Distribution and fate of macrophagic ameboid cells in the rat brain

Following an intravenous injection of a fluorescent dye, primuline, into neonatal rats, the distribution of ameboid cells and their fate in the central nervous system were examined under incident fluorescence, light and electron microscopes. In the ameboid cells labeled with primuline, the ingested fluorogen spread throughout the cytoplasm, in which it appeared as a whitish yellow fluorescence. Such labeled cells displayed increased acid phosphatase (ACPase) activity. Combining primuline labeling with the ACPase reaction allowed for the identification of ameboid cells even after the transformation mentioned below. Ameboid cells were distributed throughout the white matter in neonatal rats, but were few in the gray matter. Particularly, they appeared as cell clusters in the corona radiata of the corpus callosum, the subependymal layer surrounding the ventricles, the fornical commissure, the internal and external capsules, the cerebellar peduncles and the medulla. Around 8 to 14 days after birth, the ameboid cells which had so far appeared spherical became gradually elongated with a few branched cytoplasmic processes, and scattered in the white matter. Thereafter, most primuline labeled cells transformed into cells with a dark nucleus and a thin cytoplasm including small phagosomes, i.e., into cells with features of microglia. However, a small number of ameboid cells underwent degeneration. The data indicate that the ameboid cells observed in the white matter during the postnatal period transform into microglia. Since other investigations suggest that the ameboid cells are derived from blood monocytes, microglia could initially come from monocytes.