The effects of early hypo- and hyperthyroidism on the development of rat cerebellar cortex. I. Cell proliferation and differentiation

Abstract

Cell proliferation and differentiation were studied in the cerebellar cortex of rats made hypo- and hyperthyroid from birth. Hyperthyroidism caused: (1) decreased body, brain and cerebellar weight, (2) early termination of cell proliferation in the external granular layer accompanied by early disappearance of this layer, (3) early cell differentiation in the molecular and internal granular layers, (4) terminal decrease in granule cells, basket cells, and astrocytes. Hypothyroidism caused: (1) decreased body, brain and cerebellar weight, (2) prolonged cell proliferation in the external granular layer and retarded disappearance of this layer, (3) retarded cell differentiation in the molecular and internal granular layers, (4) terminal increase in granule cells and astrocytes, and decrease in basket cells. Hypo- and hyperthyroidism had different effects on the inner and outer halves of the molecular layer, which differentiate sequentially. Hypothyroidism caused a retardation of the peak of cell differentiation in the inner half from 5 to 10 days whereas hyperthyroidism caused an overall acceleration of differentiation in both zones, with an early peak of differentiation in the outer zone at 5 days. These results are discussed with regard to mechanisms of differentiation of cells derived from the external granular layer, and the role played by thyroid hormone in the initiation of cell differentiation in the central nervous system.