Chapter 5 - Cartilage and Bone
Cartilage and bone are specialized connective tissues that provide support to other tissues and organs. Cartilage occurs where flexibility is required, while bone resists deformation.
CARTILAGE
Cartilage is composed of cells, fibers, and a highly hydrated ground substance. The high content of water provides resistance to compression, while the fibers provide tensile strength and resilience.
Three types of cartilage are recognized based on differences in fiber composition:
- Hyaline cartilage - type II collagen
- Elastic cartilage - elastic fibers and type II collagen
- Fibrocartilage - type I and II collagens
Cartilage is avascular, and its cells rely on diffusion for nutrients.
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 provides support and protection for the organs of the body. Bone is hard and rigid because of mineralization of the extracellular matrix. Bone also serves as a reservoir for calcium.
Bone has a rich vascular supply (unlike cartilage).
Bone tissue is classified morphologically into two types:
- Spongy bone (also known as cancellous or trabecular bone)
- Compact bone (also known as cortical bone)
Spongy Bone
Most bones are composed of both compact and spongy bone.
Compact Bone
Compact bone forms a dense layer on the outside of bones. It is composed of cylindrical units, known as osteon (Haversian systems), that are usually aligned with the long axis of the bone. An osteon is composed of concentric rings of bone (lamellae) surrounding a central channel.
Osteocytes maintain the mineral content of bone, detect mechanical stress and signal bone formation or resorption as needed.
Bone Development
Osteogenesis (or ossification) is the process of bone formation.
- Intramembranous Ossification - bone tissue forms directly from mesenchymal stem cells without a prior cartilage stage. This occurs primarily in the flat bones of the skull, the mandible, and the clavicles.
- Endochondral Ossification - bone tissue forms by replacing cartilage. The cartilage serves a template to be completely replaced by new bone.
The epiphyseal plate is a thin layer of hyaline cartilage found in the epiphysis, of a growing long bone.
