Molecular control of the implantation window

Human endometrium is the end organ of the hypothalamic-pituitary-ovarian axis. Therefore, endometrium is susceptible to changes in the cases of infertility that originate from disturbances in the normal functioning of this axis. In addition, some cases of unexplained infertility may be due to altered endometrial function. This disturbed endometrial function may originate from lesions in the molecular repertoire that are crucial to implantation. Human endometrium becomes receptive to implantation by the blastocyst within a defined period during the menstrual cycle. The duration of this so-called 'endometrial receptivity' or 'implantation' period seems to span from few days after ovulation to several days prior to menstruation. Successful implantation results from a co-ordinated series of events that would allow establishment of a timely dialogue between a receptive endometrium and an intrusive blastocyst. The members of the molecular repertoire that make endometrium receptive to implantation are gradually being recognized. Among these are the cytokines, integrins, heat shock proteins, tastin and trophinin. In addition, the expression of a second set of genes including tumour necrosis factor alpha (TNF-alpha) and ebaf, may be the appropriate signal for the closure of the 'implantation window', for making the endometrium refractory to implantation and for preparing it for the menstrual shedding.