ON THE FORMATION OF THE NEURAL KEEL AND NEURAL-TUBE IN THE ZEBRAFISH DANIO (BRACHYDANIO) RERIO

Morphogenetic movements accompanying formation of the neural keel and neural tube in the zebrafish Danino (Brachydanio) rerio were studied by labelling single neural plate cells with fluoresceinated dextran (FDA) during late gastrula stages (95-100% epiboly) and localizing their progeny with an anti-fluorescein antibody on histological sections throughout neurulation. The mediolateral extent of the neural plate correlates directly with the dorso-ventral extent of the neural tube. That is to say, the progeny of cells located medially in the neural plate come to lie ventrally in the neural tube; cells located laterally in the neural plate give rise to progeny that populate dorsal levels in the neural tube. Fixation of labelled cells at various stages reveals that neural keel and nerve rod are organized as monostratified epithelia and that they maintain this organization during neurulation. These observations strongly suggest that the neural keel in the zebrafish forms by way of infolding of the neural plate and, therefore, utilizes a mechanism similar to primary neurulation in other vertebrates. The folding process juxtaposes the apical surfaces of both flanks of the neural plate at the midline. Mitoses occur preferentially in this zone, leading very frequently to formation of bilaterally symmetrical clones of progeny cells. The size of the clones that develop from injected cells suggests that neural plate cells divide an 1.5 times on average between late gastrula and the end of neurulation.