The process of ossification allows bones to form while a fetus is still in the womb. The process converts various types of connective tissue into bone. The two main processes of ossification are intra-membranous and intra-cartilaginous, depending on the area of the body in which the cartilage is located.
Intra-membranous ossification is also known as mesenchymal ossification because in this type of ossification, bone is ossified from mesenchymal tissues. Bones formed from intra-membranous ossification include membranous bones or dermal bones.
The other type of ossification is intra-cartilaginous ossification. During this process, the mesenchyme has already been converted into cartilaginous structures. The process of ossification begins with the cartilaginous structures. These structures undergo a process known as chondrification, in which the cartilaginous structures begin to grow harder and convert into bones. Intra-cartilaginous ossification occurs during the second month of fetal life and continues throughout the pregnancy.
Both of these types of ossification form bones through cells known as osteoblasts. These are bone-forming cells that secrete collagen and other compounds to form the ground substance of the bone. These cells are found at sites in the body known as centers of ossification. Primary centers of ossification form during fetal life, while secondary centers of ossification appear after birth to reform bones during childhood and adulthood.