INTRODUCTION Gastrulation in the mouse embryo involves the active ingression of epiblast cells through the primitive streak and the reassortment of cell populations to the definitive germ layers

Gastrulation in the mouse embryo involves the active ingression of epiblast cells through the primitive streak and the reassortment of cell populations to the definitive germ layers (Hashimoto and Nakatusji; 1989, Tam and Beddington, 1987; Tam et al., 1993). It is not known how the different populations of presumptive mesodermal and endodermal cells are sorted during germ layer morphogenesis. Fate-mapping studies using either cell-marking or cell transplantation techniques have revealed that both the position of the cells in the proximaldistal axis and their proximity to the primitive streak are major determinants for the patterning of the embryonic mesoderm (Lawson et al., 1991; Parameswaran and Tam, 1995; Tam et al., 1997). According to these results, the first wave of epiblast cells that ingress through the primitive streak are those giving rise to extraembryonic mesoderm, followed by epiblast cells destined for the heart and cranial mesoderm. Thus, cells that will form the mesoderm of the yolk sac and the amnion make up a major part of the mesodermal layer of the midprimitivestreak-stage embryo. By the late-primitive-streak stage, the mesodermal layer contains only the precursors of the embryonic mesoderm. So far, however, no direct evidence has been presented that proves the results of cell-fate-mapping studies because no suitable genetic markers have been available to trace the cell fate from the beginning of gastrulation to the particular cell lineage. MesP1, belonging to the bHLH transcription factor family, was found to be expressed in the early mesoderm at the onset of gastrulation (Saga et al., 1996). Since the expression was rapidly down-regulated after 7.0 dpc (days postcoitum), only mesodermal cells that ingressed through the primitive streak in the early stage, between 6.5 and 7.0 dpc, expressed Mesp1. Homozygous Mesp1 null mice exhibited growth retardation after 7.5 dpc and died before 10.5 dpc with many developmental defects (Saga, 1998). The defects included 3437 Development 126, 3437-3447 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV4114