Distribution of α-MSH -like immunoreactive cells in the nervous system, Hatschek’s pit and other tissues of amphioxus, Branchiostoma belcheri

Immunohistochemical localization of α-melanocyte-stimulating hormone (α-MSH) in the nervous system, Hatschek’s pit and other tissues of amphioxus (Branchiostoma belcheri) was performed using the antibody against synthetic α-MSH. The results revealed that α-MSH-like immunoreactive cells were distributed at the dorsal side and ventral side of the brain vesicle, the dorsal side and the surrounding of central tube in the nerve tube, the epithelial cells of Hatschek’s pit, and zones 1, 3 and 6 in endostyle and hindgut. The immunoreactive substance was also found in the primary oocytes of the small and large growth stage of ovary and early stage spermatogenic cells in testis. These findings indicate that α-MSH is an ancient and highly conserved hormone and it is extensively distributed in amphioxus. Although Hatschek’s pit in amphioxus do not have a structure of the intermediate lobe of vertebrate adenohypophysis, it has already hosted α-MSH-like endocrine cells, implying that the functional differentiation of α-MSH-like cells occurred earlier than the differentiation of the tissue structure. The results of present study provided new evidence for the endocrinology of Hatschek’s pit and the origin and evolution of vertebrate adenphypophysis.