Thyroid deficiency and cell death in the rat cerebellum during development.

The [3H]-DNA content of the cerebellum remained constant during a period from 4 h to 8--9 days after the administration of [3H]-thymidine both to normal rats at 6 and 12 days of age and to thyroid deficient rats at 6 days. On the other hand, when [3H]-thymidine was given to 12-day-old thyroid deficient rats, a progressive decrease in the cerebellar content of [3H]-DNA was observed during the first 4 days after the injection. These findings, together with previous results showing a markedly elevated pyknotic index in the internal granular layer of the 12-day-old hypothyroid cerebellum, are consistent with there being an increased death of newly formed and differentiating granule cells in thyroid deficiency towards the end of the second week of postnatal life.