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

The effects of hypoand hyperthyroidism on the development of the brain have been widely studied. Both conditions produce decreases in adult cerebral and cerebellar weights 6--s,l°-a2,16,2a,z4, but for apparently different reasons. In hypothyroidism, the cerebral weight deficit is partly the result of decreased cell size 7,12-14,z4 with a concomitant increase in packing density 7,12,t3,21,24. In the cerebellum there is also a transient decrease in cell number as well as a more permanent reduction in cell size and increase in packing density 7. This transient decrease in cell number is presumably due to a reduced rate of cell proliferation in the external granular layer. In hyperthyroidism the cerebral and cerebellar weight deficits are attributed to a permanent decrease in total cell numbera, 29 with no concomitant change in packing density or cell size 5, perhaps reflecting premature termination of cell proliferation due to early initiation of cell differentiation6,1s, ~9. It appears, therefore, that thyroid hormone plays a key role in the process of cell differentiation, since hypothyroidism produces smaller, more tightly packed (less differentiated) cells, while hyperthyroidism produces fewer cells due to early differentiation and associated termination of cell proliferation. The purpose of the present study was to specify the effects of hypoand hyperthyroidism on cell proliferation and differentiation in the rat cerebellar cortex by means of quantitative histological methods and to correlate these effects with changes in specific cell populations. A subsequent study 23 explores the effects of these treatments on synaptogenesis in the molecular layer.