Ossification is defined as the making of bone. In the adult mammal, two types of bone are distinguished, by reason of two different modes of ossification. Cartilage bones are formed by replacement of cartilage; membrane bones are developed directly from the embryonic connective tissue. In the first category we have the bones of the limbs, vertebral column and girdles (except the clavicle); in the second category are most of the bones of the skull, and the clavicles. The two types are formed respectively by endochondral and intra-membranous ossification.
Before the process begins, we have a cartilage surrounded by a perichondrium. A primary ossification centre appears in the middle of the shaft. The chondroblasts are arranged in rows; they become hypertrophied and lay calcium salt In the matrix. The process spreads from the centre outwards. At this stage we have calcified cartilage. It must be noted that the perichodrium is now called the periosteum.
Another region of ossification arises in the shaft, just beneath the periosteum. Cells called osteoblasts, derived from fibroblasts of the periosteum, proceed to lay down fibres upon the surface of the cartilage. This region also becomes calcified and will gradually form compact bone.
Now cells called osteoclasts eat their way through the sub-periosteal bone into the cartilage. They are followed by vascular tissue. There are formed a series of mainly longitudinal spaces filled with connective tissue, blood-vessels and osteoblast. On the wall of the space, osteoblasts deposit bone. Some of the osteoblasts become enveloped by the bone salts and so are trapped in lacunae, maintaining connexion with other osteoblasts by fine protoplasmic fibrils in bone canaliculi.
From the primary ossification centre, the process proceeds longitudinally towards both ends in this sequence: arrangement in rows; hypertrophy of cartilage cells; deposition of calcium salts forming calcified cartilage; erosion by osteoclast; deposition of calcium salt forming calcified cartilage; erosion by osteoclast; deposition of bone by osteoblast. Thus, the bone in the shaft is deposited in the form of hollow bars called trabeculae; these give it its spongy nature. In long bones the trabeculae are eroded to form the marrow cavity. This spongy bone is surrounded by the compact sub-periosteal bone. Later, osteoclasts erode the Haversian canals; they are followed by the vascular and connective tissue.
This process of ossification has so far affected only the shaft; the two ends of the structure are still cartilaginous. Later, ossification centres appears in each epiphysis and eventually epiphseal bone is formed. It is separated from the bone of the diaphysis by the epiphyseal cartilages. When growth is completed, the epiphyseal cartilage are ossified and the bone will grow no longer.
Growth in length of bones is due to the epiphyseal ossification centre; growth in girth is due to sub-periosteal ossification. The final shape is modeled in conformity with strains imposed by muscle pull. In every case where alteration in shape is effected, the sequence of processes is the same. The portion concerned is eroded by osteoclasts, then bone is deposited by osteoblasts. In cases of fracture, the repair is due mainly to the activity of the perioteum. Fibroblasts multiply and become osteoblasts. They lay down bony trabeculae and form a swollen callus. Later, this is gradually eroded and replaced, until finally little trace of it remains. A phosphatase enzyme working at an optimum pH of 9 is necessary for the formation of bone. The enzyme catalyses the liberation of phosphoric acid from phosphoric eaters.
The dermal embryonic connective tissue in the region of bone formation becomes very vascular. Fine bundles of fibres are laid down in the matrix. Calcium salts are next deposited around the fibre bundles and thus the ossification centre is established. Osteoblasts now form bony trabeculae after erosion of the calcified matrix by osteoclast. The periosteum becomes differentiated as a tough sheet of white fibrous connective tissue.
The skull grows by deposition of new bone under the periosteum. Simultaneously the inner region of bone abutting on the dura mater is absorbed by osteoclasts. Hence the structure grows in area without much increase in weight.