Influence of thyroliberin on the rat pituitary cell type differentiation: an in vitro study.

The effects of TRH on the primordia pituitary cell differentiation, in from 11-day-old rat embryos cultured in a synthetic medium, were studied by immunocytochemistry. The expression of different pituitary phenotypes were measured as the percentage of immunoreactive area per section of cultured pituitary. Addition of TRH on day 1 of culture induced the thyrotroph and gonadotroph differentiation in a dose-dependent fashion. TRH promoted a fewer differentiation of gonadotrophs than GnRH. Much smaller increase in gonadotropic cell area was observed when 10(-9) M TRH was added on day 4 of culture instead of day 1. This suggests that gonadotroph differentiation can be only transiently induced during early embryonic period. Moreover, addition of TRH on day 1 of culture induces a differentiation of few lactotrophs. Contrary to gonadotrophs, the number of lactotrophs was higher when TRH was supplemented on day 4. As previous reports have demonstrated that the lactotroph differentiation is controlled by the alpha-subunit of glycoproteic hormones, further studies are necessary to determine whether the effect of TRH is direct. TRH had no effects on the differentiation of corticotrophs or somatotrophs. These results in vitro suggest that the availability of TRH in fetal hypothalamus at early embryonic stage may affect the pituitary differentiation in a subpopulation of precursor cells.