Uterine sensitivity for blastocyst implantation

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Attempting to analyse the role of the ovarian hormones upon the onset, magnitude and loss of uterine receptivity/sensitivity, particular emphasis is given to uterine vascular changes. Information concerning the modulation by hormones of uterine micro-circulation appears essential for the understanding of the receptivity/sensitivity uterine changes. The generation, storage and release of vasoactive mediators and prostaglandins appear involved. As shown in the rat recently, the onset of uterine receptivity/sensitivity is temporarily correlated with the appearance of endometrial PGE binding sites under hormonal control. On the other hand catecholamines may also modulate the uterine vascular functions. Endometrial monoamine oxidase and catechol O-methyltransferase two enzymatic activities involved in catecholamine deactivation show hormone dependant changes parallel to the manifestation of uterine receptivity/sensitivity. The precise role of these phenomena is discussed. Irrespective of the highly variable way in which egg-implantation occurs in different species, all the events related to this process appear to follow a well-timed schedule. Thus, around the time at which the fertilized egg reaches the blastocyst stage, the uterus enters a phase of optimal conditions for co-participation. We present here several criteria characterizing this uterine phase of receptivity for blastocyst implantation as defined mainly in rats. Among them we will more particularly describe the endometrial vascular reactivity towards the blastocyst stimulus or towards artificial stimuli leading to a decidual response. This reactivity is expressed by an increase in capillary permeability and appears to be, in all the species studied, a regular index of endometrial receptivity/sensitivity. It may be the result of a cascade-like effect involving various vasoactive tissular mediators, such as histamine and/or certain prostaglandins, just as a common inflammatory reaction. The existence of an optimal time for the application of the stimulus which will initiate a decidual response is known since the classical studies of Loeb (1908). It has been further shown by De Feo (1967) that under controlled light conditions, the uteri of pseudopregnant rats acquire sensitivity to a knife-scratch trauma between days 5 and 6 of pseudopregnancy. By using the intraluminal injection of chemical inducers instead of a knife-scratch, De Feo demonstrated further that a peak of sensitivity appears around noon of day 5. Histological examination of rat uteri fixed around noon of day 5 of pregnancy or pseudopregnancy shows an oedematous swelling of the endometrial stroma (Psychoyos, 1967). This transient stromal oedema is one of the phenomena which have to be considered as indicating the manifestation of optimal uterine conditions for