THE ALLOCATION OF EPIBLAST CELLS TO ECTODERMAL AND GERM-LINE LINEAGESIS INFLUENCED BY THE POSITION OF THE CELLS IN THE GASTRULATING MOUSE EMBRYO

The developmental potency of cells in the proximal and distal regions of the epiblast of pre- and early-primitive-streak-stage mouse embryos was assessed by their differentiation in the host embryo following or thotopic and heterotopic cell transplantation. Normally, cells in the distal epiblast differentiate predominantly into neuroectoderm and sur face ectoderm. However, when they were transplanted to proximal region s of the epiblast, distal epiblast cells behaved Like proximal epiblas t cells: they colonised the extraembryonic mesoderm and other mesoderm al tissues in the posterior region of the host embryo, In addition, ab out 3.7% of the transplanted distal epiblast cells differentiated into primordial germ cells. This proportion is comparable to the 3.9% of o rthotopically transplanted proximal epiblast cells that became primord ial germ cells. When proximal epiblast cells were transplanted heterot opically to distal sites, their descendants were generally absent from the extraembryonic mesoderm and the germ cell population of the host embryo. Like cells in the distal epiblast, they mostly colonised the n eural plate and surface ectoderm. This plasticity of cell fate suggest s that the epiblast cells are not irreversibly allocated to any specif ic lineages, including the germ line. The adoption of developmental fa te that is typical of the cell population at the site of transplantati on suggests that the specification of cell lineages is subject to cert ain site-specific influences in the epiblast. Allocation of cells to t he ectodermal and germ cell lineages may be subject to local tissue in teractions and the restriction of morphogenetic tissue movement of dif ferent epiblast cell populations during gastrulation. (C) 1996 Academi c Press, Inc.