Early eye morphogenesis in the zebrafish, Brachydanio rerio

Early eye morphogenesis in the zebrafish between 12 and 36 hours postfertilization was studied by light- and scanning electron microscopy. Overall, early eye morphogenesis in the zebrafish is similar to that of other vertebrates even though the optic primordia evaginate from the forebrain as solid masses of cells. After initial evagination (6-7 somite stage [SS]), the optic primordia take on a wing-like shape (8-9 SS). Subsequently, they bend ventrally and rotate slightly in an anterior direction (10-12 SS). These changes serve to bring the primordia from a horizontal to a more vertical orientation in relation to the embryonic neural axis. Invagination commences from the center of each primordium (14 SS) and progresses symmetrically out towards the periphery (14-20 SS). The choroid fissure forms by an involution along the anterior region of the eyecup (18-20 SS). By 24 hours postfertilization (pf), the eyecups are well formed. Between 24 and 36 hours pf, the eyes rotate further in relation to the axis of the embryo, and this repositions the choroid fissue to a typical ventral location by 36 hours pf. Because of the two rotations of the eye during early morphogenesis, particularly the later one, the anterior-posterior orientation of the emerging optic primordium ultimately becomes the ventral-dorsal axis of the completed eyecup.