Immunocytochemical and ultrastructural characterization of somatolactin cells from the gilthead sea bream (Sparus aurata L., Teleostei): an ontogenicstudy (from newly hatched to adults)

For the first time, somatolactin (SL) cells have been ultrastructurally ide ntified and characterized during the ontogeny of gilthead sea bream, Sparus aurata, using specimens ranging in age from hatching to 15 months. The SL cells were identified by an immunogold method using anti-cod SL serum. The SL-immunoreactivity was mostly located on the secretory granules of the cel ls, although some vesicles of variable size and shape with a medium electro n-dense content, and some irregular secretory granules and polymorphic or v ery irregular masses that can arise from the fusion of several secretory gr anules, also presented immunogold labeling. In adults, the SL cells were ma inly found in the pars intermedia, where they were organized in discontinuo us cell cords lying against the neurohypophysis or surrounding the neurohyp ophyseal branches. Some SL cells, however, appeared isolated or in small gr oups in the pars intermedia, in the proximal pars distalis and, rarely, in the rostral pars distalis. The SL cells were variable in shape, with proces ses directed towards the neurohypophysis or blood vessels, or intermingling among other adenohypophyseal cells. The secretory granules were mostly rou nd, although some were oval, bilobate or pear-shaped, with a homogeneous, v ery electron-dense content and a narrow, dense or clear, halo. Different SL cell populations can be distinguished according to secretory granule size. Our findings indicate that SL is stored in the secretory granules and rele ased by exocytosis. SL cells showing involutive features were only found in adults. SL cells can be ultrastructurally identified in one-day-old larvae although similar characteristics to those found in adults can be positivel y identified only after 4 days. Secretory granules increased in number, siz e and heterogeneity during development. Synaptic-like structures between ax on terminals of the neurohypophysis and the SL cells were found in larvae f rom one-day-old onwards. In juveniles of 118 days of age, two different pop ulations of secretory granules (immunogold-labeled and non-immunogold-label ed) can be found in the same or different SL cells, findings that suggest t he existence of two different molecular forms of SL at this age. There was a clear increase in the complexity of the pituitary gland and in the hetero geneity of the SL cells during development, the latter observation probably reflecting different functional cell stages or production of SL molecules.