Embryonic lens induction: more than meets the optic vesicle.

The classic model of lens induction stipulated that the optic vesicle is both a necessary and sufficient inductor of the lens in amphibian development. Although this view has subsequently been modified to encompass the contributions of earlier inductors, such as the involuting endo-mesoderm, it is still widely reported that the optic vesicle alone is able to elicit lens formation from ectoderm. Recent work, which has employed a host- and donor-marking scheme, has shown the optic vesicle to be a weak inductor of the lens, involved only in the final rather than the initial phases of determination. In addition, a review of the literature substantiates this conclusion since many of the transplantation experiments arguing for the sufficiency of the optic vesicle are characterized by the lack of adequate criteria for judging the authenticity of the resulting lens responses, particularly the absence of a host- and donor-marking strategy. This analysis of the literature, together with our own results, lead us to propose a new model of lens determination in which tissue interactions during gastrulation are required to confer a lens-forming bias upon a large area of head ectoderm allowing the optic vesicle to induce lens formation in a defined area of this primed ectoderm. Data from studies on mesoderm and neural induction are also beginning to suggest a multistep model involving the initial establishment of bias and subsequent interactions resulting in determination, and we propose that this framework will serve as a general paradigm for embryonic induction.