Cerebellar granule cell genesis in the hydrocortisone-treated rats.

The development of the cerebellar cortex was studied by quantitative light microscopic methods in rats treated with hydrocortisone on postnatal days 7-18. This treatment resulted in a decreased number of cells in the external granular layer (EGL) and an early disappearance of the EGL. In the adult cerebellum of these neonatally treated animals, the total number of granule cells in lobule VIII was decreased by 41% and that of molecular layer microneurons by 28%. Autoradiographic determinations of the 'birthdays' (time of origin) of cerebellar microneurons showed that the peaks of cell formation in the internal granular and molecular layers occurred at earlier ages than in controls. In the granule cell population, this observation was the result of both decreased cell proliferation during the treatment and premature cessation of division in granule cell precursors in the EGL. The morphological effects of 'late' hydrocortisone treatment on the cerebellum observed in this study are compared with those of 'early' hydrocortisone treatment on days 1-4 and those of thyroid hormone from previous studies. It is suggested that endogenous levels of both thyroid and glucocorticoid hormones during the neonatal period may influence the rate of neurogenesis in the postnatal rat cerebellum.