An enzyme cytochemical study of the endocytic pathways in anterior pituitary cells of the mouse in vivo.

Abstract

The endocytic pathways within the 5 types of secretory cells in the mouse anterior pituitary gland were studied in vivo using the enzymatic tracer horseradish peroxidase (HRP); cytochemical techniques for demonstration of thiamine pyrophosphatase (TPPase) and acid hydrolase activities were employed to distinguish Golgi saccules from GERL [membrane adjacent to Golgi apparatus connected to endoplasmic reticulum giving rise to lysosomes]. Mice were injected i.v. with 30-90 mg HRP and killed 5 min-12 h later. The anterior pituitaries from noninjected mice were prepared for morphological examination and demonstration of TPPase and acid hydrolase activities. In all preparations, comparisons were made among somatotrophs, mammotrophs gonadotrophs, thyrotrophs and corticotrophs. Each cell type exhibited secretory granule formation predominantly from GERL. No significant differences in the localizations of TPPase or acid hydrolase activities were noted among the 5 cell types. TPPase activity appeared in 1 or 2 Golgi saccules and less frequently in GERL and secretory granules. Acid hydrolase activity was present in GERL, nascent and immature secretory granules, and secondary lysosomes, which included dense bodies, multivesicular bodies, and elongated tubules. HRP was localized in coated and smooth surfaced vesicles and in the various forms of secondary lysosomes in each cell type; however, in somatotrophs only, HRP also accumulated in a Golgi-related cisterna that corresponded morphologically to GERL. Internalization of cell surface membrane into GERL of somatotrophs may reflect a recycling of this membrane for secretory granule production. Thus, the sequestration of internalized HRP into GERL in the anterior pituitary of the mouse may be dependent upon the cell type.