The pituitary gland is present in all vertebrates, from agnathans (jawless fishes) to mammals, but not in invertebrates. Hagfishes, which lack both jaws and vertebrae, are considered the most primitive vertebrate known, living or extinct. Accordingly, studies on hagfishes are indispensable for understanding the origin and evolution of the pituitary hormones. Nevertheless, little is known about the hagfish adenohypophysial hormones. Our recent immunohistochemical and lectin histochemical studies have revealed that gonadotropin (GTH), adrenocorticotropin (ACTH), and growth hormone (GH) are present in the hagfish pituitary gland. This review summarizes the latest data regarding the hagfish adenohypophysial hormones from an evolutionary point of view.
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