Journal of Histochemistry and Cytochemistry, Vol. 45, 1673-1682, Copyright © 1997 by The Histochemical Society, Inc.
The Secretory Granule and Pro-opiomelanocortin Processing in Xenopus Melanotrope Cells During Background Adaptation
Constance A.F.M. Berghsa,
Shigeyasu Tanaka1,a,
Frank J.C. Van Striena,
Shingo Kurabuchib, and
Eric W. Roubosa
a Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, University of Nijmegen, Nijmegen, The Netherlands
b Department of Histology, The Nippon Dental University School of Dentistry, Chiyoda-ku, Tokyo, Japan
Correspondence to:
Constance A.F.M. Berghs, Dept. of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, Faculty of Science, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands.
In this immunocytochemical study, we used light and electron microscopic observations in combination with morphometry to analyze the processing of pro-opiomelanocortin (POMC) in melanotrope cells of the intermediate pituitary of Xenopus laevis adapted to either a white or a black background. An antiserum was raised against a synthetic peptide including the cleavage site between ACTH and ß-lipotropic hormone in Xenopus. Western blotting revealed that this antiserum recognizes only a 38-kD protein, the POMC prohormone, from extracts of Xenopus neurointermediate pituitary. Light immunocytochemistry showed differential immunostaining for anti-POMC compared to anti- -MSH. Anti-POMC was predominantly found in the perinuclear region, whereas anti- -MSH yielded staining throughout the cytoplasm. Immunogold double labeling revealed that electron-dense secretory granules (DGs) show high immunoreactivity for anti-POMC and low immunoreactivity for anti- -MSH. Electron-lucent granules (LGs) are immunoreactive to anti- -MSH only. Moderately electron-dense granules (MGs) revealed intermediate reactivity compared to DGs and LGs. Background light intensity has significant effects on the morphology and the immunoreactivity of the secretory granules. Black-adapted animals have 4.5 times as many DGs and MGs as white-adapted animals. In addition, the MGs in black animals show 42% more anti- -MSH immunogold than the MGs in white animals. Together, these findings indicate that the three granule types represent subsequent stages in granule maturation. Adaptation to a black background stimulates the formation of young immature granules, while at the same time the processing rate during granule maturation increases. (J Histochem Cytochem 45:1673-1682, 1997)
Key Words:
pro-opiomelanocortin, -MSH, melanotrope cells, quantitative immunoelectron, microscopy, Xenopus laevis

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