Practice question

Retinoic acid signaling synthesized by retinal pigment epithelium via Raldh1-3 enzymes plays pivotal role in retinal differentiation patterning. Retinoic acid gradient along dorsal-ventral axis regulates expression of Tbx5, Vax2, and Cyp26 to specify dorsal-ventral retinal identity, promote photoreceptor differentiation, and stimulate neurite outgrowth. RA also induces Pax2 in optic stalk and supports vascular development. It does not primarily induce lens which depends on BMP-FGF, nor optic vesicle formation which precedes RA, nor cornea formation dependent on surface ectoderm, but fine-tunes retinal progenitor competence and patterning.