Interlaminar astroglial processes in the cerebral cortex of adult monkeys but not of adult rats.

Three adult Cebus apella and two Saimiri sciureus monkeys, as well as four adult rat brains were processed for glial fibrillary acid protein (GFAP) immunocytochemistry. Irregular patches with relatively abundant, long astrocytic processes were found in monkey but not in rat brains, along the cingulate, as well as orbital, medial and dorsolateral cortices of the frontal lobe, and in temporal lobe cortical areas. The observed long astrocytic processes alternated in an apparent irregular fashion with others with a predominantly clear background and scattered GFAP-immunoreactivity (IR) astrocytes or with a predominant GFAP-IR-dense fibrillary meshwork. They usually stemmed from lamina I or II and coursed through deep laminae. Entorhinal cortex in rat brains showed occasional astroglial processes somewhat longer than those commonly observed in other cortical regions, albeit significantly shorter than those observed in Ceboidea monkeys and not truly interlaminar. These observations indicate a clear morphological difference in astroglial organization between nonhuman primates and a rodent brain. They further show that 'interlaminar astrocytes' are distributed nonhomogeneously in the frontal cortex of New World monkeys. No interlaminar processes were observed in cortical areas of rat brains. Even though the physiological role of interlaminar astrocytes in a primate brain remains unknown, their significant departure from the morphology of classical, stellate astrocytes suggest that they may subserve different cortical requirements.