Abstract
The fetal development of the mammalian eyelid involves the expansion of the epithelium over the developing cornea, fusion into a continuous sheet covering the eye, and a splitting event several weeks later that results in the formation of the upper and lower eyelids. Recent studies have revealed a significant number of molecular signaling components that are essential mediators of eyelid development. Receptor-mediated sphingosine 1-phosphate (S1P) signaling is known to influence diverse biological processes, but its involvement in eyelid development has not been reported. Here, we show that two S1P receptors, S1P2 and S1P3, are collectively essential mediators of eyelid closure during murine development. Homozygous deletion of the gene encoding either receptor has no apparent effect on eyelid development, but double-null embryos are born with an "eyes open at birth" defect due to a delay in epithelial sheet extension. Both receptors are expressed in the advancing epithelial sheet during the critical period of extension. Fibroblasts derived from double-null embryos have a deficient response to epidermal growth factor, suggesting that S1P2 and S1P3 modulate this essential signaling pathway during eyelid closure.