Ossification of Bones

Ossification is the process by which bone is formed. The formation of bone is in fact conversion of other types of connective tissues into bone. Based on the type of tissue converted into bone, the process of ossification is of three types. These types with appropriate detail are explained in the lines below.

Intra-membranous ossification

This type of ossification is also known as mesenchymal ossification. In this type the bone is ossified from mesenchymal condensations. The bones formed by this process of ossification are known as membranous bones or dermal bones.

Intra-cartilaginous ossification

In this type of ossification, the mesenchyme has been converted to cartilaginous models and the process of ossification starts in these cartilaginous models. The process of conversion of mesenchymal condensations in cartilage is known as chondrification and this process takes place during the second month of intrauterine life. This indicates that the bones which start ossifying before second month are membrane bones and the bones which start ossifying after 2nd month of intrauterine life are cartilaginous bones.
There is another type of ossification in which a bone partly ossifies from membrane and partly from cartilage. These bones are known as membro-cartilaginous bones.

Process of ossification

With the explanation of different types of ossification in bones it is also important to explain what ossification is. Ossification is the process by which bone is formed. It is started at certain sites known as centers of ossification each of which is a point where lying down of lamellae (bone formation) is started by the activity of osteoblasts. Osteoblasts are bone forming cells and secrete collagen and other substances that form the ground substance of bone. The centers of ossification may be primary or secondary. The primary centers of ossification appear before birth and are the first to start the process of ossification. The secondary centers of ossification mostly appear after birth but there are few exceptions to this that is some secondary centers do appear before birth. The secondary centers are sites where process of ossification starts after it has started in primary centers.3