Distribution of immunoreactive adenohypophysial cell types in the pituitaries of the Atlantic and the Pacific hagfish, Myxine glutinosa and Eptatretus burgeri.

The hagfish is considered the most primitive vertebrate known, living or extinct. It remains an enigma whether adenohypophysial hormones similar to those of more advanced vertebrates are present in the hagfish pituitary gland or not. The present study aimed to detect immunoreactive adenohypophysial hormones in the hagfish pituitary gland, using antisera to tetrapod and fish adenohypophysial hormones as immunohistochemical probes. For this purpose, two species of hagfish, the Atlantic hagfish, Myxine glutinosa, and the Pacific hagfish, Eptatretus burgeri, were used. In both species, three different types of immunoreactive cells were detected in the adenohypophysis. (1) The first type of cells was gonadotropin (GTH)-like cells which were stained by antisera to LH-related GTHs, such as ovine LHbeta, human LHbeta, bullfrog LH, salmon LHbeta and sturgeon LHbeta in both species of hagfish. (2) The second type of cells that were detected was growth hormone (GH)/prolactin (PRL)-like cells. In M. glutinosa the cells were stained by antisera to salmon GH, salmon PRL, sturgeon GH, sturgeon PRL, blue shark GH, and lamprey GH. In E. burgeri the cells were only stained by anti-human GH and anti-sturgeon PRL. (3) The last type of cells was adrenocorticotropin (ACTH)-like cells. These cells were stained by antisera to lamprey ACTH and human beta-endorphin. In both species of hagfish, GTH-like cells were relatively abundant, and were distributed throughout the adenohypophysis, whereas GH/PRL-like and ACTH-like cells were few in number in the adenohypophysis. Based on these findings, we suggest that hagfish may have retained ancestral characteristics of key anterior pituitary hormones.