The process of bone formation by osteoblasts is called Ossification. Ossification begins approximately six weeks after fertilization in an embryo. Before this time, the embryonic skeleton is completely made of fibrous membranes and hyaline cartilage. This growth of bones from fibrous membranes is called intramembranous ossification. The growth of bones from hyaline cartilage is called endochondral ossification. Bone growth lasts until the age of 25.
Intramembranous Ossification
The process of bone development from fibrous membranes is known as Intramembranous Ossification. This process involves the formation of the flat bones of the skull, mandible, and the clavicles. They then differentiate into osteoblasts at the ossification center. Osteoblasts secrete extracellular matrix and the calcium they deposit, hardens the matrix. The non-mineralized portion of the bone continues to form around blood vessels and thus forms a spongy bone. The spongy bone is then transformed into a thin layer of compact bone.
Endochondral Ossification
The process of bone development from hyaline cartilage is known as Endochondral Ossification. Except for the flat bones of the skull, mandible, and clavicles, all other bones of the body are formed through endochondral ossification. Chondrocytes form a template of the hyaline cartilage diaphysis in long bones. The matrix begins to calcify according to the response of complex developmental signals. Calcification prevents diffusion of nutrients into the matrix. And it results in chondrocytes dying and the opening up of cavities in the diaphysis cartilage.
Growth of Bone
Long bones continue to lengthen through the addition of bone tissue at the epiphyseal plate until the adolescent stage. They also continue to widen through appositional growth.
Lengthening of Long Bones
The cells, which are pushed from the epiphysis, mature and are destroyed by calcification. This process replaces cartilage with bone on the diaphyseal side of the plate and thus, results in the lengthening of the bones. Bones stop growing approximately at the age of 18 in females and at the age of 21 in males in a process called epiphyseal plate closure.
Thickening of Long Bones
The increase in the diameter of bones by the addition of bony tissue at the surface of bones is called appositional growth. Osteoblasts at the bone surface secrete bone matrix, and on the inner surface osteoclasts break down bone. The osteoblasts metamorphose into osteocytes. A balance between these two processes allows the bones to thicken without becoming hefty.