Immunohistochemical studies of blood monocytes infiltrating into the neonatal rat brain.

Brains of normal rats ranging in age from newborn to adult were observed with immunofluorescence technique using anti- granulomonocyte antiserum. For the first 10 days after birth, many cells with positive fluorescence were found in the white matter, the subependyma , the extra-parenchymal spaces, and the leptomeninx , but very few in the gray matter. They were mononuclear, rich in cytoplasm, and globular or irregular in shape. After about day 10 p.n., the positive cells decreased in number and became slender. However, there was no change in the distribution pattern. After about 3 weeks of age, no positive cells were detected in the brain parenchyma, except for very rare necrobiotic ones. It was suggested that blood monocytes infiltrate into the brain parenchyma of normal neonatal rat, but only for a while in the limited areas (white matter and subependyma ). They have the morphology and distribution of the "ameboid microglia" of neonatal brain. These monocytes disappear from the brain finally by the end of month 1 p.n.