RETINOIC ACID ESTABLISHES VENTRAL RETINAL CHARACTERISTICS

The developing eye is known to be rich in retinoic acid (RA), and pert urbations in RA levels during formation of the optic primordia, as wel l as RA receptor mutations, cause retinal malformations, especially in ventral eye regions. To test the hypothesis that RA plays a role in t he establishment of ventral retinal characteristics, we examined sever al dorsal and ventral ocular markers in RA-treated zebrafish. The opti c stalk represents the ventral-most region of the early eye field. Dur ing normal development, the optic stalks constrict, decreasing in widt h and are gradually replaced by the optic nerve. Systemic high RA leve ls cause an expansion in the optic stalk with an increased cell conten t and a patent lumen. In addition, the stalks do not constrict and per sist into later stages of development indicating an enhancement of ear ly ventral eye characteristics. Expression of the transcription factor pax[b], normally confined to the ventral retina, expands into the dor sal retina following RA treatment, whereas msh[c], normally expressed in the dorsal retinal pole, disappears. Activity of an aldehyde dehydr ogenase that normally occupies the dorsal third of the retina is reduc ed or abolished following high systemic RA. When a localized RA source , an RA-soaked bead, is placed next to the developing eye, a fissure r esembling the choroid fissure appears in the eye facing the bead. Take n together, these observations suggest that RA is involved in the dete rmination of the ventral retina.