REGIONALIZATION OF CELL FATE AND MORPHOGENETIC MOVEMENT OF THE MESODERM DURING MOUSE GASTRULATION

The developmental fate of cells in the epiblast of early-primitive-str eak-stage mouse embryos was assessed by studying the pattern of tissue colonisation displayed by lac Z-expressing cells grafted orthotopical ly to nontransgenic embryos. Results of these fate-mapping experiments revealed that the lateral and posterior epiblast contain cells that w ill give rise predominantly to mesodermal derivatives. The various mes odermal populations are distributed in overlapping domains in the late ral and posterior epiblast with the embryonic mesoderm such as heart, lateral, and paraxial mesoderm occupying a more distal position than t he extraembryonic mesoderm. Heterotopic grafting of presumptive mesode rmal cells results in the grafted cells adopting the fate appropriate to the new site, reflecting a plasticity of cell fate determination be fore ingression. The first wave of epiblast cells that ingress through the primitive streak are those giving rise to extraembryonic mesoderm . Cells that will form the mesoderm of the yolk sac and the amnion mak e vp a major part of the mesodermal layer of the midprimitive-streak-s tage embryo. Cells that are destined for embryonic mesoderm are still found within the epiblast, but some have been recruited to the distal portion of the mesoderm. By the late-primitive-streak-stage, the mesod ermal layer contains only the precursors of embryonic mesoderm. This s uggests that there has been a progressive displacement of the midstrea k mesoderm to extraembryonic sites, which is reminiscent of that occur ring in the overlying endodermal tissue. The regionalisation of cell f ate in the late-primitive-streak mesoderm bears the same spatial relat ionship as their ancestors in the epiblast prior to cell ingression. T his implies that both the position of the cells in the proximal-distal axis and their proximity to the primitive streak are major determinan ts for the patterning of the embryonic mesoderm. (C) 1995 Wiley-Liss, Inc.