VAN 211: Veterinary Histology and Embryology (2+2)

INTRAMEMBRANOUS OSSIFICATION

• In an area where bone is going to develop, the mesenchymal cells are connected with one another by their processes and delicate bundles of white fibres run in all directions between them. In between the cells and bundles is a semifluid intercellular substance
• At one or more central points, ossification begins by appearance of osteoblasts, which promptly deposit bone matrix in the form, of acidophil bars or spicules, which unite a meshwork of trabeculae. As the matrix is laid down some cells remain imprisoned in the meshes as osteoblasts or bone producing cells, which takes up calcium, salts from the blood and deposit the same in the network of acidophil bars, thus converting them into plates of bone. Thus Ossification starts from the centre and proceeds towards the periphery.
• As this proceeds, the surrounding mesenchyme furnishes more and more of membrane for ossification and this results in an increase in the size of bone. The osteoblasts at the periphery arrange themselves in a continuous layer and these also produce parallel plates of bone at the periphery. The mesenchyme in the region forms a fibrous membrane enclosing the bone the periosteum.
• Bone formed in this process has a spongy character and consists of irregular plates of bone, which branch and unite with one another and in the meshes is enclosed embryonic connective tissue rich in blood vessels forming the red marrow.