Optic cup formation: a calcium-regulated process.

Invagination of the optic vesicle to form the optic cup is an important event in the formation of the eye in the early embryo. To obtain support for earlier conclusions that a contractile process is involved, calcium dependency of optic cup formation was tested. Heads were excised from chicken embryos at the optic vesicle stage of development (stage 13) and incubated in nutrient medium containing antagonists or agonists of calcium transport. Invagination was reversibly inhibited by the Ca2+ antagonists verapamil and papaverine. It was initiated in a precocious fashion by the Ca2+ ionophore A23187 but only in the presence of external Ca2+. Neither caffeine, theophylline, nor A23187 (in the absence of external Ca2+) were able to initiate precocious optic cup formation. Trifluoperazine and chlorpromazine reversibly inhibited optic cup formation while chlorpromazine sulfoxide had no effect at the concentrations used. The binding of [3H]trifluoperazine to isolated stage 13 heads revealed a class of Ca2+-dependent binding sites having a Kd similar to that of calmodulin. These results indicate a Ca2+-dependence for optic cup formation and that the source of the Ca2+ may be extracellular. This Ca2+ dependence probably is conferred to the system by calmodulin.