Two types of secretory granules in gonadotrophs: Discrimination by the simultaneous EM immunocytochemical localization of serotonin and ?-follicle stimulating hormone

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Abstract
The hypothesis that the secretory granules of mammalian gonadotrophs are heterogeneous was tested. Previous studies had shown that all of the granules contain β‐luteinizing hormone (β‐LH) immunoreactivity, and some contain β‐follicle stimulating hormone (β‐FSH) or 5‐HT immunoreactivity. Moreover, differential release of β‐LH and β‐FSH has also been demonstrated. In the current study the pituitary glands of mice were investigated immunocytochemically at the ultrastructural level with antisera directed against human growth hormone (GH), β‐LH, β‐FSH, and 5‐HT. The immunoreactivities of β‐LH, β‐FSH, and 5‐HT were restricted to gonadotrophs. No 5‐HT immunoreactivity was seen in somatotrophs, identified by the immunoreactivity of GH in the secretory granules of these cells. Antisera to β‐LH labeled all gonadotroph granules; however, anti‐β‐FSH and anti‐5‐HT sera labeled only subsets of the granules. The proportion of granules labeled could not be increased by doubling the concentration of anti‐β‐FSH serum. The incidence of double labeling of granules by antisera to β‐FSH and 5‐HT was significantly less than that predicted from the incidence of granule labeling by either reagent alone. It is concluded that β‐FSH and 5‐HT immunoreactivities do not co‐exist in the same secretory granules of gonadotrophs; therefore, these granules are heterogeneous and there must be at least two types of granules. It is possible that the two types of granules may be responsive to different second messengers, thereby explaining the differential release of LH and FSH.