The Location and Morphology of Astrocyte, Tancyte, and Radial Glial Cells in the Neonatal Rat Brain
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Studies have been done which examined the development and differentiation of both neuronal and glial (astrocyte) cells in only certain areas of the brain, including the cerebral ventricular zone of the fetal monkey (Levitt, Cooper, and Rakic, 1981), the superficial layers of early postnatal mouse neocortex (Konig & Schachner, 1981), and the rat embryo (Raju, Bignami, & Dahl, 1981). This study is the first in which the location, morphology, and degree of immunostaining of astrocyte, tanycyte, and radial glial cells were examined in the entire postnatal brain in 2, 5, and 10 day old rats through the development of glial fibrillary acidic (GFA) protein. GFA protein is a major component of filaments in fibrous astroglial cells of the CNS, and is also found in tanycyte and radial cells. Normal white rats at 2, 5, and 10 days of age were sacrificed and their brains were cut transversely with a vibrotome into 60 urn thick serial sections. The unlabeled antibody peroxidase-anti peroxidase (PAP) method was the immunocytochemical technique used to stain the GFAP-containing glial cells in the tissue. Generally, white matter fiber tracts, including the corpus callosum, optic tract, ventral hippocampal commissure, and anterior commissure at 10 days of age contained a large number of well differentiated astrocytes with many well branched processes emanating from their cell bodies. Further, these astrocyte cell bodies were present in rows which ran parallel to the axons of the tracts. At 2 and 5 days of age, though, white fiber tracts contained a smaller number of astrocytes. These astrocytes were truncate in shape with very few processes. Thus, a great deal of glial differentiation and maturation occurs between 5 and 10 days of age. Ependymal tanycytes were present in the dorso-medial corners of the lateral ventricles (at the level of the lateral septal nucleus) in 2, 5, and 10 day old rats. However, there were fewer and smaller tanycytes in the 2 day old animals in this area; in the older animals the tanycytes were thicker, longer, and were running in one direction toward the midline. Also, the processes of astrocytes located in the ventral portion of the corpus callosum came into contact with the ependymal tanycyte processes of the lateral ventricles. Thus, there may be an astrocyte-tanycyte interaction. Radial glia processes were observed in the area of the rat cortex above the corpus callosum at 2 and 5 days of age. At 10 days of age, though, no radial glia processes were seen in this area; instead, small but well branched astrocytes were present. Thus, it was thought that the radial glia cells transformed postnatally into astrocytes. During embryonal life, the radial glia may serve as guides for migrating microneurons in the cortex.