The histology and chronology of thymic development in sea bass were described.
Day 27 post-hatch: the thymus appears as a paired anlage on dorsal caudal portion of both gill chambers, predominantly filled with lymphoid cells associated with the pharyngeal epithelium.
Day 38 ph: the penetration into the parenchyma of connective trabeculae begins to divide it into lobes and carries the vascular supply. This process is accompanied by the development of a subcapsular, inner thymic region.
Day 53 ph: the active lymphopoiesis and enlargement of the outer region, filled by numerous small lymphocytes, define evident regional differences in the thymus, that are retained in older stages.
In one-year old bass, the organ is completely encapsulated by connective, the pharyngeal epithelium detaches from the lymphoid tissue. The thymus, which still retains a superficial position, does not show evident signs of regression
