Effects Of Tributyltin (TBT) On Histological Structure In Xenopus Tropicalis Embryos

Tributyltin (TBT) has been widely used as a biocide in antifouling paints. In our previous study, we found that unique malformations are induced in Xenopus tropicalis embryos by TBT at environmentally relevant concentrations after 24,36 and 48 h of exposure. The most obvious alterations were abnormal eyes, enlarged proctodaeums, narrow fins and skin hypopigmentaion. The reasons leading to the external changes, however, are not well known.Based on the findings of morphological malformations above, we exposed embryos of X. tropicalis to 50,100 and 200 ng·L-1 tributyltin chloride (TBTC1) for 36 and 72 h, and the histological changes were observed on the main phenotypes of malformations. Our aim was to determine the histological abnormalities induced by TBT and to reveal the internal relationship between histological changes and the external malformations.The embryos treated with TBT showed multiple malformations such as abnormal eyes, enlarged trunks, enlarged proctodaeums and narrow fins. The histological changes were further observed on these main malformations induced by TBT. The lens and the retinal layers of abnormal eyes were slightly or barely differentiated, and that the pigment epithelium was neither continuous nor smooth. The external gills were not absorbed, and the inner gills were not well developed. The velar plates developed in a deformed way, what is worse, some were absent in the pharynx. The abdomens were full of undifferentiated gut tissue with yolk-rich inclusions in the tadpoles with enlarged trunks. The proctodaeums formed a bump-like or columnar structure. The mass of yolk-rich cells occupied the lumen, blocked the opening and even turned inside out of the proctodaeum. Both the ventral and dorsal fins in trunks and tails became narrow or even disappeared totally.Our results suggest that great changes of histology took place corresponding to the unique phenotypes. All the embryonic cells inherit yolk platelets from the egg cytoplasm and consumed them intracellularly during embryogenesis. The yolk-rich mass in TBT treatment tadpoles suggest that the gut tissue was poorly differentiated, which led to the failed elongation of the guts and subsequently the enlarged trunks. In TBT treatment groups, both the large intestine and proctodaeum were rich in yolk granules, sharing the similar cell types. Therefore, the thick proctodaeums still came from the original undifferentiated gut tissues. The enlarged proctodaeums were due to the undifferentiation of inner layer, the expansion of outer epidermal part and the absence of fins around them.The narrow fins or absence of fins in X. tropicalis larvae might be due to the apoptosis induced by TBT. The fin defects were always companied by enlarged protodaeums in this study. The development of proctodaeums and ventral fins are closely related to bone morphologenetic protein (Bmp) signaling. In brief, the histological observations provided insights into the reason of the unique external malformations in some degree.