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The relation conjugated/unconjugated estrogens associated with reproductive processes has brought about the interest to study the biological role and regulation of the estrogen sulfotransferase and estrogen sulfatase which participate in the formation and hydrolysis of estrogen 3-sulfates, respectively. In this paper, both activities were measured through the reciprocal conversion of 3H-estrone sulfate and 3H-unconjugated estrogen during in vitro incubation with implantation sites (SI) and non-implanted sites (SNI) from the rat uterus, during the process of embryo implantation. Contrasting enzyme activities were found in these tissues. While sulfotransferase activity was higher in SI than in SNI (0.205 vs 0.144 pmol of E1S formed/mg protein/h, the inverse was found for the sulfatase (1.470 vs 1.977 pmol of E1 formed/mg protein/h). These results indicate the presence of both enzymes in the rat uterus and suggest the existence of a mechanism in SI that locally regulate the concentration of free and sulfoconjugated estrogens in which these enzymes participate.

In the early stages of pregnancy, embryo development and preparation of the endometrium must be synchronized in a very precise manner, in order to let blastocyst establishment. This state results from the interactions of steroid hormones, progesterone and/or estrogens. The hormonal requirements for embryo implantation vary considerably among different mammals. Systemic progesterone intervention is common to the various species; however, the requirement for ovarian estrogen seems not to be. In this paper, certain new aspects of the hormonal requirements for blastocyst implantation are reviewed.

Establishment of pregnancy in mammals is a process resulting of closely concerted events. The participation of the embryo through its development and secretory activity on the one hand, and on the other, the oviductal and endometrial tissues, by means of their secretions, give the optimal conditions to let the embryo establish creating a proper milieu for its nutrition and development. In this review, we analyze the influence of the oviductal and uterine secretions upon early embryonic development and differentiation and some transport systems of nutritional elements required in the early stages of the blastogenesis.