.

• Circumferential Lamellae: Lamellae encircling the entire bone beneath the periosteum or medullary cavity.

Spongy Bone Tissue

Spongy bone is lighter and less dense than compact bone, consisting of a network of trabeculae that provide structural support and house bone marrow.

• Trabeculae: Irregular lattice of thin columns; spaces between trabeculae contain red bone marrow.

• Canaliculi: Radiate outward, allowing for nutrient and waste exchange.

• Location: Found in the interior of short, flat, and irregular bones, and at the ends of long bones.

• Function: Supports and protects bone marrow; site of hematopoiesis in adults.

Comparison Table: Compact vs. Spongy Bone

Key Equations

• Hydroxyapatite Formation:

Summary

The skeletal system is a complex network of bones, cells, and tissues that provide structure, protection, movement, and metabolic functions. Understanding the anatomy and physiology of bone tissue is essential for comprehending how the body maintains its form and function.