Identification of mammosomatotropes, growth hormone cells and prolactin cells in the pituitary gland of the gilthead sea bream (Sparus aurata L., Teleostei) using light immunocytochemical methods: an ontogenetic study.

Growth hormone (GH) and prolactin (PRL) immunoreactivities in the adenohypophysis of Sparus aurata specimens from newly hatched until 48-months-old were detected using the peroxidase-antiperoxidase method. GH cells and PRL cells, and cells that were immunoreactive to both GH and PRL antisera, called mammosomatotropes (MS cells), were found. This is the first report on the identification of MS cells in fish, which were found in newly hatched and older larvae and juvenile specimens. GH and PRL cells appeared from two days after hatching. MS cells were first located in the central region of the adenohypophysis and afterwards in the rostral pars distalis. The GH cells were first identified in the dorsal and ventral areas of the middle-posterior part, and the PRL cells in the ventral region of the middle-anterior part. Later, during development, the sequence of appearance of the GH cells was proximal pars distalis, pars intermedia and rostral pars distalis, while for the PRL cells sequence was rostral pars distalis, proximal pars distalis and pars intermedia. This expansion pattern could be due to a GH- and PRL-cell migration although independent cell differentiation may occur in each region. The present results suggest that GH and PRL cells arise from MS cells at the outset of pituitary development, while MS cells proceed from PRL cells in old larvae and juveniles.