Distinct morphologies of fusion and closure of the choroid fissure
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Coloboma is a disorder characterized by the lack of fusion of the embryonic choroid fissure, a transient cleft along the ventral midline of the eye. Traditionally, work in this field has treated the appearance of continuous tissue along the ventral midline as a fused eye. We report that along the proximodistal axis of the chick and the mouse eye, there are different morphologies accounting for this ventrally continuous tissue. We show that the eye has to modes of achieving ventral continuity: closure, involving a ventrally continuous tissue because of the intercalation of the optic nerve between the choroid fissure folds, and fusion, involving the fusion of the basement membranes of the fissure margins and/or the fusion of the margins themselves. We demonstrate that the chick exhibits this closure morphology proximally, fused basement membranes medially and unfused margins because of the intercalated pecten, and fused margins distally. We show that for most of the mouse eye, the ventral midline is fused, except at the optic disc, where optic nerve intercalation yields a closed eye. Thus, choroid fissure closure results in different morphologies that are species-dependent and that vary along the axis of a single eye.