Y M Sin

The morphologic transformation of the developing zebrafish thymus from 1 week postfertilization (wpf) to 15 wpf is described. The thymus overall morphology changed from a small pouch‐like shape at 1 wpf to a conical shape between 2 and 3 wpf before acquiring a more complex shape from 4 wpf onward. Rapid growth rate along the lateral axis at the region near the pharyngeal epithelium occurred between 1 and 2 wpf, whereas rapid growth rate along the dorsal‐ventral axis occurred between 3 and 6 wpf. Expansion of thymocyte population beginning from 1 wpf became more evident by 2 to 3 wpf, as indicated by the apparent increase of different sizes of lymphocytes, recombination activating gene‐1 (rag‐1), and T‐cell antigen receptor alpha chain constant region (TCRAC) ‐positive cells. Tissue section in situ hybridization (ISH) analysis with rag‐1 probe reveals that cortex‐medullary regionalization has begun between 1 and 2 wpf as rag‐1 expression clearly demarcated the cortex, whereas the medulla was rag‐1 negative. The presence of TCRAC‐positive cells in the medulla by 2–3 wpf, suggests that the thymic selection processes had begun. The zebrafish thymus is morphologically mature by 3 wpf. Early signs of thymic involution were observed in zebrafish aged 15 wpf.