CELL FATE AND MORPHOGENETIC MOVEMENT IN THE LATE MOUSE PRIMITIVE STREAK

A prospective fate map of the late gastrulation mouse primitive streak has been charted in 8.5 dpc mouse embryos developed in culture, using the lineage marker DiI to label groups of cells. As at earlier stages , the fate of cells in the 8.5 dpc primitive streak is regionalised su ch that successively more caudal regions of the streak give rise to mo re lateral mesoderm. While most labelled cells over a 24 or 48 h cultu re period exit from the primitive streak, some are consistently found to remain within it. The most conspicuous resident population is prese nt in the node. To determine when ingression of ectoderm through the s treak ceases, ectoderm cells of the streak and posterior neuropore of 8.5-10.0 dpc embryos were labelled. Involution of surface cells to for m mesoderm continues until closure of the posterior neuropore but is n ot seen thereafter.