EMBRYO-UTERINE INTERACTIONS IN PIGS DURING WEEK-2 OF PREGNANCY

The second week of pregnancy is a particularly critical period for emb ryonic survival in pigs. Within that time, conceptus oestrogen synthes is is initiated, spacing and final placement of conceptuses is complet ed, and the signal for extending the functional lifespan of the corpor a lutea is received by the mother. There is also a marked increase in blood flow to the uterus and the uterine endometrium produces and secr etes nutrient histotrophe. Conceptus-derived oestrogen has been implic ated in many of these events. It is also during this period that the t rophoblast elongates dramatically and the inner cell mass starts to di fferentiate into the embryo proper. Here, we critically review the evi dence that oestrogen is the sole factor initiating long-term corpus lu teum maintenance in pigs. We also review the functions and general pro perties of the major secretory proteins in histotrophe and the role of oestrogen in controlling their expression. It is now generally accept ed that asynchrony within a litter underlies much of the losses of con ceptuses that are otherwise genetically normal, but which are lagging in their development; however, the underlying mechanisms remain unclea r. Here we hypothesize that oestrogenic compounds derived from more ad vanced conceptuses or provided prematurely, either by injection or in the diet, trigger a massive increase in uterine expression and secreti on of retinol-binding protein laden with retinol. We propose that less developed, smaller conceptuses are least able to contend with the sud den exposure to this potential teratogen at a time when they are parti cularly susceptible to imbalance in retinol supply. Hence, even though their growth proceeds for a few days, their developmental potential i s irrevocably compromised.