Immunocytochemical demonstration of melanotropic and adrenocorticotropic cells from the gilthead sea bream (Sparus aurata L., Teleostei) by light and electron microscopy: an ontogenic study.

In the pituitary of gilthead sea bream, Sparus aurata, melanotropic (MSH) and adrenocorticotropic (ACTH) cells were identified at the light and electron microscopic levels using rabbit anti-synthetic alphaMSH (MSH) and anti-human ACTH (1-24) (ACTH) sera. The distribution of these cell types was followed from hatching to 48 months. The techniques used included the peroxidase anti-peroxidase (PAP) method, conventional electron microscopy, and an immunogold technique. Using PAP, MSH (immunoreactive to both anti-MSH and anti-ACTH) and ACTH (immunoreactive to anti-ACTH) cells were detected from hatching onward. These cells were distinguished ultrastructurally in 1-day-old larvae. Immunogold labeling was first detected in MSH cells in 5-day-old larvae, while ACTH cells were only immunogold labeled in adults. In newly hatched larvae, MSH cells were located from the middle to the posterior region of the adenohypophysis, while ACTH cells were found in the dorsoanterior region, next to the hypothalamus. At this age, both cell types were scarce. As the fish developed, these cell types progressively increased in number: MSH cells made up a layer surrounding the neurohypophysis (NH) in the pars intermedia (pi), whereas ACTH cells bordered the developing NH in the rostral pars distalis (rpd). From 82 days onward, a few MSH cells were observed in the proximal pars distalis (ppd) next to the pi and some ACTH cells were seen in the ppd next to the rpd. In adult specimens, both MSH and ACTH cells were adjacent to the stellate cells and showed processes and synaptic-like structures. MSH cells exhibited numerous round secretory granules with a granular content and of varying electron density and compactness. These granules were immunogold labeled with anti-MSH serum. Electron-dense secretory granules near the Golgi complex immunoreacted with anti-MSH, anti-ACTH, or with both antisera. ACTH cells exhibited round secretory granules with a homogeneous, high electron-dense core and a narrow, clear halo. These granules immunoreacted with anti-ACTH serum. The main ultrastructural features that characterize the MSH and ACTH cells of adults appeared early during ontogeny. Involutive MSH and ACTH cells were only observed in adult specimens.