Immunohistochemical detection of ACTH and MSH cells in the hypophysis of the hermaphroditic teleost, Diplodus sargus

Hypophyseal ACTH and MSH cells were immunohistochemically characterised in the teleost fish, Diplodus sargus, using anti-ACTH (1-24) and anti alpha -M SH polyclonal antisera. ACTH cells were found both in the pars distalis and in the pars intermedia. In the Former region, they appeared small, round-s haped and clustered; in the latter, they were either small or large and elo ngated. Moreover, a few ACTH-immunoreactive cells resembling microglia were present in the neurohypophysis. Conversely, MSH cells were found only in t he pars intermedia, and were similar to the larger ACTH cells of the same r egion. In the pars intermedia, co-localisation of ACTH and MSH immunoreacti vity in the same cell was revealed by double immunostaining, though the two hormones were also observed in distinct cell types. The distribution of AC TH cells appeared quite uniform, without any marked difference between the specimens tested. Conversely, MSH cell amount varied according to the stage of the sexual cycle of this teleost fish, which is characterised by protan drous hermaphroditism. In fact, a lower amount of MSH cells were observed i n females, whereas no significant difference was found between immature and male specimens.