Implantation in Women; Is it Unique?

Abstract

Implantation is generally considered to begin with initial apposition and attachment of the trophectoderm (specialized cells of the embryo) to the maternal endometrial epithelium and is considered complete only once the developing conceptus is adequately supplied with a blood supply from the mother. However, even prior to attachment, there is a complex interactive and highly synchronized embryo-maternal dialogue that differs between species. Establishment of pregnancy depends on these interactions and on molecular processes that occur even before attachment. While for ethical reasons it is not possible to study the precise interactions between the blastocyst and the maternal endometrium during the earliest stages of implantation in women, it is clear that women differ in a number of aspects from other mammals. This is largely because of key differences in the physiology of the reproductive tract between species and in the highly invasive capacity of human trophoblast. For example, women are unique in the extent of the endometrial preparation for implantation that occurs even in a non-conception cycle and this offers potential for molecular interactions between the blastocyst and the endometrium as early as the time the embryo first enters the uterine cavity. Furthermore, there are fundamental differences in the way in which the epithelium is modified at the time of implantation. Morphological differences are reflected at the molecular level: for example, molecules identified as important for implantation by genetic approaches in mice, are often found to be expressed in different cell types and differently regulated in women. These include a number of cytokines and proteases that are critical for implantation in mice but which have quite different patterns of expression in the endometrium of women. Given that many proteins are produced by uterine epithelium and secreted into the uterine lumen, it is likely that they contribute to the pre-attachment dialogue with the developing blastocyst.