Mice homozygous for the Extra-toes mutation (Xt/Xt) die perinatally with multiple malformations involving the brain, the eye, the nasal mucosa, the face and the skeleton. We have investigated the development of the eye defect in Xt/Xt embryos until late gestation. In early midgestation, three classes of homozygotes can be identified: some form an apparently normal optic cup and are associated with an apparently normal lens vesicle, some have distorted optic cups with abnormal and small lens vesicles, and others do not form an optic cup and have no lens placode or vesicle. In late gestation, some embryos have obviously continued to develop small eyes, which develop coloboma because the optic fissure in the optic nerve does not close. The others show degeneration of the optic nerve and eye cup, with no remnants of lenses. Even in the absence of an eye in these mutants, extra-ocular muscles develop. Upon comparison with other mouse mutants with eye defects, our data suggest that the malformation of the eye in Extra-toes homozygotes is due to the defective optic vesicle, and not as previously stated because of generic forces interfering with lens induction.
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