Chapter 6: Bones and Bone Tissue

Module 6.1: Introduction to Bones as Organs

The skeletal system is composed of bones, joints, and their associated supporting tissues. Bones are complex organs made up of osseous tissue, dense and collagenous connective tissue, and bone marrow.

• Protection: Bones protect vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Mineral Storage and Acid-Base Homeostasis: Bones store minerals such as calcium and phosphorus, and help maintain acid-base balance by absorbing or releasing alkaline salts.

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

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

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

• Support: Bones provide structural support for the body and soft tissues.

Classification of Bones by Shape

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

• Short Bones: Cube-shaped, about as long as they are wide (e.g., carpals, tarsals).

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

• Irregular Bones: Complex shapes (e.g., vertebrae, c ertain skull bones).

• Sesamoid Bones: Small, oval-shaped bones within tendons (e.g., patella).

Structure of a Long Bone

• Periosteum: Dense irregular collagenous connective tissue membrane covering the outer surface, rich in blood vessels and nerves.

• Perforating (Sharpey's) Fibers: Collagen fibers anchoring the periosteum to bone.

• Diaphysis: The shaft of the bone.

• Epiphyses: The ends of the bone, covered with articular (hyaline) cartilage.

• Medullary Cavity: Hollow cavity in the diaphysis containing red or yellow marrow.

• Compact (Cortical) Bone: Dense outer region providing strength.

• Spongy (Cancellous) Bone: Inner honeycomb-like structure housing bone marrow.

• Endosteum: Thin membrane lining the internal bone surfaces.

• Epiphyseal Lines: Remnants of growth plates separating epiphyses from diaphysis.

Structure of Short, Flat, Irregular, and Sesamoid Bones

• Covered by periosteum and perforating fibers.

• Composed of two thin layers of compact bone with a middle layer of spongy bone (diploë).

• Some skull bones contain sinuses (air-filled spaces).

Blood and Nerve Supply

• Bones are highly vascularized and innervated.

• Red Bone Marrow: Loose connective tissue supporting hematopoietic cells; in adults, found in pelvis, proximal femur and humerus, vertebrae, ribs, sternum, clavicles, scapulae, and some skull bones.

• Yellow Bone Marrow: Composed of triglycerides, blood vessels, and adipocytes.

Module 6.2: Microscopic Structure of Bone Tissue

Bone (osseous tissue) consists mainly of extracellular matrix (ECM) and a small population of cells.

• Inorganic Matrix: ~65% of bone weight; mainly calcium and phosphate salts as hydroxyapatite crystals (), providing hardness and resistance to compression.

• Organic Matrix (Osteoid): ~35% of bone weight; includes collagen fibers (main protein), proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins. Collagen provides tensile strength and flexibility.

Bone Cells

• Osteoblasts: Bone-forming cells found in periosteum and endosteum; synthesize organic matrix and facilitate mineralization.

• Osteocytes: Mature bone cells maintaining ECM; reside in lacunae.

• Osteoclasts: Large, multinucleated cells derived from bone marrow; responsible for bone resorption.

Histology of Bone Tissue

• Compact Bone: Organized into osteons (Haversian systems) with concentric lamellae, central (Haversian) canal, lacunae (housing osteocytes), and canaliculi (connecting lacunae).

• Spongy Bone: Composed of trabeculae (struts) with lamellae and osteocytes; lacks central canals, receives nutrients via diffusion from marrow spaces.

Module 6.3: Bone Formation (Ossification)

Ossification (osteogenesis) is the process of bone formation, beginning in the embryonic period and continuing through childhood.

• Primary (Woven) Bone: Immature bone with irregular collagen arrangement; later replaced by mature (lamellar) bone.

• Intramembranous Ossification: Forms flat bones (e.g., skull, clavicles) from mesenchymal membrane. Steps include mesenchymal cell differentiation, osteoid secretion, calcification, formation of spongy bone, and development of periosteum and compact bone.

• Endochondral Ossification: Forms most bones below the head (except clavicles) from a hyaline cartilage model. Steps include cartilage model formation, bone collar development, cartilage calcification, primary ossification center formation, medullary cavity development, and replacement of cartilage with bone.

Module 6.4: Bone Growth in Length and Width

Bone growth occurs by two main processes: longitudinal (length) and appositional (width) growth.

Longitudinal Growth l

• Occurs at the epiphyseal plate (hyaline cartilage) through chondrocyte division.

• Epiphyseal plate has five zones: reserve cartilage, proliferation, hypertrophy and maturation, calcification, and ossification.

• Growth continues until the epiphyseal plate is replaced by bone (epiphyseal line), typically by ages 18–21.

Appositional Growth

• Osteoblasts beneath periosteum lay down new bone, increasing diameter.

• New circumferential lamellae are formed; older lamellae may be remodeled into osteons.

• Width growth can continue after length growth ceases.

Hormonal Regulation of Bone Growth

• Growth Hormone: Stimulates protein synthesis and cell division in bone.

• Testosterone: Increases appositional growth, accelerates closure of epiphyseal plate.

• Estrogen: Increases longitudinal growth, but also accelerates epiphyseal plate closure.

Module 6.5: Bone Remodeling and Repair

Bone remodeling is a continuous process involving bone deposition by osteoblasts and bone resorption by osteoclasts. It maintains calcium homeostasis, replaces old bone, and adapts to stress.

Bone Remodeling

• Deposition: Osteoblasts build new bone matrix.

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

• Remodeling is influenced by mechanical stress, hormones (testosterone, estrogen), age, dietary calcium and vitamins (A, C, D), and protein intake.

Calcium Ion Homeostasis

• Regulated by a negative feedback loop involving parathyroid hormone (PTH) and calcitonin.

• Low Blood Calcium: Detected by parathyroid cells, which release PTH. PTH increases bone resorption, releasing calcium into blood.

• High Blood Calcium: Calcitonin from the thyroid gland promotes bone deposition, lowering blood calcium.

• Vitamin D enhances calcium absorption from the gut.

Bone Repair

• Fractures: Simple (closed) fractures do not break the skin; compound (open) fractures do.

• Healing Process:

  1. Hematoma formation (blood clot at fracture site).

  2. Soft callus formation (fibroblasts and chondroblasts form fibrocartilaginous tissue).

  3. Bone callus formation (osteoblasts build new bone).

  4. Remodeling (primary bone replaced by secondary bone).

Table: Classification of Bone Fractures

Key Equations

• Hydroxyapatite Formula:

Additional info: Bone health is influenced by lifestyle factors such as physical activity, nutrition, and hormonal balance. Disorders of bone remodeling include osteoporosis (excessive bone resorption) and osteopetrosis (excessive bone deposition).