Ossification is the process of turning cartilage into bone. Bone may be synthesized by intramembranous ossification, endochronal ossification, or a combination of both.
In intramembranous ossification, embryo cells are transformed into bone. Specifically, mesenchyme cells, which are found in the mesoderm of the embryo, develop into connective tissue like bone and blood. The bones of the skull are developed through intramembranous ossification.
Endochronal ossification is the gradual replacement of cartilage by bone during growth. Much of the skeleton is formed through this process. Actively dividing bone-generating cells called osteoblasts emerge in areas of cartilage. The osteoblasts turn into osteocytes (mature bone cells) and become part of the hardened bone matrix.
Again, most bones arise from a combination of intramembranous and endochondral ossification. In this process, mesenchyme cells develop into chondroblasts and multiply by cell division. Next, the chondroblasts grow larger and generate a matrix which grows harder due to presence of inorganic minerals. After this, chambers are formed in the matrix and osteoblasts enter these chambers. Osteoblasts then secrete minerals to form the bone matrix.