Morphologic transformation of the thymus in developing zebrafish

Abstract

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 TCRACpositive 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.

In vivo tracking of T cell development, ablation, and engraftment in transgenic zebrafish

“…Immunohistochemical studies of rag2-GFP transgenic fish reveal that GFP staining is detected in both the cortical and medullary regions of the thymus. This finding contrasts with the restricted expression of rag1 and rag2 mRNA expression in immature cortical thymocytes (20). These results can be explained by the stability of the GFP fluorophore and indicate that GFP protein expression may persist longer than the highly labile rag1 and rag2 proteins.…”

“…Our experiments show that the zebrafish absorb the steroid from the water and respond to treatment within 3 days. Because fish must pass water over the gills to obtain oxygen and because the zebrafish thymus remains contiguous with the gill arches throughout development until 15 weeks postfertilization (20), the thymus is likely constantly exposed to environmental agents, and thus, thymocytes may directly absorb dexamethasone from the water.…”

“…lck transcripts were detected in cells throughout the thymus (Fig. 10, which is published as supporting information on the PNAS web site), whereas rag1 and rag2 transcripts were selectively expressed in the cortex, and TCR-␣ was predominantly localized to cells found in the medulla and the corticomedullary junction (20) (Fig. 10).…”

“…The zebrafish thymic rudiment is completely developed by 60 h postfertilization (hpf) and by 68 hpf is colonized by T lymphocyte progenitors, which begin to transcribe rag1 and rag2 by 72 hpf (15,(17)(18)(19). As in mice and humans, mature T cells localize to the medulla of the adult zebrafish thymus, whereas immature T cells localize to the cortex (20).…”

NOTCH1-induced T-cell leukemia in transgenic zebrafish

“…20 The zebrafish exhibit thymic architecture similar to mammals, with a cortical and medullary region compartmentalizing T cells as they mature. 21 T-and B-cell development in the zebrafish relies on many of the same molecules and pathways used in mammalian lymphopoiesis. 22,23 Similarly, lymphoid-specific genes, including ikaros, rag-1, rag-2 and lck, have also been identified in the zebrafish, and these genes exhibit appropriate spatiotemporal expression patterns.…”