Extrinsic, migratory mesenchymal cells are essential for the development and morphogenesis of many epithelial organs. In mouse, early eye defects have been described in a neural crest mutant, but the cellular and molecular mechanisms underlying aberrant optic cup morphology and patterning are unknown. Using zebrafish molecular genetics and multidimensional timelapse imaging, we sought to determine when and how neural crest migrates around the early eye, as well as exactly when and how eye defects arise in the absence of neural crest.
We find that at the earliest stages of optic vesicle formation, sox10-positive neural crest cells are in direct contact with the dorsal, posterior optic vesicle; these neural crest cells then migrate ventrally and anteriorly to enwrap the developing optic cup. By the end of optic cup morphogenesis, neural crest derived cells encapsulate the optic stalk and all but the distal, lens-facing side of the optic cup. Therefore, neural crest is present at the right time and place to influence patterning and morphogenesis of specific regions of the developing eye.
To determine precisely when and where eye defects arise in the absence of neural crest, we utilize the zebrafish tfap2ats213;foxd3zdf10 double mutant which exhibits a complete loss of neural crest. In the absence of neural crest, we find dramatic optic cup morphogenesis defects including a misshapen retina and coloboma, suggesting that neural crest is required for optic cup invagination and formation of the optic fissure. Similar results were seen using an independent neural crest mutant (paf1z24). In addition to the morphological defects, we began to characterize defects in tissue patterning: as in mouse Tcfap2a knockouts, tfap2a;foxd3 mutant zebrafish display an aberrant expansion of Pax2a expression into ectopic dorsal and posterior domains. This suggests an evolutionarily conserved role for neural crest in optic stalk and optic cup patterning and control of morphogenetic movements critical for optic cup formation.
Using computational cell tracking tools combined with our multidimensional imaging datasets, we are now pinpointing the precise cell movements disrupted by loss of neural crest as well as examining the specific signaling pathways dysregulated in the absence of neural crest. These results highlight the diverse roles mesenchyme can play in epithelial organ development and morphogenesis.