Cytokines in implantation.

The endometrium of most species is now recognized as an important site of production of cytokines and their receptors. The cellular origin of the cytokines varies but many predominate in the uterine glandular or luminal epithelium or in the decidualized stromal cells. From studies in genetically modified mice it is clear that implantation of the blastocyst can proceed in the absence of most individual cytokines, although leukemia inhibitory factor and interleukin-11 have indisputable roles in this process. In other cases, such as CSF-1, GM-CSF, IL-1, and IL-6, the numbers of implantation sites or litter sizes are reduced when the cytokine is absent. The same cytokines that are implicated in implantation in mice are generally maximally expressed in human endometrium with maximal production in the secretory phase, particularly during the "window of implantation," but functional studies of their role in implantation in women and other primates are still required.