Bones and Bone Tissue: Structure, Function, and Physiology

Bones and Bone Tissue

Introduction to Bones as Organs

The skeletal system is a complex organ system that provides structure, protection, and support for the human body. Bones are dynamic organs composed of several tissue types and perform a variety of essential functions.

• Functions of the Skeletal System:

  • Support: Provides a framework for the body and supports soft tissues.

  • 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 such as calcium and phosphate, which can be released into the bloodstream as needed.

  • Blood Cell Production: Houses red bone marrow, which produces blood cells (hematopoiesis).

  • Fat Storage: Yellow bone marrow stores triglycerides as an energy reserve.

• Classification of Bones by Shape:

  • Long Bones: Longer than they are wide (e.g., femur, humerus).

  • Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).

  • Flat Bones: Thin and broad (e.g., sternum, skull bones).

  • Irregular Bones: Complex shapes (e.g., vertebrae, certain facial bones).

  • Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

• Gross Structure of Bones:

  • Long Bones: Consist of a diaphysis (shaft), epiphyses (ends), and medullary cavity.

  • Short, Flat, Irregular, and Sesamoid Bones: Generally composed of a thin layer of compact bone surrounding spongy bone.

• Red vs. Yellow Bone Marrow:

  • Red Bone Marrow: Site of hematopoiesis; found mainly in flat bones and the ends of long bones in adults.

  • Yellow Bone Marrow: Consists mainly of adipose tissue; found in the medullary cavity of long bones in adults.

Microscopic Structure of Bone Tissue

Bone tissue is a specialized connective tissue with a unique extracellular matrix and several cell types that contribute to its structure and function.

• Extracellular Matrix Components:

  • Inorganic Matrix: Primarily hydroxyapatite crystals (calcium phosphate), providing hardness and strength.

  • Organic Matrix: Mainly collagen fibers and ground substance, providing flexibility and tensile strength.

• Main Cell Types in Bone Tissue:

  • Osteoblasts: Bone-forming cells that secrete bone matrix.

  • Osteocytes: Mature bone cells that maintain bone tissue.

  • Osteoclasts: Large, multinucleated cells that resorb (break down) bone matrix.

• Microscopic Structure of Compact Bone:

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

  • Lamellae: Layers of bone matrix.

  • Lacunae: Small spaces housing osteocytes.

  • Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

• Microscopic Structure of Spongy Bone:

  • Composed of a network of trabeculae (thin columns and plates of bone) with spaces filled by bone marrow.

Bone Formation: Ossification

Ossification is the process by which bone tissue forms. There are two main types: intramembranous and endochondral ossification.

• Primary vs. Secondary Bone:

  • Primary (woven) bone: First bone formed; irregular arrangement of collagen fibers.

  • Secondary (lamellar) bone: Mature bone with organized collagen fibers; replaces primary bone.

• Intramembranous Ossification:

  • Bone develops directly from mesenchymal tissue; forms most flat bones (e.g., skull, clavicle).

• Endochondral Ossification:

  • Bone develops by replacing hyaline cartilage; forms most bones of the body, especially long bones.

Bone Growth in Length and Width

Bones grow in both length and width through distinct processes regulated by hormones and other factors.

• Longitudinal Growth:

  • Occurs at the epiphyseal (growth) plates through endochondral ossification.

• Appositional Growth:

  • Increase in bone width by addition of new bone tissue at the surface.

• Hormonal Regulation:

  • Growth hormone, thyroid hormone, and sex hormones (estrogen and testosterone) play key roles in bone growth.

Bone Remodeling and Repair

Bone is a dynamic tissue that undergoes continuous remodeling and can repair itself after injury.

• Bone Resorption and Deposition:

  • Resorption: Osteoclasts break down bone matrix, releasing minerals into the blood.

  • Deposition: Osteoblasts form new bone matrix.

• Factors Influencing Bone Remodeling:

  • Physical: Mechanical stress and exercise stimulate bone formation.

  • Hormonal: Parathyroid hormone (PTH), calcitonin, and vitamin D regulate calcium homeostasis and bone metabolism.

  • Dietary: Adequate intake of calcium, vitamin D, and protein is essential for healthy bone remodeling.

• Role of Hormones in Bone Remodeling:

  • Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

  • Calcitonin: Lowers blood calcium by inhibiting bone resorption.

  • Vitamin D: Promotes calcium absorption in the gut and supports bone mineralization.

• Bone Repair:

  • Involves hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.