Chapter 6: Bones and Skeletal Tissues

6.0 Overview of Bones

Bones are the organs of the skeletal system, providing support, protection, and movement for the body. They also serve as sites for blood cell production and mineral storage.

• Functions of Bones: Support, protection, movement, mineral storage (especially calcium and phosphate), and blood cell formation (hematopoiesis).

• Types of Bone Tissue: Compact bone (dense outer layer) and spongy bone (internal, honeycomb structure).

• Cartilage: Provides support and flexibility; types include hyaline, elastic, and fibrocartilage.

6.1 Skeletal Cartilages

Cartilage is a resilient, semi-rigid form of connective tissue found in areas such as joints, rib cage, ear, nose, bronchial tubes, and intervertebral discs. It is essential for bone growth and repair.

• Types of Cartilage:

  • Hyaline cartilage: Most abundant; provides support with flexibility and resilience. Found in articular surfaces, costal cartilages, respiratory tract, and nasal cartilage.

  • Elastic cartilage: Contains more elastic fibers; found in the ear and epiglottis.

  • Fibrocartilage: Highly compressible with great tensile strength; found in intervertebral discs and menisci of the knee.

• Growth of Cartilage: Appositional (growth from outside) and interstitial (growth from within).

6.2 Bone Classification and Functions

Bones are classified by their shape and function. They support the body, protect organs, anchor muscles, and store minerals.

• Bone Shapes:

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

  • Short bones: Cube-shaped (e.g., carpals, tarsals).

  • Flat bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones).

  • Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).

• Bone Functions: Support, protection, movement, mineral and growth factor storage, blood cell formation, triglyceride (fat) storage, and hormone production.

6.3 Bone Structure

The human skeleton consists of 206 bones, divided into axial and appendicular skeletons. Bones are complex organs containing bone tissue, cartilage, nerves, and blood vessels.

• Gross Anatomy:

  • Compact bone: Dense outer layer; smooth and solid.

  • Spongy bone: Internal honeycomb of trabeculae filled with marrow.

• Structure of Long Bones: Diaphysis (shaft), epiphyses (ends), membranes (periosteum and endosteum).

• Structure of Short, Irregular, and Flat Bones: Thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone inside; no shaft or epiphyses.

Summary: 206 bones in the human body are classified as axial or appendicular. There are four bone categories: long, short, flat, and irregular.

6.4 Microscopic Structure of Bone

Bone tissue is a dynamic tissue composed of several cell types and a matrix of collagen fibers and inorganic salts.

• Cells of Bone Tissue:

  • Osteogenic cells: Stem cells that differentiate into osteoblasts.

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

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

  • Osteoclasts: Large cells that resorb or break down bone matrix.

• Bone Matrix: Consists of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components.

• Microscopic Anatomy:

  • Osteon (Haversian system): Structural unit of compact bone.

  • Lamellae: Concentric rings of bone matrix.

  • Central (Haversian) canal: Contains blood vessels and nerves.

  • Canaliculi: Small channels connecting lacunae and osteocytes.

6.5 Bone Development (Ossification)

Ossification is the process of bone tissue formation. It occurs during embryonic development, growth, and bone remodeling.

• Types of Ossification:

  • Intramembranous ossification: Bone develops from a fibrous membrane (e.g., flat bones of the skull).

  • Endochondral ossification: Bone forms by replacing hyaline cartilage (e.g., long bones).

• Growth in Length: Occurs at the epiphyseal plate through proliferation, hypertrophy, calcification, and ossification of cartilage cells.

• Growth in Thickness: Appositional growth adds bone to the surface.

6.6 Bone Remodeling and Repair

Bone remodeling is a continuous process of bone resorption and deposition, essential for bone health and calcium homeostasis.

• Bone Resorption: Carried out by osteoclasts, which break down bone matrix and release minerals into the blood.

• Bone Deposition: Performed by osteoblasts, which build new bone matrix.

• Hormonal Regulation: Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts.

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

6.7 Factors Affecting Bone Health

Bone health depends on nutrition, hormones, physical activity, and age. Imbalances can lead to disorders such as osteoporosis.

• Calcium and Vitamin D: Essential for bone strength and density.

• Hormones: Growth hormone, thyroid hormone, sex hormones, and PTH regulate bone growth and remodeling.

• Physical Activity: Weight-bearing exercise stimulates bone formation.

6.8 Bone Disorders

Common bone disorders include osteoporosis, osteomalacia, and Paget's disease.

• Osteoporosis: Characterized by decreased bone mass and increased fracture risk; often due to aging, hormonal changes, or calcium/vitamin D deficiency.

• Osteomalacia: Softening of bones due to inadequate mineralization; often caused by vitamin D deficiency.

• Paget's Disease: Excessive and disorganized bone remodeling; bones become enlarged and deformed.

Summary: Osteoporosis and osteomalacia are both forms of bone weakness. Osteoporosis involves reduced bone density, while osteomalacia involves poor bone mineralization. Paget's disease is marked by abnormal bone remodeling.