Lipocortin 1 immunoreactivity identifies microglia in adult rat brain

AbstractImmunohistochemical localization of two Ca++‐binding proteins, Lipocortin 1 (LC1) and S100‐β, demonstrates two distinct classes of primitive glia in the floor plate of rat embryos. With proper fixation (formalin‐lysine‐periodate‐acetic acid), dendritic glia in the CNS of adult rats also apparently stain for either LC1 or S100‐β in the ratio of 1:3. In order to further distinguish and identify these two glial classes, we have examined their population density, topography, and responses to localized neuron death. Neurons of the ipsilateral thalamus undergo apoptosis following cortical ablation; the contralateral thalamus serves as control. By eight days post‐lesion, the number of LC1 cells in the ipsilateral thalamus has increased >4‐fold, the increase comprising primarily activated phagocytes adjacent to degenerating neurons. The S100‐β glia in the same region are virtual‐ ly indistinguishable from control; but background staining (apparently representing extracellular S100‐β) is increased. Thus, the responses of dendritic LC1 glia resemble those previously described for microglia and are quite different from the astrocytes identified by S100‐β immunoreactivity. Both dend