Chick embryos exposed to trichloroethylene in an ex ovo culture model show selective defects in early endocardial cushion tissue formation

Abstract

Formation of the primitive heart is a critical step for establishing a competent circulatory system necessary for continued morphogenesis, and as such has significant potential as a target for environmental insult. The goal of this study was to identify the initial cellular events that precede more superficially observable abnormalities resulting from exposing early chick embryos to trichloroethylene (TCE). A whole embryo culture method was used to assess the susceptibility of endocardial epithelial-mesenchymal transformation in the early chick heart to TCE. This method has the benefits of maintaining the anatomical relationships of developing tissues and organs, instantaneously exposing precisely staged embryos to quantifiable levels of TCE in a protein-free medium, and the ability to directly monitor developmental morphology. A minority of embryos (Hamburger and Hamilton [HH] stage 13-14) exposed to TCE (10-80 ppm) were not viable after 24 hr in culture and exhibited a variety of gross malformations in a dose-dependent fashion. However, the majority of treated embryos remained viable and developed into HH stage 17 embryos that were superficially indistinguishable from vehicle-treated controls. Further analysis of the hearts of these superficially normal embryos by whole-mount confocal microscopy revealed selective reduction in the number of atrioventricular canal mesenchymal cells. Additionally, those mesenchymal cells that did develop migrated abnormally as long thin cords of adherent cells. The regional selectivity of these effects in the chick heart suggests a critical window of susceptibility to TCE in the epithelial-mesenchymal transformation of atrioventricular canal endocardium.