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 support in various parts of the skeleton.

• 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 childhood and adolescence.

• Costal Cartilage: Connects ribs to the sternum, allowing for expansion during breathing.

• Fibrocartilage: Found in intervertebral discs (between vertebrae), meniscus of the knee, and pubic symphysis (between pelvic bones). Provides strong support and withstands pressure.

Classes of Bone

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

• Compact Bone: Dense and forms the outer layer of bones. Provides strength where stresses come 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: Nearly 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). Protect nervous tissue and provide multiple anchor points for muscles.

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

Long Bone Structure

Long bones have a unique structure that supports their function in movement and weight-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 diaphysis and epiphysis, contains the epiphyseal plate in growing bones.

• Articular Cartilage: Hyaline cartilage covering joint surfaces.

• 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 that break down bone matrix (osteolysis).

• Extracellular Matrix:

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

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

Compact Bone Structure

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

• Osteon: Cylindrical structure containing concentric lamellae around a central (Haversian) canal.

• Lamellae: Layers of bone matrix.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

• Perforating (Volkmann's) Canals: Connect adjacent osteons and 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.

• Spaces: Filled with red bone marrow, site of hematopoiesis.

Periosteum and Endosteum

These connective tissue membranes cover and line bone surfaces.

• Periosteum: Covers outer bone surface; consists of a fibrous layer (dense irregular CT) 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 osteoclasts.

Bone Marrow

Bone marrow is a soft tissue found within bone cavities, essential for blood cell production and fat storage.

• Red Bone Marrow: Hematopoietic tissue, produces blood cells. Found in flat bones and ends of long bones.

• Yellow Bone Marrow: Primarily adipose tissue, stores triglycerides. Can convert to red marrow if needed.

Bone Growth

Bones grow in length and diameter through distinct processes during development.

• Interstitial Growth: Occurs at the epiphyseal plate, increases bone length via chondrocyte division and matrix production.

• Appositional Growth: Addition of new bone tissue to the surface, increasing 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, calcium reabsorption in kidneys, and 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 distal tibia and fibula.

Fracture Repair Process

Bone healing involves several stages:

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

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

3. Bony Callus Formation: Spongy bone replaces 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 (in adults); in children, called rickets.

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

Table: Comparison of Bone Types

Key Equations

• Hydroxyapatite Formation:

• Calcium Homeostasis:

Additional info: Some details were inferred and expanded for completeness, such as the full process of fracture repair and the hormonal regulation of calcium homeostasis.