Unbiased estimate of cerebellar granule cell numbers in hypothyroid and in sex-age-matched control rats.

Regardless the enormous amount of morphometric data related to the influence of thyroid hormones deficiency on the cerebellar cortex histogenesis, it remains controversial whether, in fully developed hypothyroid rats, the number of granule cells is actually altered. In order to get a quantitative unbiased view of its number at the end of the neurogenic period, we studied, using recently described stereological techniques, the granular layer of groups of 30 days old rats rendered hypothyroid from the day of birth. The results were compared with age- and sex-matched controls, as sex dimorphic differences were previously described for cerebellar granule cells. The granule cell packing density was increased independently of changes of their nuclear diameters. Its numerical density was similar in hypothyroid and controls, although a significant decrease in their total number was depicted as a consequence of the volumetric reduction of the granular layer in the hypothyroid groups. When male and female control groups were compared, the number of granule cells was found to be significantly smaller in females. This sex dimorphic difference was not detected between male and female hypothyroid groups, which suggests that, to be manifested, the presence of thyroid hormones is a necessary condition.