In this PS you can see a fairly well developed MARROW CAVITY. Observe the numerous basophilic bone marrow cells. Remember that the cells are supported by a delicate, fishnet, reticular tissue. Endothelial lined sinusoidal spaces are present. A large megakaryocyte is present in the marrow.
Boney spicules are still present but these will subsequently be eroded away by osteoclastic activity. Can you see an osteoclast? Note that there is still calcified cartilage in the boney spicule and that there is quite a bit of bone matrix around the calcified cartilage. Within the acidophilic bone matrix you can barely see at this magnification, cells (osteocytes) which are completely surrounded by matrix.
IMPORTANT: OSTEOCYTES ARE FORMED FROM OSTEOBLASTS. Very simply, osteoblasts form on the surface of developing bone and literally bury themselves with their own secretory product. When completely surrounded by matrix, the osteoblast is termed an osteocyte. Osteoblasts continue to divide at the surface and add new layers of bone. This is APPOSITIONAL GROWTH. Osteocytes do not divide; therefore, interstitial growth is not possible in bone as it is in cartilage.
You should now have identified in your slides the three cells associated with bone and bone development, the OSTEOBLAST, the OSTEOCYTE, and the OSTEOCLAST. Remember the osteoblast and osteocyte develop from a common osteogenic cell. The osteoclast probably arises from the mononuclear phagocyte precursor (monocyte origin).
