Widespread lateral processes of glial cells in the immature striate cortex of the cat

The topographies of intrinsic tangential connections of cells in the striate cortex of cats were determined in animals ranging in age between 1 day and 30 days. Implants of the carbocyanin dye, DiI, were found to label cellular elements which are classified by morphological criteria as astrocytes, in addition to neurons. Labeled astrocytes are seen in all cortical layers and in the superficial and deep white matter. Most labeled astrocytes occur underneath the DiI implant, but a number of them are also located horizontally at various distances from the implant. The horizontal span of the glial processes is the same as that for neuronal processes; in contrast to neurons however, the laterally distributed astrocytes assume a flat and unclustered distribution in the horizontal plane. Our observations suggest that the program for determining the span of laterally directed projections in the cat striate cortex is likely to be the same for neurons and astrocyte-like neuroglia.