Role of the polar trophectoderm in determining the pattern of early post-implantation morphogenesis in mammals: evidence from development of the short-tailed field vole, Microtus agrestis.

The peri-implantational embryogenesis in the field vole, Microtus agrestis, is described. Implantation is interstitial, as it is in the mouse, but egg cylinder formation occurs by invagination of the blastocyst's embryonic pole and not (as in the mouse) by formation of a multilayered extra-embryonic ectoderm. This difference can be attributed to loss in the field vole of the central portion of the polar trophectoderm at the time of blastocyst attachment. In comparing early postimplantation development of mammalian species, three morphogenetic variables should be considered: (i) continued proliferation of polar trophectoderm; (ii) mechanical constraints on the direction of its growth; (iii) variations in the degree to which polar trophectoderm is maintained intact after implantation.