Summary

We have previously shown that one of two chicken engrailed-like genes, chick En-2, is expressed in a restricted region of the early chick embryo brain: the mes/metencephalon (Gardner et al. 1988). In this study, we examine the role of the cellular environment in regulation of engrailed-like (En) protein expression in quail-chick chimeric embryos. Two types of transplant surgery were performed at the 9–15 somite stage to produce chimeric embryos. In the first, the mid-mesencephalic vesicle or caudal mesencephalic vesicle alar plate (which is En protein-positive) was transplanted from a quail embryo into an En protein-negative region of chick neuroepithelium, the prosencephalon (mMP and cMP grafts, respectively). In the second reciprocal surgery, prosencephalic alar plate which is En protein-negative, was transplanted into the En protein-positive mesencephalic vesicle (PM grafts). A polyclonal antiserum, alpha Enhb-1, which recognizes chick En proteins (Davis et al. 1991) was used to identify En-positive cells 48 h after surgery. In mMP embryos, 71% of integrated grafts had lost En expression (n = 17). In contrast, in cMP grafts, 93% of integrated grafts continued to stain with the antiserum (n = 14). In addition, in 86% of these embryos, the graft induced adjacent chick host diencephalic cells to become En protein-positive as well. All PM grafts contained aEnhb-1-positive cells; such cells never expressed this protein in their normal environment. These early changes in En protein expression correlate well with the morphological changes observed in similar graft surgeries assayed later in development. Thus, our results are consistent with the hypothesis that En genes play a role in the regionalization of the early cranial neuroepithelium.