Sustained BMP signaling induces the lens placode

Abstract

Abstract Purpose Fate maps at late gastrula and head fold stages show that prospective olfactory and lens placodal cells are intermingled in a domain of the anterior border region between the neural plate and the future epidermis. During embryonic development Bone Morphogenetic Protein (BMP) signals represent one class of secreted signals that are implicated in lens specification. When and how BMP signals is involved in the initial specification of the lens placode remains to be defined. Methods We have established explant and whole embryo assays of placodal cell differentiation in the chick embryo, and analyzed a panel of markers that are expressed in differentiated olfactory and lens placodal cells. Results We provide evidence that cells in the anterior border region have become specified as olfactory and lens placodal cells at the late gastrula stage, and that BMP signals are required and sufficient to induce olfactory and lens placodal cells from these progenitor cells. We also provide evidence that time of exposure of progenitor cells to BMP signals plays a key role during the differential specification of olfactory and lens placodal cells. By the neural fold stage, the generation of lens cells requires continued exposure to BMP signals, whereas the generation of olfactory placodal cells has become independent of further exposure to BMP signals. Moreover, prolonged exposure of BMP signals promotes the generation of lens cells at the expense of olfactory placodal cells. Conclusion Our results provide evidence that the specification of the lens placode requires sustained BMP signaling. Thus, our results support the emerging idea that time of exposure of progenitor cells to patterning signals play important roles during cell fate specification in different tissues in vertebrate embryos.