Temporal and spatial effects of Sonic hedgehog signaling in chick eye morphogenesis

Proper dorsal--ventral pattern formation of the optic cup is essential for vertebrate eye morphogenesis and retinotectal topographic mapping. Previous studies have suggested that midline tissue-derived Sonic hedgehog (Shh) molecules play critical roles in establishing the bilateral eye fields and in determining the proximal--distal axis of the eye primordium. Here, we have examined the temporal requirements for Shh during the optic vesicle to optic cup transition and after early optic cup formation in chick embryos. Both misexpressing Shh by virus and blocking Shh activity by antibodies resulted in disruption of ventral ocular tissues. Decreasing endogenous Shh signals unexpectedly revealed a sharp morphological boundary subdividing dorsal and ventral portions of the optic cup. In addition, Shh signals differentially influenced expression patterns of genes involved in ocular tissue specification (Pax6, Pax2, and Otx2) and dorsal--ventral patterning (cVax) within the ventral but not dorsal optic cup. Ectopic Shh suppressed expression of Bone Morphogenetic Protein 4 (BMP4) in the dorsal retina, whereas reducing endogenous Sonic hedgehog activity resulted in a ventral expansion of BMP4 territory. These results demonstrate that temporal requirements for Shh signals persist after the formation of the optic cup and suggest that the early vertebrate optic primordium may be subdivided into dorsal and ventral compartments. We propose a model in which ventrally derived Shh signals and dorsally restricted BMP4 signals act antagonistically to regulate the growth and specification of the optic primordium.