Histological and cytological studies on the developing thymus of sharpsnout seabream, Diplodus puntazzo

The structure of the developing thymus of the marine teleost, Diplodus punt azzo, was studied by light and transmission electron microscopy. The first anlage of the thymus developed by d 20 postfertilisation (p.f.) as a group of undifferentiated cells dorsal to the epithelium of the branchial chamber . The organ increased significantly in size around d 51-66 p.f. and differe ntiation of cortex and medulla occurred concomitantly. On the basis of thei r localisation, 4 main types of epithelial cell were distinguished: (1) lim iting, adjacent to the connective capsule; (2) medullary and cortical retic ular cells; (3) nurse cells, located in the corticomedullary boundary; (4) Hassall-like corpuscles. The majority of medium to large blast-like lymphoi d cells were localised in the medulla, while small lymphocytes were housed in the cortical region. These morphological features were maintained at lat er stages. However, in juveniles in the medulla we observed reticular epith elial cells with cysts and rare Hassall-like corpuscles. The study was desi gned to obtain more information concerning the histology of the developing thymus of sharpsnout seabream and give a concise description of the differe ntiation of epithelial cells and lymphoid cells in the thymic parenchyma.