A single eye or two fused eyes exist in some human fetuses suffering from holoprosencephaly ( Cohen, 1989 Sperber et al. Interest in how two eyes form in normal embryogenesis has, in part, been motivated by attempts to understand developmental origins of cyclopean eyes in abnormal situations. Here we address the question how two symmetric eyes form in the anterior neural plate. Formation of a single heart, on the other hand, results from the fusion of two originally separate primordia ( Copenhaver, 1926 Jacobs and Fraser, 1994). Most of the structures on both sides of an animal, such as limbs, emerge independently from separate primordia on each side of the embryo. It is known that several strategies are used to form these structures in the embryo.
Our results provide direct evidence for the existence of a single retina field, indicate that the retina field is resolved by suppression of retina formation in the median region of the field, and demonstrate that the prechordal plate plays a primary signaling role in retina field resolution.įormation of bilaterally symmetric and asymmetric structures have been studied in vertebrates since the beginning of experimental embryology. Transplantation experiments in chick embryos indicate that the prechordal plate is able to suppress Pax-6 expression. Removal of the prechordal mesoderm led to formation of a single retina both in chick embryos and in Xenopus explants. Lineage tracing experiments demonstrate that retina field resolution is not due to migration of cells in the median region to the lateral parts of the field. We report here that the expression pattern of a new gene ET reveals that there is a single retina field which resolves into two separate primordia, a suggestion supported by the expression pattern of the Xenopus Pax-6 gene. An interesting question is whether both eyes share a common developmental origin or they originate separately.
Two bilaterally symmetric eyes arise from the anterior neural plate in vertebrate embryos.
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