Chapter 5 - Cartilage and Bone
Cartilage and bone are specialized connective tissues that serve structural and supportive functions. Each is uniquely adapted to distinct mechanical demands. Cartilage is found in locations requiring flexibility, resilience, and shock absorption, such as joints, the ear, and the respiratory tract. In contrast, bone provides rigid support and resists deformation, forming the structural framework of the skeleton that protects organs, anchors muscles, and bears the body's weight.
CARTILAGE
Cartilage consists of three primary components: chondrocytes , extracellular fibers (collagen and/or elastic fibers), and a hydrated ground substance rich in proteoglycans and glycosaminoglycans. Its high-water content—up to 80% by weight—gives exceptional resistance to compressive forces, acting as a cushion and shock absorber. Meanwhile, the fibrous component provides tensile strength and elasticity, enabling cartilage to withstand stretching and return to its original shape after deformation.
Three Types of Cartilage:
- Hyaline Cartilage : Primarily type II collagen fibers embedded in abundant ground substance
- Elastic cartilage : Contains elastic fibers in addition to type II collagen, providing greater flexibility
- Fibrocartilage : Contains dense bundles of type I collagen along with type II collagen, providing exceptional tensile strength
Cartilage is avascular and its cells rely on diffusion for nutrients. Because of this, damaged cartilage heals poorly after injury.
Hyaline Cartilage
Hyaline cartilage is composed of type II collagen fibers and ground substance. It is the most common cartilage and is associated with articular surfaces of bone, walls of the respiratory system (trachea and bronchi), and growth plates.
BONE
Bone serves three critical functions in the body: providing structural support, protecting vital organs, and acting as a calcium reservoir. The exceptional hardness and rigidity of bone result from mineralization of its extracellular matrix, where calcium phosphate crystals are deposited onto a collagen framework.
Morphologically, bone tissue exists in two distinct forms:
- Spongy bone (also called cancellous or trabecular bone): Porous, lattice-like structure found primarily at the ends of long bones and within vertebrae
- Compact bone (also called as cortical bone): dense outer shell of all bones and the shafts of long bones
Most bones in the human skeleton contain both compact and spongy bone tissue, strategically distributed to optimize strength while minimizing weight.
Unlike the avascular cartilage, bone maintains a rich vascular supply that supports its metabolic activity and remodeling capacity.
Spongy Bone
Spongy bone consists of a three-dimensional network of thin, interconnecting plates and rods called trabeculae. This porous structure is predominantly found in the interior of bones.
Compact Bone
Compact bone forms the dense, solid outer layer of all bones and comprises the entire shaft (diaphysis) of long bones. Its structure is highly organized into cylindrical units called osteons or Haversian systems , which align parallel to the long axis of the bone to resist bending and torsional forces. Each osteon consists of concentric rings of mineralized bone matrix called lamellae, arranged around a central Haversian canal that contains blood vessels and nerves.
Osteocytes are mature bone cells within the bone matrix housed in small spaced called lacunae between the lamellae. These cells extend slender processes through tiny channels ( canaliculi ) that connect adjacent lacunae, forming an extensive communication network throughout the bone.
Osteocytes detect mechanical stress and strain during physical activity, and they regulate bone homeostasis by signaling osteoblasts to form new bone or osteoclasts to resorb existing bone to maintain optimal bone strength and mineral balance.
BONE DEVELOPMENT
Osteogenesis is the process of bone formation and development. It involves cell migration, differentiation, extracellular matrix deposition, and mineralization.
Main Types:
- Intramembranous ossification - bone forms directly from mesenchymal tissue without a cartilage intermediate
- Endochondral ossification - bone develops by replacing a cartilage model
The epiphyseal plate , also known as the growth plate, is a specialized region of hyaline cartilage located between the epiphysis (end) and diaphysis (shaft) of growing long bones in children and adolescents. It is the primary site of longitudinal bone growth, where cartilage cells continuously proliferate and are replaced by bone tissue through endochondral ossification.
Once skeletal maturity is reached, the epiphyseal plate ossifies completely and is replaced by the epiphyseal line, marking the cessation of further bone lengthening.