Morphogenetic domains in the yolk syncytial layer of axiating zebrafish embryos

The yolk syncytial layer (YSL) of the teleostean yolk cell is known to play important roles in the induction of cellular mesendoderm, as well as the p atterning of dorsal tissues. To determine how this extraembryonic endoderma l compartment is subdivided and morphologically transformed during early de velopment, we have examined collective movements of vitally stained YSL nuc lei in axiating zebrafish embryos by using four-dimensional confocal micros copy. During blastulation, gastrulation, and early segmentation, zebrafish YSL nuclei display several highly patterned movements, which are organized into spatially distinct morphogenetic domains along the anterior-posterior and dorsal-ventral axes. During the late blastula period, with the onset of epiboly, nuclei throughout the YSL initiate longitudinal movements that ar e directed along the animal-vegetal axis. As epiboly progresses, nuclei pro gressively recede from the advancing margin of the epibolic YSL. However, a small group of nuclei is retained at the YSL margin to form a constricting blastoporal ring. During mid-gastrulation, YSL nuclei undergo convergent-e xtension behavior toward the dorsal midline, with a subset of nuclei formin g an axial domain that underlies the notochord. These highly patterned move ments of YSL nuclei share remarkable similarities to the morphogenetic move ments of deep cells in the overlying zebrafish blastoderm. The macroscopic shape changes of the zebrafish yolk cell, as well as the morphogenetic move ments of its YSL nuclei, are homologous to several morphogenetic behaviors that are regionally expressed within the vegetal endodermal cell mass of ga strulating Xenopus embryos. In contrast to the cellular endoderm. of Xenopu s, the dynamics of zebrafish YSL show that a syncytial endodermal germ laye r can express a temporal sequence of morphogenetic domains without undergoi ng progressive steps of cell fate restriction. (C) 2001 Wiley-Liss, Inc.