ORIENTED CELL DIVISIONS AND CELLULAR MORPHOGENESIS IN THE ZEBRAFISH GASTRULA AND NEURULA - A TIME-LAPSE ANALYSIS

We have taken advantage of the optical transparency of zebrafish embry os to investigate the patterns of cell division, movement and shape du ring early stages of development of the central nervous system. The su rface-most epiblast cells of gastrula and neurula stage embryos were i maged and analysed using a computer-based, time-lapse acquisition syst em attached to a differential interference contrast (DIC) microscope. We find that the onset of gastrulation is accompanied by major changes in cell behaviour. Cells collect into a cohesive sheet, apparently lo sing independent motility and integrating their behaviour to move cohe rently over the yolk in a direction that is the result of two influenc es: towards the vegetal pole in the movements of epiboly and towards t he dorsal midline in convergent movements that strengthen throughout g astrulation. Coincidentally, the plane of cell division becomes aligne d to the surface plane of the embryo and oriented in the anterior-post erior (AP) direction. These behaviours begin at the blastoderm margin and propagate in a gradient towards the animal pole. Later in gastrula tion, cells undergo increasingly mediolateral- and autonomous converge nce movements towards the dorsal midline leading to an enormous extens ion of the neural axis. Around the equator and along the dorsal midlin e of the gastrula, persistent AP orientation of divisions suggests tha t a common mechanism may be involved but that neither oriented cell mo vements nor shape can account for this alignment. When the neural plat e begins to differentiate, there is a gradual transition in the direct ion of cell division from AP to the mediolateral circumference (ML). M L divisions occur in both the ventral epidermis and dorsal neural plat e. In the neural plate, ML becomes the predominant orientation of divi sion during neural keel and nerve rod stages and, from late neural kee l stage, divisions are concentrated at the dorsal midline and generate bilateral progeny (C. Papan and J. A. Campos-Ortega (1994) Roux's Arc h. Dev. Biol. 203, 178-186), Coincidentally, cells on the ventral surf ace also orient their divisions in the ML direction, cleaving perpendi cular to the direction in which they are elongated. The ML alignment o f epidermal divisions is well correlated with cell shape but ML divisi ons within the neuroepithelium appear to be better correlated with cha nges in tissue morphology associated with neurulation.