As we get older, tremendous increase in size of the bones is an important process in development. Bone growth usually continues until age 25. However, do you ever wonder how our body makes the bones or transforms cartilage into bones? This process of replacing tissues with bones is known as ossification. Ossification is further split into two categories: endochondral ossification where the cartilage is replaced by bone and intramembranous ossification where bone forms from mesenchyme (loose connective tissue derived from the germ layers from an embryo) or fibrous connective tissue.
Endochondral Ossification: Most bones begin as hyaline cartilage and are converted to bones through endochondral ossification.
- Chondrocytes, cartilage cells, in the center of bone begin to enlarge, causing the cartilage itself to increase in size. As this occurs, the matrix begins to calcify thwarting diffusion of nutrients through the calcified cartilage. As a result, the chondrocytes die and disintegrate due to the lack of nourishment.
- Soon blood vessels grow and spread around the edges of the cartilage, also known as perichondrium. This results in the cells within perichondrium to differentiate into osteoblasts which work to create a thin layer of bone around the shaft. Osteoblasts are responsible for manufacturing and releasing the organic and protein components of bone matrix. The perichondrium then turns into periosteum since it has transformed from cartilage to bone. Periosteum is a fibrous outer layer and a cellular inner layer which wraps around the bone.
- The increase in activity in the periosteum causes blood supply to increase. Soon the capillaries and fibroblasts invade the center of the cartilage causing the calcified matrix (in the center) to break down. The fibroblasts also specialize to form osteoblasts which turn the matrix into spongy bone. The center where all this is taking place is known as the primary ossification center which soon spreads outward.
- As the bone continues to grown, marrow cavity is formed through the process of remodeling. The osteoclasts secrete acids and proteolytic enzymes to dissolve the matrix, resulting in the formation of marrow cavity. The bone grows both in length and in width.
- Soon secondary ossification center forms as the ends of the bones which become calcified. The capillaries and osteoblasts migrate into the center of the epiphyses, ends of the bones. The same process that occurred at the primary ossification center occurs in the epiphyses.
- The epiphysis is occupied with spongy bone; however, some of the original cartilage remains exposed to the joint cavity called articular cartilage. The articular cartilage thwarts damage to bones when they are in contact. The epiphysis is separated from the diaphysis (shaft of the bone) by epiphyseal cartilage or epiphyseal plate. The epiphyseal plate is usually located at the metaphysis (where epiphysis and diaphysis come together).
Intramembranous Ossification: This process occurs when osteoblasts differentiate within mesenchymal or fibrous connective tissue to form bones.
- Mesenchymal cells start the process of ossification as they gather together and produce organic components of matrix, resulting in mineralization of the osteoid (tissue which turns into bone). These mesenchymal cells then differentiate to form osteoblasts while calcification takes place. Calcification is the process of depositing calcium or calcium salts. The bone then grows out from the ossification center in spicules which are small struts. Some of the osteoblasts become trapped in the process of ossification, forming osteocytes.
- In order for the bones to continue growing and for the cells to stay alive, they require oxygen and nutrients. As a result, the fusing of spicules traps some of the blood vessels within the bone.
- At first, the bone is composed of only spongy bone. However, through remodeling, compact bone and marrow cavity could be formed.