Coloboma is a defect in the morphogenesis of the eye that is a consequence of failure of choroid fissure fusion. It is among the most common congenital defects in humans and can significantly impact vision. However, very little is known about the cellular mechanisms that regulate choroid fissure closure. Using high-resolution confocal imaging of the zebrafish optic cup, we find that apico-basal polarity is re-modeled in cells lining the fissure in proximal to distal and inner to outer gradients during fusion. This process is accompanied by cell proliferation, displacement of vasculature, and contact between cells lining the choroid fissure and periocular mesenchyme (POM). To investigate the role of POM cells in closure of the fissure, we transplanted optic vesicles onto the yolk, allowing them to develop in a situation where they are depleted of POM. The choroid fissure forms normally in ectopic eyes but fusion fails in this condition, despite timely apposition of the nasal and temporal lips of the retina. This study resolves some of the cell behaviors underlying choroid fissure fusion and supports a role for POM in choroid fissure fusion.

Vertebrate eye formation is a multistep process requiring coordinated inductive interactions between neural and non-neural ectoderm and underlying mesendoderm. The induction and shaping of the eyes involves an elaborate cellular choreography characterized by precise changes in cell shape coupled with complex cellular and epithelial movements. Consequently, the forming eye is an excellent model to study the cellular mechanisms underlying complex tissue morphogenesis. Using examples largely drawn from recent studies of optic vesicle formation in zebrafish and in cultured embryonic stem cells, in this short review, we highlight some recent advances in our understanding of the events that shape the vertebrate eye.