Immunohistochemical localization of the S-100β protein in postnatal cat visual cortex: spatial and temporal patterns of expression in cortical and subcortical glia

The ontogenic expression of the glial-specific protein S100β was examined in postnatal cat visual cortex using immunocytochemical methods. Astrocytes in visual cortex and oligodendrocytes in the subcortical white matter exhibited distinct spatio-temporal gradients in their expression of the S100β protein. In the visual cortex, S100β-immunoreactivity was detected in astroglial cytoplasm, as well as in the extracellular interstitium, in a lamina-specific manner throughout postnatal development. Using double labeling procedures, the S100β protein was found to be strictly colocalized with GFAP-immunoreactive astroycytes when GFAP was present. The glial fibrillary acidic protein (GFAP), a marker of mature astrocytes, was not present at high levels until the 4th postnatal week. From the 2nd through 5th postnatal weeks, the expression of S100β was highest in the thalamocortical recipient, layer IV, of visual cortical areas 17 and 18. At ages beyond 6 postnatal weeks, S100β-immunoreactivity increased disproportionately in supra- and infragranular layers such that areas 17 and 18 were demarcated from adjacent cortices by lower levels in layer IV. The S100β protein was also highly expressed in oligodendroglial somata and processes in the subcortical white matter between the 2nd and 6th postnatal weeks. The levels of S100β in the subcortical white matter progressively diminished to adult levels, where it was localized only to a few remaining oligodendroglial somata. The differential laminar expression of the S100β protein in astrocytes during the period within which the visual cortex exhibits input- and experience-dependent synaptic modifications suggests that astrocytes, possibly via their release of S100β, may play a special role in mediating plasticity in visual cortical development. A consistent feature of the appearance of the S100β protein was its expression in immature astroglia and oligodendroglia, well before they are considered morphologically mature. This characteristic underscores the potential of S100β as a marker of distinct populations of glial cells and of their role in normal and abnormal development.