Immunocytochemical and ultrastructural characterization of melanotropin and adrenocorticotropin cells from the mediterranean yellowtail (Seriola dumerilii, Risso 1810)

Abstract

Background Melanotropin (MSH) and adrenocorticotropin (ACTH) are pituitary hormones derived from a common precursor: the proopiomelanocortin (POMC), which is processed differently in the melanotropic and corticotropic cells of several vertebrates. While ACTH is a major final product in corticotropes, it is further processed into α-MSH and corticotropin-like intermediate lobe peptide (CLIP) in melanotropes. Cells which are immunoreactive to ACTH (ACTH cells) and to both α-MSH and ACTH (MSH cells) have been described in a number of teleosts, including the Mediterranean yellowtail, by light microscopic immunocytochemistry. However, these cells have been ultrastructurally characterized only in a few species. In this paper, we use electron microscopy to identify and characterize the cells producing MSH and ACTH in M. yellowtail (Seriola dumerilii). Methods Pituitaries from adult specimens were dissected and processed for conventional and immunocytochemical electron microscopy. An immunogold technique was performed using anti-synthetic α-MSH and anti-human (h) ACTH (1-24) sera. Results MSH cells had round secretory granules with a granular content of varying electron density and compactness, which were immunogold-labeled with anti-α-MSH. Homogeneous and electron-dense secretory granules found in the Golgi area of these cells reacted with both anti-α-MSH and anti-hACTH (1-24). ACTH cells had round secretory granules with a homogeneous and medium or high electron-dense core and narrow clear halo, which were grouped in the cell area near the neurohypophysis (NH). Some granules showed an osmiophilic semicore in the medium electron-dense content, which has not been described in other teleost pituitary cells. Immunogold-labeling over the secretory granules only was obtained with all the antisera used. Some ACTH cells showed involutive features. Conclusions MSH and ACTH are respective final products of the POMC in two ultrastructurally different cells of the pituitary of M. yellowtail, MSH and ACTH cells. The immature granules in the Golgi area of MSH cells seem to be the site of proteolitic cleavage of ACTH into α-MSH and CLIP. Anat. Rec. 249:74–80, 1997. © 1997 Wiley-Liss, Inc.