Glial cell differentiation in neuron-free and neuron-rich regions. II. Early appearance of S-100 protein positive astrocytes in human fetal hippocampus

Abstract

The development of the human fetal hippocampus and dentate gyrus has been studied immunocytochemically. The first glial cells to appear are vimentin-positive radial glial cells. A gradual transition from vimentin to glial fibrillary acidic protein (GFAP) reactivity in the radial glial cells occurs at week 8. The GFAP-positive radial glial cells transform into astrocytes from week 14. A population of small S-100-positive somata which morphologically and spatially are distinct from GFAP-positive radial glial cells and their transformed progeny, are found as early as week 9.5 in the hippocampus during the period of peak neurogenesis. The well-defined immunoreactivity of the morphologically homogenous cell subpopulation for S-100 protein, which has been used as an astrocytic marker in the adult hippocampus, indicates that astrocytes may differentiate at very early gestational ages in human fetuses. The S-100-positive astrocytes are thought to be derived from ventricular zone cells, which at the time of their appearance do not express any of the applied astrocytic markers (S-100, GFAP, vimentin). It is suggested that the S-100-positive astrocytic cell population interacts with the first incoming projection fibers, so modulating the pattern of connectivity.