Embryonic signaling at implantation in the mouse

Signaling between the preimplantation embryo and its mother is generally presumed to be important for successful establishment of pregnancy in mammals. However, with the exception of the large domestic species, little information is available about either the mechanisms for embryonic signaling or the nature and significance of maternal responses. In mice it is the localized decidual transformation of endometrium, characterized in the early stages by development of the so called Pontamine Blue reaction, that is the most completely described maternal response to the embryo at the time of implantation. Although much interest has been focused on the induction of decidualization as an important example of embryonic signaling, the reaction can be elicited by a variety of nonspecific stimuli as well as by an implanting blastocyst and little progress has been made toward identifying the authentic signal. The present work was prompted by the recent finding that there are embryo dependent differences between normal implantation sites and experimentally induced deciduomata with respect to the patterns of production of uterine secreted proteins (i.e., stimulation of a subset of proteins with Mr 14,000-20,000). (Weitlauf and Suda-Hartman, 1988). It was reasoned that it should be possible to use this specific pattern of uterine protein production to dissect out and characterize the authentic signal factor from embryos. Blastocysts were recovered from delayed implanting mice after reactivation with estrogen and incubated in microdrops of tissue culture medium. The resulting embryo conditioned medium (CME) was instilled into the uteri of virgin mice that had been ovariectomized and given appropriate hormone replacement. Medium similarly conditioned by incubation without blastocysts was used as a control. The uteri were removed and incubated with 3H methionine (CME) or 35S methionine (control). The resulting samples of differentially labeled uterine proteins mixed together and subjected to two dimensional polyacrylamide gel electrophoresis (2D-PAGE). The radioactivity in individual spots was determined and patterns of relative differences in their production was determined by a dual label ratio method. It was found that the pattern of uterine protein production typically associated with development of a normal embryo dependent implantation site was induced by instillation of CME and that this response was absent with control medium. It is concluded that the mouse blastocyst, like that of the sheep, synthesizes and releases one or more soluble factors in the peri-implantation period that influence metabolic activity in the uterus.(ABSTRACT TRUNCATED AT 400 WORDS)