Bones and Skeletal Tissue: Structure, Function, and Disorders

Bones and Skeletal Tissue

Overview of Skeletal Cartilages

The human skeleton initially consists of cartilage, which is gradually replaced by bone except in areas requiring flexibility. Cartilage is a resilient and flexible connective tissue found in various parts of the body.

• Hyaline cartilage: Provides support, flexibility, and resilience; most abundant; contains only collagen fibers. Found in articular (joints), costal (ribs), respiratory (larynx), and nasal (nose tip) regions.

• Elastic cartilage: Similar to hyaline but contains elastic fibers; found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers; has great tensile strength; found in menisci of knee and intervertebral discs.

Functions of Bones

Bones serve multiple essential functions in the human body, contributing to both structure and physiology.

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

• Protection: Shields the brain, spinal cord, and vital organs.

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities.

• Triglyceride storage: Fat stored in bone cavities serves as an energy source.

• Hormone production: Osteocalcin regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

The human skeleton contains 206 named bones, classified by location and shape.

• Axial skeleton: Long axis of the body; includes skull, vertebral column, and rib cage.

• Appendicular skeleton: Bones of limbs and girdles attaching limbs to axial skeleton.

Bones are also classified by shape:

• Long bones: Longer than wide; found in limbs.

• Short bones: Cube-shaped; found in wrist and ankle; sesamoid bones form within tendons (e.g., patella).

• Flat bones: Thin, flat, slightly curved; includes sternum, scapulae, ribs, and most skull bones.

• Irregular bones: Complicated shapes; includes vertebrae and hip bones.

Bone Structure

Gross Anatomy of Bone

Bones are organs composed of various tissues, including bone tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels. Bone structure is studied at three levels: gross, microscopic, and chemical.

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

• Spongy bone: Honeycomb structure of trabeculae; spaces filled with red or yellow bone marrow.

Structure of Long Bones

Long bones have a shaft (diaphysis), bone ends (epiphyses), and membranes.

• Diaphysis: Tubular shaft; compact bone surrounds medullary cavity filled with yellow marrow in adults.

• Epiphyses: Ends of long bones; compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of childhood epiphyseal plate where bone growth occurs.

Hematopoietic Tissue in Bones

Red marrow is found within trabecular cavities of spongy bone and diploë of flat bones. In newborns, all spongy bone and medullary cavities contain red marrow; in adults, red marrow is mainly in flat bone diploë and some irregular bones. Yellow marrow can convert to red if needed.

Microscopic Anatomy of Bone

Cells of Bone Tissue

Bone tissue contains five major cell types, each specialized for specific functions:

• Osteogenic cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells; secrete osteoid (unmineralized bone matrix).

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors.

• Bone-lining cells: Flat cells on bone surfaces; help maintain matrix.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Structure of Compact Bone

Compact bone, also called lamellar bone, consists of osteons (Haversian systems), canals, canaliculi, and lamellae.

• Osteon: Structural unit; elongated cylinder running parallel to bone axis; consists of concentric rings (lamellae) with collagen fibers in alternating directions for strength.

• Canals: Central (Haversian) canal contains blood vessels and nerves; perforating (Volkmann’s) canals connect periosteum, medullary cavity, and central canal.

• Lacunae and canaliculi: Lacunae house osteocytes; canaliculi connect lacunae, allowing communication and nutrient/waste exchange.

Structure of Spongy Bone

Spongy bone is organized along lines of stress, with trabeculae conferring strength. It lacks osteons but contains lamellae and osteocytes interconnected by canaliculi. Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

Organic components include cells (osteogenic, osteoblasts, osteocytes, bone-lining, osteoclasts) and osteoid (ground substance and collagen fibers). Osteoid provides tensile strength and flexibility.

Inorganic Components

Inorganic components are mainly hydroxyapatites (mineral salts), which make up 65% of bone mass. These calcium phosphate crystals provide hardness and resistance to compression.

• Bones are half as strong as steel in resisting compression and as strong as steel in resisting tension.

• Mineral composition allows bones to last long after death and reveal information about ancient populations.

Formation of the Bony Skeleton

Endochondral Ossification

Endochondral ossification is the process by which bone forms by replacing hyaline cartilage. Most bones of the skeleton are formed this way.

Intramembranous Ossification

Intramembranous ossification begins within fibrous connective tissue membranes formed by mesenchymal cells. It forms flat bones such as the frontal, parietal, occipital, temporal, and clavicle bones.

Bone Disorders

Osteomalacia and Rickets

Osteomalacia is characterized by poorly mineralized bones, resulting in soft, weak bones and pain upon bearing weight. Rickets is the childhood form, causing bowed legs and bone deformities due to vitamin D deficiency or insufficient dietary calcium.

Osteoporosis

Osteoporosis is a group of diseases where bone resorption exceeds deposition, leading to decreased bone mass. The spongy bone of the spine and neck of femur is most susceptible, with vertebral and hip fractures being common.

Paget’s Disease

Paget’s disease involves abnormal bone remodeling, resulting in bones that are structurally disorganized and weak.

Bone Disorder	Main Features	Causes
Osteomalacia	Soft, weak bones; pain	Vitamin D deficiency, low calcium
Rickets	Bowed legs, bone deformities	Vitamin D deficiency, low calcium
Osteoporosis	Decreased bone mass, fractures	Imbalance in bone resorption/deposit
Paget’s Disease	Disorganized, weak bone	Abnormal bone remodeling

Additional info: Bone health is influenced by nutrition, hormonal balance, and physical activity. Prevention and treatment of bone disorders often involve dietary supplementation, medication, and weight-bearing exercise.