Cell movements in Xenopus eye development

Abstract

The vertebrate eye develops morphological markers, such as the ventral choroid fissure, which define its anatomical polarity in relation to the body. Also, retinal nerve fibres behave as though regionally differentiated by forming ordered, topographical maps of the visual field in the brain. It has long been held that these two features of eye polarity--its structure and the neural specificity of the ganglion cells--are labile in the early eye vesicle and become determined relative to body axes at a subsequent stage of development. Contrary to these earlier findings, recent experiments show that the structure and connectivity of the eye are already determined by the time it first appears as a distinct eye vesicle; surgical construction of eye pigmentation chimaeras suggested that normal tectal maps, previously reported after early eye rotations, may have arisen from neural retina cells recruited from the optic stalk after the operation. In agreement, using a radioactive tag which marks neural retina as well as pigment epithelium, I report here that during normal development in Xenopus laevis, cells in the ventral retina, where the choroid fissure forms, move into position from the optic stalk region during eye-cup formation. Depending on developmental stage, surgical eye rotations may intercept their movement and this provides a simple explanation of results previously taken to indicate a change from a labile to a determined state of eye polarity.