THE ALLOCATION OF EPIBLAST CELLS TO THE EMBRYONIC HEART AND OTHER MESODERMAL LINEAGES - THE ROLE OF INGRESSION AND TISSUE MOVEMENT DURING GASTRULATION

The cardiogenic potency of cells in the epiblast of the early primitiv e-streak stage (early PS) embryo was tested by heterotopic transplanta tion. The results of this study show that cells in the anterior and po sterior epiblast of the early PS-stage embryos have similar cardiogeni c potency, and that they differentiated to heart cells after they were transplanted directly to the heart field of the late PS embryo. That the epiblast cells can acquire a cardiac fate without any prior act of ingression through the primitive streak or movement within the mesode rm suggests that neither morphogenetic event is critical for the speci fication of the cardiogenic fate, The mesodermal cells that have recen tly ingressed through the primitive streak can express a broad cell fa te that is characteristic of the pre-ingressed cells in the host when they were returned to the epiblast. However, mesoderm cells that have ingressed through the primitive streak did not contribute to the later al plate mesoderm after transplantation back to the epiblast, implying that some restriction of lineage potency may have occurred during ing ression. Early PS stage epiblast cells that were transplanted to the e piblast of the mid PS host embryos colonised the embryonic mesoderm bu t not the extraembryonic mesoderm. This departure from the normal cell fate indicates that the allocation of epiblast cells to the mesoderma l lineages is dependent on the timing of their recruitment to the prim itive streak and the morphogenetic options that are available to the i ngressing cells at that instance.