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 is composed of cells, fibers, and a highly-hydrated ground substance. The fibers provide tensile strength, while proteoglycans in the ground substance make cartilage resilient by trapping water.
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. Because of this, damaged cartilage heals poorly after injury.
Hyaline cartilage contains type II collagen fibers and a highly-hydrated ground substance. It is the most common cartilage and is found on articular surfaces of bone, walls of the respiratory system (trachea and bronchi), and epiphyseal plates.
Elastic cartilage is similar to hyaline cartilage but also contains elastic fibers. It occurs where flexibility is required, such as the epiglottis, external ear, and auditory tubes.
Fibrocartilage contains a mixture of hyaline cartilage and dense regular connective tissue. It combines the tensile strength of collagen fibers with the resistance to compression of cartilage. It is found where tendons attach to bones, menisci and intervertebral discs.
Chondrogenesis is the process by which cartilage is formed from condensed mesenchyme, differentiation into chondroblasts, and deposition of the extracellular matrix.
Bone provides support and protection for the organs of the body. It 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
Most bones are composed of both compact and spongy bone.
Spongy bone forms a network of anastomosing trabecula (spicules) that form interconnecting spaces containing bone marrow.
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.
Osteogenesis is a complex process consisting of cell migration, differentiation, extracellular deposition, and mineralization.
Two processes result in bone formation:
- Intramembranous ossification - direct formation of bone
- Endochondral ossification - involves cartilage as a precursor
Intramembranous ossification begins during fetal development with differentiation of mesenchymal cells into osteoblasts, secretion of osteoid, which undergoes calcification to produce bone. This process forms most flat bones of the skull, mandible, and clavicles.
Endochondral ossification begins with the formation of a hyaline cartilage model, which is later replaced by bone. It occurs during the development of most bones and at epiphyseal plates that enable long bones to grow in length.
Tooth development (odontogenesis) is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth.