Connective Tissue: Cartilage

Classes and General Features of Cartilage

Cartilage is a specialized connective tissue that provides support and flexibility to various parts of the body. It is classified into three main types based on fiber composition and function.

• Hyaline cartilage

• Elastic cartilage

• Fibrocartilage

• Chondroblasts are the actively dividing cells that synthesize the cartilage matrix.

• Chondrocytes are mature cartilage cells embedded in lacunae (small spaces) within the matrix.

• Cartilage is avascular (lacks blood vessels) and lacks nerve fibers.

• Nutrients are supplied by diffusion from capillaries in the perichondrium, a dense connective tissue layer surrounding cartilage.

• Cartilage has a weak capacity to regenerate and heals poorly if injured.

Types of Cartilage

Hyaline Cartilage

Hyaline cartilage is the most abundant type of cartilage in the body, providing firm support with some flexibility.

• Structure: Contains numerous fine collagen fibers; amorphous but firm matrix.

• Function: Supports and reinforces; serves as a resilient cushion; resists compressive stress.

• Locations: Forms most of the embryonic skeleton, covers the ends of long bones in joint cavities, forms costal cartilages of the ribs, cartilages of the nose, trachea, and larynx.

Elastic Cartilage

Elastic cartilage contains more elastic fibers than hyaline cartilage, providing strength and exceptional flexibility.

• Structure: Similar to hyaline cartilage but with a higher content of elastic fibers.

• Function: Maintains the shape of a structure while allowing great flexibility.

• Locations: External ear (pinna), epiglottis.

Fibrocartilage

Fibrocartilage is characterized by thick bundles of collagen fibers, making it highly compressible and able to withstand heavy pressure.

• Structure: Alternating rows of chondrocytes and thick collagen fibers.

• Function: Acts as a shock absorber; resists both compression and tension.

• Locations: Intervertebral discs, pubic symphysis, menisci of the knee.

Growth of Cartilage

Mechanisms of Cartilage Growth

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

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

Classification and Structure of Bones

Types of Bones by Shape

Bones are classified by their shape, which relates to their function and location in the skeleton.

• Long bones: Longer than they are wide; have a shaft (diaphysis) and two ends (epiphyses). Examples: humerus, femur, radius, ulna, tibia, fibula, metacarpals, metatarsals, phalanges.

• Short bones: Cube-like; found in the wrists (carpals), ankles (tarsals), and patella (sesamoid bones).

• Flat bones: Thin, flattened, and usually curved. Examples: sternum, ribs, scapulae, cranial bones.

• Irregular bones: Complicated shapes. Examples: vertebrae, hip bones, certain facial and cranial bones.

Types of Osseous Tissue

• Compact bone: Dense, smooth outer layer that provides strength.

• Spongy bone (cancellous bone): Internal network of trabeculae (thin beams) that provide structural support and reduce bone weight.

Structure of a Typical Long Bone

Major Regions and Membranes

• Diaphysis: Shaft of the bone; composed of compact bone surrounding a medullary cavity filled with yellow bone marrow (fat) in adults.

• Epiphyses: Bone ends; outer shell of compact bone with an inner layer of spongy bone; covered with articular cartilage to reduce friction at joints.

• Metaphyses: Narrow regions between diaphysis and epiphyses; contain the epiphyseal line (remnant of the growth plate).

• Periosteum: Double-layered connective tissue membrane covering the external surface of bone except at joint surfaces; contains osteogenic cells, nerves, and blood vessels.

• Endosteum: Delicate connective tissue membrane lining internal bone surfaces, including the medullary cavity and trabeculae of spongy bone.

Bone Cells and Bone Remodeling

Types of Bone Cells

• Osteoprogenitor (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; reside in lacunae and communicate via canaliculi.

• Osteoclasts: Bone-resorbing cells that break down bone matrix, releasing calcium into the blood.

Bone remodeling is a continuous process involving bone resorption by osteoclasts and bone deposition by osteoblasts, maintaining bone strength and mineral homeostasis.

Microscopic Anatomy of Bone

Compact Bone Structure

• Osteon (Haversian system): The structural unit of compact bone, consisting of concentric lamellae (layers) surrounding a central canal containing blood vessels and nerves.

• Lacunae: Small cavities containing osteocytes.

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

• Perforating (Volkmann's) canals: Canals perpendicular to the osteons, connecting blood vessels and nerves of the periosteum to those in the central canals.

Spongy Bone Structure

• Trabeculae: Thin beams of bone that form a lattice-like network, providing strength while reducing weight.

• Spaces between trabeculae are filled with red or yellow bone marrow.

Bone Development and Growth

Ossification (Osteogenesis)

Ossification is the process of bone formation, occurring in two main ways:

• Intramembranous ossification: Bone develops from a fibrous membrane; forms flat bones of the skull and clavicles.

• Endochondral ossification: Bone develops by replacing hyaline cartilage; forms most bones below the base of the skull.

Bone Growth in Length and Width

• Length: Long bones lengthen at the epiphyseal plates through endochondral ossification during childhood and adolescence.

• Width (appositional growth): Osteoblasts in the periosteum add new bone tissue to the outer surface, while osteoclasts in the endosteum resorb bone from the inner surface, maintaining proper bone shape and weight.

Bone Remodeling and Homeostasis

Bone Remodeling

• Bone is continuously remodeled in response to mechanical stress and hormonal signals.

• In healthy adults, the rate of bone resorption equals the rate of bone deposition, maintaining bone mass.

• Imbalances can lead to conditions such as osteoporosis (severe bone density loss).

Fractures and Bone Repair

Classification of Fractures

• Position of bone ends: Nondisplaced (ends retain normal position) vs. displaced (ends out of alignment).

• Completeness: Complete (broken all the way through) vs. incomplete.

• Skin penetration: Open (compound, skin penetrated) vs. closed (simple, skin not penetrated).

Bone Repair Process

1. Hematoma formation: Blood vessels break, forming a mass of clotted blood (hematoma) at the fracture site; inflammation occurs.

2. Fibrocartilaginous callus formation: New blood vessels grow; fibroblasts and chondroblasts produce collagen fibers and cartilage matrix, connecting bone ends.

3. Bony callus formation: Osteoblasts form new spongy bone, converting the callus into a hard (bony) callus.

4. Bone remodeling: Osteoclasts and osteoblasts remodel the bony callus, restoring the bone's original shape and structure.

Summary Table: Types of Cartilage

Key Equations

• Bone Remodeling Balance:

• Osteon Structure:

Clinical Application: Osteoporosis

Osteoporosis is a condition characterized by severe loss of bone density, increasing the risk of fractures. It is most common in postmenopausal women due to decreased estrogen levels, but can affect both sexes with age or inactivity.