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.
We have previously demonstrated that TRH induces several hypophyseal cell type differentiation. In the present study, we reevaluated the effect of GnRH on gonadotroph differentiation, and we determined whether gonadotropin is implicated in the appearance of other pituitary cell types. These effects were compared to the influence of TRH on LH and TSH cell differentiation. Pituitary primordia of 11 day-old rat embryos were cultured in a synthetic medium until the equivalent of term, and the LH and TSH cell differentiation was visualized using an immunohistochemical technique. The effect of GnRH on pituitary hormone cells was evaluated by measuring the percentage of immunoreactive area per section.GnRH, when added on the first day of culture, induced LH and TSH cell differentiation in a dose-dependent manner. GnRH was more effective than TRH in inducting gonadotropic phenotype expression. This suggests that GnRH, which evokes LH release, is the most effective factor inducing gonadotroph differentiation in early fetal life. However, GnRH was as effective as TRH in thyrotroph differentiation. Lactotrophs were detected in primordia cultured with GnRH. As lactotroph phenotype differentiation is controlled by the cc-subunit of glycoproteic hormone, further studies are required to determine whether or not GnRH effect on lactotroph is direct. GnRH did not affect the corticotroph and somatotroph differentiation. These results confirm that hypothalamic factors such as TRH or GnRH are capable of inducing more than one pituitary cell phenotypes.
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.
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