COMPLETE DISSIPATION OF COHERENT CLONAL GROWTH OCCURS BEFORE GASTRULATION IN MOUSE EPIBLAST

Observations on chimaeric mice argue that there must be considerable d ispersal and intermingling of the clonal descendants of epiblast found er cells at an early stage in the development of the tissue. However, it has not been established when or how this occurs. Here we have used a genetic marker that enables donor cells to be visualized in situ to examine the early postimplantation distribution of clones obtained by transplanting epiblast founder cells into host blastocysts. We have a lso determined the spatial relationship between sister cells in non-ch imaeric postimplantation epiblast by ionophoretic injection of a fluor escent macromolecule. Both experimental approaches support the conclus ion that breakdown of coherent clonal growth accompanies epithelializa tion of the epiblast and is essentially complete by the onset of gastr ulation. Furthermore, the clonal analysis shows that descendants of di fferent epiblast founder cells continue to intermingle extensively wel l into organogenesis. We suggest that this sustained intermingling of cells in the epithelial epiblast, which does not occur in the adjacent visceral endoderm, depends on cells losing contact with the basal lam ina when they divide. These findings have implications both for patter ning of the early amniote embryo and for the growth of tall columnar e pithelia in general.