Generation of a second eye by embryonic transplantation of the antero-ventral hemicephalon.

Vertebrate ocular morphogenesis requires proper dorso-ventral polarity within the optic vesicle, and loss of dorso-ventral polarity results in failure of optic cup formation and domain specification, as shown by a reverse transplantation of the optic vesicle. We have shown previously that the ocular development depends not only on the signal within the antero-ventral optic vesicle but also on the extraocular signals. In the present study, using embryonic transplantation of a discrete portion of the embryonic chick brain, we demonstrate formation of a second eye from the antero-ventral hemicephalon when it was transplanted in the antero-dorsal hemicephalon of the host embryo. The transplant consists of an antero-ventral quadrant of the optic vesicle and the surrounding part of the anterior cephalon. The original dorso-ventral polarity of the transplant was once cancelled and re-established in accordance with that of the host embryo. Neither dorsal nor ventral cephalic halves in isolation did not develop into entire eye structures under the culture condition; the dorsal halves developed merely into the retinal pigmented epithelium and the ventral halves into the neural retina alone. The present study clearly suggests that extraocular dorsal and ventral signals counterbalance each other to specify the polarity of the optic vesicle.