The glia types inthe visual system of adult rats, their shape variability, distribution patterns, and their lightoptically visible contacts to other tissue structures.

In silver impregnated coronal sections of adult rat brains the glia types of the Corpus geniculatum laterale, pars dorsalis (CGLd) an in area 17 were registered considering their form variants (FV), their intraareal distribution and their light optically demonstrable connexions to other tissue structures. Compared with the shape-determining light optically visible processes and perikarya, astrocytes show the lowest, microglial cells the greatest from variability. Transitional forms between the three glia types were not detected, but between elongated astrocytes and fiber astrocytes. An accumulation of elongated astrocytes, fiber astrocytes, and oligodendrocytes was found in the lateral zone of the CGLd, of protoplasmic astrocytes in the lateral and intermediary zones and at the rostral and caudal poles of the griseum, too. In the cortex distribution differences in the single laminae became also evident. Astrocytes were most concentrated in L I, oligodendrocytes and microglia, however, in L V. A few FV of the microglia were found to show a predominant localisation in specific laminae. Neither the total glia number nor the single glia types correlate with the neuron packing density. The frequency distribution of the FV of oligodendroglia in the CGLd and the visual cortex is similar, that of microglia, however, significantly deviating. Compared with the CGLd the oligodendrocytes of area 17 have smaller somata. Astrocytes, oligodendrocytes and microglial cells reveal structural contacts to different parts of the neurons, to blood vessels and other glial cells, too. Contacts between oligodendrocytes and microglial cells as well as astrocytes in satellite position could never be observed. In the cortex a few FV of microglia show typical connexions.