Gonadotropin-like and adrenocorticotropin-like cells in the pituitary gland of hagfish, Paramyxine atami; immunohistochemistry in combination with lectin histochemistry

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Abstract
The pituitary system of the hagfish remains an enigma. The present study has aimed to detect possible adenohypophysial hormones in the pituitary gland of the brown hagfish, Paramyxine atami, by means of immunohistochemistry in combination with lectin histochemistry. Rabbit antisera raised against ovine luteinizing hormone (LH)β, proopiomelanocortin (POMC)-related peptides, and the growth hormone/prolactin family of tetrapod and fish species were used, and 25 kinds of lectins were tested. Three different types of adenohypophysial cells were revealed in the pituitary of brown hagfish. The first was stained with both anti-ovine LHβ and several D-mannose-binding lectins, such as Lens culinaris agglutinin and Pisum sativum agglutinin. This cell type predominated in the adenohypophysis in adults with developing gonads and thus appeared to be involved in the regulation of gonadal functions. The second was negative for anti-ovine LHβ but was stained with several N-acetylglucosamine-binding lectins, such as wheat germ agglutinin and Lycopersicon esculentum lectin. This cell type exhibited a weak positive reaction with anti-lamprey adrenocorticotropin (ACTH) and thus appeared to be related to POMC-like cells. The second cell type was found in the adenohypophysis regardless of the developmental state of the gonads. The third cell type was negative for both antisera and lectins. Since this cell type was numerous in juveniles and adults without developing gonads, most cells of this type were probably undifferentiated. These findings suggest that GTH and ACTH are major adenohypophysial hormones in the hagfish.