Degeneration of osteoblasts involved in intramembranous ossification of fetal rat calvaria.

Ossification of calvariae from day-21 rat fetuses was reinvestigated by electron microscopy using different fixation techniques (glutaraldehyde/OsO4, tannic acid, ruthenium red, K-pyroantimonate). An osteoid layer with scattered mineral deposits was found at the mineralization front. Directly beyond this layer, a sheet of one to two layers of necrotic and degenerating osteoblasts was present. Above this sheet, normal and healthy cells were seen, formed by six to eight layers of flattened cells, embedded in a collagenous matrix. The osteoblasts on the less mineralizing opposite side of the calcified calvariae and the osteocytes embedded in the calcified calvariae appeared healthy. Closer inspection of the necrotic zone revealed apatite crystals in vesicles which most probably originated from mitochondria of the degenerated cells. Large K-pyroantimonate deposits were found throughout the osteoid and the necrotic zone, whereas only small granules were scattered in the cytoplasm and at the plasma membrane of the healthy cells directly adjacent to the necrotic zone. A concept of intramembranous mineralization is outlined, according to which osteoblasts store enormous amounts of calcium, which are liberated by physiological cell death in the vicinity of the mineralizing front.