Bones and Bone Tissue: Structure, Function, and Physiology

Chapter 6: Bones and Bone Tissue

Introduction to Bones as Organs

The skeletal system is a dynamic organ system that provides structure, protection, and support for the human body. Bones are classified by their shape and internal structure, and they contain specialized tissues such as red and yellow bone marrow.

• Functions of the Skeletal System:

  • Support: Provides a rigid 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, some facial bones).

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

• Gross Structure of Bones:

  • Long bones have a diaphysis (shaft), epiphyses (ends), and a medullary cavity.

  • Short, flat, irregular, and sesamoid bones have varying structures but generally consist of compact bone surrounding spongy bone.

• Red vs. Yellow Bone Marrow:

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

  • Yellow Bone Marrow: Primarily 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 maintain bone health and structure.

• Extracellular Matrix Components:

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

  • Organic Matrix (Osteoid): Composed of collagen fibers and ground substance, providing flexibility and tensile strength.

• Main Cell Types in Bone Tissue:

  • Osteoblasts: Bone-forming cells that secrete osteoid.

  • Osteocytes: Mature bone cells that maintain the bone matrix.

  • Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown).

• Microscopic Structure of Compact Bone:

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

  • Central Canal: Contains blood vessels and nerves.

  • Lacunae: Small spaces housing osteocytes.

  • Canaliculi: Tiny channels connecting lacunae, allowing communication between osteocytes.

• Microscopic Structure of Spongy Bone:

  • Composed of trabeculae (lattice-like network) with spaces filled by bone marrow.

  • Provides strength while reducing bone weight.

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; organized collagen fibers in lamellae.

• 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 length and width through distinct processes regulated by hormones and other factors.

• Longitudinal Growth:

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

  • Chondrocytes proliferate, hypertrophy, and are replaced by bone tissue.

• Appositional Growth:

  • Bone increases in diameter as new bone is added to the surface by osteoblasts.

• Hormonal Regulation:

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

Bone Remodeling and Repair

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

• Bone Remodeling:

  • Involves bone resorption (by osteoclasts) and bone deposition (by osteoblasts).

  • Maintains bone strength and mineral homeostasis.

• Factors Influencing Remodeling:

  • Physical: Mechanical stress stimulates bone formation.

  • Hormonal: Parathyroid hormone (PTH) increases blood calcium by stimulating bone resorption; calcitonin lowers blood calcium by inhibiting resorption; vitamin D promotes calcium absorption.

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

• Bone Repair:

  • Occurs in stages: hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Key Hormones in Bone Remodeling and Calcium Homeostasis

Hormone	Source	Main Effect on Bone	Effect on Blood Calcium
Parathyroid Hormone (PTH)	Parathyroid glands	Stimulates osteoclasts (bone resorption)	Increases
Calcitonin	Thyroid gland	Inhibits osteoclasts (reduces resorption)	Decreases
Vitamin D (Calcitriol)	Kidneys (activated form)	Promotes calcium absorption in intestines	Increases

Example: When blood calcium levels drop, parathyroid hormone is released, stimulating osteoclast activity to release calcium from bone into the bloodstream.

Additional info: Bone remodeling is essential for adapting to mechanical stress, repairing microfractures, and regulating mineral balance throughout life.