The role of the endometrium during embryo implantation.

The endometrium undergoes cyclic growth and development with the sole purpose of successful establishment of pregnancy. As more is known about the gene products of the endometrium, it appears that many of the secreted products of the glandular epithelium function to support the nascent embryo and begin the early communication that continues into pregnancy. Maternal endometrial cells are regulated directly by ovarian steroids and indirectly by various growth factors and cytokines. The time of maximal uterine receptivity is now thought to arise on cycle days 20-24 and is manifest by the expression of many different endometrial products. These proteins can serve as markers of uterine receptivity and have been used to identify women at risk for implantation failure. The use of marker proteins promises to promote our understanding of the mechanism of implantation while providing clues into the causes of some types of infertility. Ultimately, the endometrium provides the opportunity for the embryo while at the same time maintaining constraints on uncontrolled invasion of the 'tumour-like' placenta. Understanding this subject in greater detail will likely improve health care opportunities for the infertile couple and provide new insights into contraception targeting the endometrium and embryo-endometrial interactions.