Fig. 2

Light micrographs of eye sections from control (A) and retinoic acid-treated (B-D) zebraflish embryos. The treated embryos were exposed to 1 μM retinoic acid at the 2- to 3-somite stage for 2 hr. Drawings illustrating the key features of the photomicrographs are shown below. (A) Control showing a normally developing eyecup at the 17-somite stage. The thin (and difficult to see) optic lumen separates the pigment epithelial cell layer (PE) from the neural retinal epithelium (NR). Some mitotic figures are seen in the neural retina (arrow). (B) In the retinoic acid-treated embryo at the 17-somite stage, a dramatic expansion of the epithelial cell layer in the ventral portion of the eyecup is seen. Several mitotic figures are observed in this region (arrow). (C) By the 22-somite stage of development in a retinoic acid-treated embryo, two retinal epithelial cell layers can be distinguished. The optic lumen separates the dorsal (D) and ventral (V) retinas and the neural retinas from the pigment epithelial cells (PE). (D) By 24 hr after fertilization, the ventral retina (V) has assumed the same orientation as the dorsal retina (D). The calibration bar in A is 40 μm and applies to all parts of the figure.