Abstract
The optic fissure is a transient gap in the developing optic cup of vertebrates. Persisting optic fissures, coloboma, are a frequent reason for blindness in children. Although many genes have been linked to coloboma, it has remained unclear how the two bi-layered epithelia comprising the optic fissure margins are fusing to form a continuous neuroretina and retinal pigmented epithelium (RPE) respectively. Besides, highly variable morphologies of coloboma phenotypes strongly argue for a diverse set of underlying pathomechanisms.
Here we investigated the contribution of the individual cell types with 4D in vivo time-lapse analyses using zebrafish (Danio rerio). This allowed defining the respective roles of the participating tissues and cell populations and their activities during fissure morphogenesis, contact formation between the margins as well as during fusion.
We show that optic fissure closure is initiated by a bilateral tissue flow partially in continuation of the dynamic optic cup morphogenesis but additionally including a tissue flow from the optic stalk. This process is followed by the setup of specific fissure margins by a distinct cell population translocating from of the optic stalk. The morphological fusion is triggered by in an EMT-like disassembly of the fissure margin driven by bi-potential pioneer cells that ultimately take the fate of both, neuroretina and RPE respectively. The consecutive fusion and re-epithelialization transforms the two initially separated epithelial bilayers into the two continuous layers of neuroretina and RPE. The processes described here in detail represents a fundamental mechanism of the seamless connection of adjacent multilayered epithelia and is highly reminiscent of other fusion processes, like palatal shelf fusion with key relevance for development and growth.
