The structure of the developing thymus of the marine teleost, Diplodus
puntazzo, 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 differentiation of
cortex and medulla occurred concomitantly.
On the basis of their localisation, 4 main types of epithelial cell were
distinguished: (1) limiting, adjacent to
the connective capsule; (2) medullary and cortical reticular cells; (3)
nurse cells, located in the
corticomedullary boundary; (4) Hassall-like corpuscles. The majority of
medium to large blast-like lymphoid
cells were localised in the medulla, while small lymphocytes were housed
in the cortical region. These
morphological features were maintained at later stages. However, in juveniles
in the medulla we observed
reticular epithelial cells with cysts and rare Hassall-like corpuscles.
The study was designed to obtain more
information concerning the histology of the developing thymus of sharpsnout
seabream and give a concise
description of the differentiation of epithelial cells and lymphoid cells
in the thymic parenchyma.