Immunostaining with antisera to oLH, hCG, hLH, pLHbeta, hFSH, hFSHbeta, hTSHalpha and bTSH was used to delineate the gonadotropic and thyrotropic cells of the human fetal anterior pituitary. Hypophyses from 29 normal fetuses, 3 newborn infants, and 5 totally ancencephalic fetuses were used. Several controls to check for the specificty of the immunocytological reaction were made. In normal fetuses, observations showed that: 1) the alpha subunit was detected from the eighth week and throughout gestation without sex differences; 2) intact LH was detected during the third month, however, age and sex differences were observed during the fourth and fifth months; 3) intact FSH was detected in female fetuses from the beginning of the fourth month, a sex difference was observed; 4) LH and FSH were detected in the same cells; 5) the thyrotropic cells were detectable from 15 weeks of gestation and their number increased during gestation without sex difference; 6) at birth the gonadotropic cells were scarce and were located in the ventromedian zone of the anterior pituitary, while the thyrotopic cells remained numerous and were located in the dorsomedian zone. In amencephalic fetuses: 1) the alpha subunit existed at each stage studies; 2) the reaction induced by anti-pLHbeta and anti-hFSHbeta sera was alwys very weak regardless of sex or age; 3) the thyrotropic cells were more numerous in comparison to the gonadotropic cells. These data are discussed in terms of the relationship of the hypophysiotropic hypothalamic factors to the appearance and evolution of the glycoprotein hormones and their subunits.

On the other hand, both eel and salmon calcitonin I were inactivated less markedly and in the similar manner. The relationship between the hypocalcemic effect 01' calcitonins and the inactivation is discussed.

The in vivo internalization of thyrotropin-releasing hormone (TRH) was studied by using a semiquantitative immunoelectron microscopic method. Pituitary glands of normal male rats or of rats intravenously injected with 100 ng TRH and sacrificed after 5–60 min were used. Ultrathin sections were obtained by cryoultramicrotomy of fixed pituitary glands. Pituitary cellular types were identified by appropriate antiserums. An antiserum specifically directed against TRH was used. TRH-like immunoreactivity due to endogenous TRH was observed in thyrotropes and prolactin cells, but never in somatotropes, gonadotropes or corticotropes. At the subcellular level, the reaction was detected within the cytoplasmic matrix, the secretory granules, and the nucleus but only occasionally at the plasma membrane. After in vivo injection of TRH, the immunocytochemical reaction (1) was still restricted to thyrotropes and prolactin cells, (2) increased with time elapsed after injection up to 15–30 min and then returned to basal intensity in cytoplasm, secretory granules, and nucleus, and (3) became very frequent at the plasma membrane. These data (1) provide evidence for endogenous TRH within thyrotropes and prolactin cells, i.e., in physiological target cells for TRH, and (2) support the hypothesis that normal TRH target cells can, in vivo, internalize exogenous as well as endogenous TRH into several subcellular compartments including the nucleus.

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