Amoeboid Microglial Cells in the Corpus Callosum of Neonatal Rats

Abstract

The corpus callosum of neonatal rats (5-day-old) has been investigated by electron microscopy.Free subependymal cells (spongioblasts), astrocytes, microglia and oligodendrocytes are identified with their characteristic features. In addition to these, a fifth cell type is identified in the maturing corpus callosum and is referred to here as the amoeboid microglial cell. The amoeboid microglial cell is characterized by a vacuolated cytoplasm loaded with numerous cytoplasmic granules. Two types of amoeboid microglial cells could be differentiated according to their cytoplasmic granularities: (a) cells with electron dense granules and (b) cells with electron lucent granules. Mitotic figures may occur in these cells. The morphological evidence shows that they are capable of phagocytosis and ingesting degenerating cells or debris.The result of cell enumeration with 0.5μ semithin araldite sections stained with methylene blue shows that the amoeboid microglial cells constitute approximately 6% of the total glial population in the corpus callosum of 5-day-old rats. The percentage declines markedly with age and by the 15th postnatal day, all the amoeboid microglial cells have disappeared. Conversely, microglia which are absent in 5-day-old rats, make up approximately 6% of the glial population of 15-day-old rats suggesting that they may have been derived from the amoeboid microglial cells. Amoeboid microglial cells are absent in the cortex, caudate nucleus, and spinal cord at the ages examined.Finally, the question of the mode of formation and fate of the amoeboid microglial cells is discussed.