Bones and Skeletal Tissues: Structure, Function, Growth, and Disorders

Bones and Cartilages of the Human Skeleton

Overview of Skeletal Components

The human skeleton is composed of bones and cartilages, which provide structural support, protection, and facilitate movement. Cartilage is found in various locations, including articular surfaces, intervertebral discs, and respiratory structures.

• Axial skeleton: Includes the skull, vertebral column, and rib cage.

• Appendicular skeleton: Comprises the limbs and girdles attaching them to the axial skeleton.

• Types of cartilage: Hyaline, elastic, and fibrocartilage, each with distinct properties and functions.

Growth of Cartilage

Mechanisms of Cartilage Growth

Cartilage grows through two primary mechanisms, both essential during embryogenesis and throughout life.

• Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix on the external surface of existing cartilage.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

• Calcification: Occurs during normal bone growth and aging, but calcified cartilage is not equivalent to bone.

Functions of Bones

Major Roles of Bone Tissue

Bones serve several critical functions in the body, contributing to overall health and homeostasis.

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

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

• Anchorage: Acts as levers for muscle action.

• Mineral storage: Reservoir for calcium and phosphorus.

• 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 and metabolism.

Classification of Bones

Bone Groups and Shapes

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

• Axial skeleton: Long axis of the body (skull, vertebral column, rib cage).

• Appendicular skeleton: Limbs and girdles.

• Shape classification: Long, short, flat, and irregular bones.

Gross Anatomy of Bone

Compact and Spongy Bone

Bones are composed of two types of tissue: compact bone and spongy bone. Compact bone forms the dense outer layer, while spongy bone consists of a network of trabeculae.

• Compact bone: Dense, smooth, and solid; also called cortical bone.

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

Structure of Short, Irregular, and Flat Bones

These bones consist of thin plates of spongy bone covered by compact bone, with periosteum and endosteum membranes.

• Bone marrow: Scattered throughout spongy bone; no defined cavity.

• Hyaline cartilage: Covers areas involved in movable joints.

Structure of a Long Bone

Long bones have a shaft (diaphysis), ends (epiphyses), and membranes. The diaphysis surrounds the medullary cavity, which contains marrow.

• Diaphysis: Tubular shaft forming the long axis.

• Epiphyses: Ends of the bone, with compact bone externally and spongy bone internally.

• Metaphysis: Region between diaphysis and epiphysis; includes the epiphyseal plate/line.

Membranes in Bones

Bones are covered by two main membranes: periosteum and endosteum.

• Periosteum: Double-layered membrane covering external surfaces (except joints); contains nerve fibers, blood vessels, and osteogenic cells.

• Endosteum: Delicate membrane covering internal surfaces, including trabeculae and canals.

Hematopoietic Tissue in Bones

Red marrow is found in trabecular cavities and medullary cavities, with distribution changing from infancy to adulthood.

• Newborns: Red marrow in all spongy bone and medullary cavities.

• Adults: Red marrow mainly in flat bones and some irregular bones.

• Yellow marrow: Can convert to red marrow if needed (e.g., anemia).

Microscopic Anatomy of Bone

Bone Cells

Bone tissue contains five major cell types, each with specialized functions.

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete osteoid (collagen and proteins).

• Osteocytes: Mature bone cells in lacunae; maintain matrix and sense mechanical stress.

• Bone-lining cells: Maintain bone matrix; called periosteal or endosteal cells depending on location.

• Osteoclasts: Multinucleate cells responsible for bone resorption.

Compact Bone Structure

Compact bone is organized into osteons (Haversian systems), which are cylindrical structures running parallel to the bone's axis.

• Osteon: Structural unit; consists of concentric lamellae.

• Lamellae: Rings of bone matrix with collagen fibers oriented in different directions for strength.

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

• Perforating (Volkmann's) canals: Connect periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae, allowing communication and nutrient/waste exchange.

Spongy Bone Structure

Spongy bone is organized along lines of stress, with trabeculae providing strength and flexibility.

• Trabeculae: Irregularly arranged lamellae and osteocytes interconnected by canaliculi.

• Capillaries: Supply nutrients via endosteum.

Chemical Composition of Bone

Organic and Inorganic Components

Bones are composed of both organic and inorganic materials, each contributing to their properties.

• Organic: Cells (osteogenic, osteoblasts, osteocytes, bone-lining, osteoclasts) and osteoid (collagen, ground substance).

• Inorganic: Hydroxyapatites (calcium phosphate crystals), responsible for hardness and compression resistance.

Postnatal Bone Growth

Growth in Length and Width

Long bones grow in length via interstitial growth at the epiphyseal plate and in width via appositional growth.

• Epiphyseal plate: Maintains thickness by balancing cartilage growth and bone replacement.

• Appositional growth: Osteoblasts add bone matrix externally; osteoclasts remove bone internally.

Hormonal Regulation of Bone Growth

Bone growth is regulated by several hormones, ensuring proper development and proportions.

• Growth hormone: Stimulates epiphyseal plate activity.

• Thyroid hormone: Modulates growth hormone effects.

• Sex hormones: Induce growth spurts and epiphyseal plate closure.

Bone Remodeling

Bone Deposit and Resorption

Bone remodeling is a continuous process involving the coordinated actions of osteoblasts and osteoclasts.

• Bone deposit: Osteoblasts lay down new matrix; calcification occurs at the osteoid seam.

• Bone resorption: Osteoclasts break down matrix, releasing calcium into the blood.

Control of Remodeling

Remodeling is regulated by hormonal and mechanical factors.

• Hormonal control: Parathyroid hormone (PTH) increases bone resorption when blood calcium is low; calcitonin has minor effects.

• Mechanical stress: Bones adapt to stresses by remodeling; electrical signals and fluid flow changes stimulate this process.

Bone Repair

Fracture Classification and Healing

Fractures are classified by position, completeness, and skin penetration. Bone repair occurs in four stages:

1. Hematoma formation: Blood vessels rupture, forming a hematoma.

2. Fibrocartilaginous callus formation: Fibroblasts and chondrocytes reconstruct bone.

3. Bony callus formation: New trabeculae form, converting callus to spongy bone.

4. Bone remodeling: Excess material is removed, and compact bone is restored.

Bone Disorders

Common Skeletal Diseases

Imbalances in bone deposit and resorption lead to various disorders.

• Osteomalacia: Poor mineralization; soft, weak bones due to inadequate calcium deposition.

• Rickets: Osteomalacia in children; results in bone deformities, often due to vitamin D deficiency.

• Osteoporosis: Bone resorption exceeds deposit; bone mass declines, increasing fracture risk.

Disorder	Cause	Effect
Osteomalacia	Insufficient calcium/vitamin D	Soft, weak bones
Rickets	Vitamin D deficiency in children	Bowed legs, bone deformities
Osteoporosis	Excess bone resorption	Decreased bone mass, fractures

Additional info: Bone remodeling and repair are essential for maintaining skeletal integrity throughout life. Hormonal and mechanical factors ensure bones adapt to physiological demands and recover from injury.