The role of embryonic polarities in preimplantation growth and implantation of mammalian embryos

Strong evidence now exists of polar axes associated with various embryologi cal, morphogenetic and developmental events in mammals from oocyte formatio n to implantation. These axes strictly regulate morphogenesis throughout th e pre; and post-implantation stages. They are expressed by gradients in gen e products and organelles, control ooplasmic and cytoplasmic rotations coup led with specific cleavage planes, and the imposition of body axes on embry o and fetus. The expression of polarities in preimplantation and implanting embryos, and their relationship with the implantation process, are discuss ed in this paper. A deeper understanding of such a fundamental aspect of th e genetic regulation of the embryo should help to clarify significant aspec ts of implantation and the clinical evaluation:of human embryos in vitro.