| Research Abstract |
Lampreys and hagfish are the only two extant representatives of the oldest class of vertebrates, Agnatha (jawless fishes). The present study aimed to elucidate the pituitary system of lampreys and hagfish by means of both immunohistochemical and molecular biological techniques. 1) Adenohypophysial cell types were studied in the sea lamprey (Petromyzon marinus) pituitary gland. It was found that ACTH-like cells were distributed in most parts of the rostral pars distalis, whereas MSH-like cells in almost all parts of the pars intermedia. In the proximal pars distalis, GH-like cells occupied the dorsal half, while GTH-like cells in the ventral half. Thus, at least four types of adenohypophysial cells are present in the sea lamprey pituitary. 2) A cDNA coding conserved amino acid sequence in gnathostome GHs has been cloned from the sea lamprey pituitary, followed by the isolation of the mature protein from the sea lamprey pituitary. That protein has been identified to be lamprey GH by demonstrating an increase of expression of lamprey IGF gene in the liver. 3) Ontogeny of adenohypophysial cell types was studied in sea lampreys at several different stages of the life cycle. Results obtained were discussed in connection with the life cycle of sea lampreys. 4) Immunoreactive adenohypophysial cell types were studied in the pituitary of the Atlantic and the Pacific hagfish, Myxine glutinosa, Eptatretus burgeri and Paramyxine atami. In all three species, three different types of hormone cells were detected in the adenohypophysis. They were ACTH-like cells, GH-like cells and GTH-like cells. It is suggested that ACTH, GH and GTH are ancient hormones and maintain their original functions throughout vertebrate evolution, while the later derived hormones, such as PRL and TSH, may have contributed to the expansion of vertebrates into new environments.
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