Skeletal System: Bone Tissue

Overview of the Skeletal System

The skeletal system provides the structural framework for the human body, protects vital organs, enables movement, and serves as a reservoir for minerals. It is composed of bones, cartilage, ligaments, and other connective tissues.

• Bones: Rigid organs that form the skeleton.

• Ligaments: Connect bone to bone, stabilizing joints.

• Cartilage: Flexible connective tissue found in joints, growth plates, and other locations.

Components of the Skeletal System

• Axial Skeleton: Consists of 80 bones including the skull, vertebrae, sternum, and ribs. Provides central support and protection for the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Composed of 126 bones including the pectoral girdle, pelvic girdle, and limb bones. Facilitates movement and interaction with the environment.

Cartilage in the Skeletal System

Cartilage is a specialized connective tissue that provides flexibility and cushioning in the skeletal system.

• Articular Cartilage: Covers the ends of bones at joints, reducing friction and absorbing shock.

• Epiphyseal (Growth) Plates: Regions of cartilage that allow for longitudinal bone growth during development.

• Costal Cartilage: Connects ribs to the sternum, contributing to the elasticity of the thoracic cage.

• Fibrocartilage: Found in structures such as the meniscus (knee), intervertebral discs (between vertebrae), and pubic symphysis (between pelvic bones). Provides strength and support in areas subject to pressure and tension.

Classes of Bone

Bones are classified based on their density and location within the skeleton.

• Compact Bone: Dense and strong, forms the outer layer of bones and the bulk of the skeleton. Designed to withstand stresses from limited directions.

• Spongy (Cancellous) Bone: Less dense, found at the ends of long bones and inside flat bones. Contains trabeculae and is adapted to withstand stresses from multiple directions.

Bone Shapes

Bones are categorized by their shapes, which relate to their functions.

• Long Bones: Longer than they are wide (e.g., femur, humerus). Function in movement and support.

• Short Bones: Approximately equal in length and width (e.g., carpals, tarsals). Provide stability and support with limited movement.

• Flat Bones: Thin and broad (e.g., skull bones, sternum). Protect internal organs and provide surfaces for muscle attachment.

• Irregular Bones: Complex shapes (e.g., vertebrae, sphenoid, ethmoid). Specialized functions.

• Sesamoid Bones: Develop within tendons (e.g., patella). Protect tendons from stress and wear.

Long Bone Structure

Long bones have a distinct anatomy that supports their function in movement and load-bearing.

• Diaphysis: Shaft of the bone, contains the medullary cavity filled with bone marrow.

• Epiphysis: Expanded ends of the bone, mostly spongy bone, covered by articular cartilage.

• Metaphysis: Region between the diaphysis and epiphysis, contains the epiphyseal plate in growing bones.

• Articular Cartilage: Hyaline cartilage covering joint surfaces, reducing friction.

• Medullary Cavity: Central cavity within the diaphysis, contains yellow or red bone marrow.

Bone Histology

Bones are dynamic tissues composed of cells and an extracellular matrix.

• Bone Cells:

  • Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Synthesize and secrete osteoid (unmineralized bone matrix).

  • Osteocytes: Mature bone cells found in lacunae, maintain bone tissue.

  • Osteoclasts: Multinucleated cells responsible for bone resorption.

• Extracellular Matrix:

  • Osteoid: Organic component, mainly collagen fibers, provides tensile strength.

  • Inorganic Component: Hydroxyapatite (calcium phosphate crystals), provides compressive strength.

Formula for Hydroxyapatite:

Compact Bone Structure

Compact bone is organized into structural units called osteons (Haversian systems).

• Osteon: Cylindrical unit containing concentric lamellae of matrix around a central (Haversian) canal.

• Central Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes.

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

• Interstitial Lamellae: Fill spaces between osteons.

• Circumferential Lamellae: Encircle the inner and outer surfaces of bone.

• Perforating (Volkmann's) Canals: Connect central canals of adjacent osteons, contain blood vessels and nerves.

Spongy Bone Structure

Spongy bone consists of a network of trabeculae, providing structural support and housing bone marrow.

• Trabeculae: Lattice-like structures aligned along lines of stress.

• Lamellae: Layers of bone matrix within trabeculae.

• Bone Marrow: Fills spaces between trabeculae, site of hematopoiesis (red marrow) or fat storage (yellow marrow).

Periosteum and Endosteum

These connective tissue membranes cover and line bone surfaces, playing roles in growth, repair, and nutrition.

• Periosteum: Outer covering of bone, consists of a fibrous layer (dense irregular connective tissue) and an inner cellular layer (osteoprogenitor cells, osteoblasts, osteoclasts). Site for tendon and ligament attachment.

• Endosteum: Lines the medullary cavity, central canals, and covers trabeculae. Contains osteoprogenitor cells and osteoblasts.

Bone Growth

Bones grow in length and diameter through two main processes.

• Interstitial Growth: Occurs at the epiphyseal plate, increases bone length via division and enlargement of chondrocytes.

• Appositional Growth: Addition of new bone matrix to the surface, increases bone diameter.

Hormonal Regulation of Bone Growth

Growth hormone (hGH) from the pituitary gland stimulates bone growth by promoting mitosis in chondrocytes and osteoprogenitor cells.

• Excess hGH before epiphyseal plate closure: Gigantism (excessive height).

• Excess hGH after plate closure: Acromegaly (enlarged bones of face, hands, feet).

Bone Remodeling

Bone tissue is continuously renewed through remodeling, balancing deposition and resorption.

• Osteoblasts: Build new bone matrix.

• Osteoclasts: Break down old bone matrix.

• Functions: Repair fractures, replace old bone, adapt to new stresses, maintain calcium homeostasis.

Calcium Homeostasis

Blood calcium levels are tightly regulated by hormones to support muscle contraction, nerve function, and bone health.

• Parathyroid Hormone (PTH): Released when blood calcium is low; increases osteoclast activity, enhances calcium reabsorption in kidneys, and stimulates calcitriol synthesis.

• Calcitonin: Released when blood calcium is high; decreases osteoclast activity, increases calcium excretion, and reduces calcitriol synthesis.

Types of Bone Fractures

Fractures are classified based on their pattern and cause.

• Avulsion Fracture: Tendon or ligament pulls off a fragment of bone.

• Comminuted Fracture: Bone breaks into multiple fragments.

• Compression Fracture: Bone is crushed under weight.

• Spiral Fracture: Caused by twisting forces.

• Greenstick Fracture: Partial fracture, common in children.

• Pott Fracture: Fracture of the distal tibia and fibula.

Fracture Repair Process

Bone healing involves several stages:

1. Hematoma Formation: Blood clot forms at the fracture site, stopping bleeding.

2. Fibrocartilaginous Callus Formation: Collagen fibers and cartilage stabilize the fracture.

3. Bony Callus Formation: Spongy bone replaces the cartilage callus.

4. Bone Remodeling: Compact bone replaces spongy bone, restoring original structure.

Bone Abnormalities and Disorders

• Osteopenia: Reduced bone deposition, leading to weak bones; common in women due to decreased estrogen.

• Osteomalacia: Soft bones due to inadequate vitamin D; called rickets in children.

• Osteogenesis Imperfecta: Genetic disorder resulting in abnormal collagen, causing brittle bones.

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.