Appearance of neuronal S-100β during development of the rat brain

Abstract In addition to being an astroglial protein, S-100β is localised in distinct populations of neurons in the adult rat hindbrain. We report, here, the expression of S-100β in both neurons and glia of the rat brain during development. Prenatally, S-100β immunoreactivity was confined to glial cells close to the germinal zone. After birth, S-100β positive glial cells were seen mainly in the brainstem and cerebellum, while only a few were detected in cerebral cortex and hippocampus. The number of S-100β containing glial cells increased steadily during the first 2 postnatal weeks after which the adult pattern was attained. No S-100β containing neurons were present prenatally. The first S-100β containing neurons were seen in the mesencephalic trigeminal nucleus at postnatal day 1 (P1), and in the motor trigeminal nucleus at P3. Neuronal S-100β immunoreactivity in other nuclei was mostly attained from the 10th to the 21st postnatal day. The neuronal S-100β immunoreactivity was first detected in the cell nuclei during development, then increased in the cytoplasm with ages. A nuclear staining in many immunoreactive neurons persisted until the adult. It usually took 1 to 2 weeks for neuronal S-100β to attain the adult staining pattern, i.e., heavy staining of the cytoplasm and processes, after its appearance. The forebrain never contained S-100β positive neurons. The S-100β is first expressed in glial cells, suggesting it is primarily of the glial origin. Coupled with neurotrophic effects of the protein, the time course of neuronal S-100β expression during the critical period of neuronal development implies that it may be involved in neuronal differentiation and maturation.