Embryo-uterine interactions in pigs during week 2 of pregnancy

The second week of pregnancy is a particularly critical period for embryonic survival in pigs. Within that time, conceptus oestrogen synthesis is initiated, spacing and final placement of conceptuses is completed, and the signal for extending the functional lifespan of the corpora lutea is received by the mother. There is also a marked increase in blood flow to the uterus and the uterine endometrium produces and secretes nutrient histotrophe. Conceptus-derived oestrogen has been implicated in many of these events. It is also during this period that the trophoblast elongates dramatically and the inner cell mass starts to differentiate into the embryo proper. Here, we critically review the evidence that oestrogen is the sole factor initiating long-term corpus luteum maintenance in pigs. We also review the functions and general properties of the major secretory proteins in histotrophe and the role of oestrogen in controlling their expression. It is now generally accepted that asynchrony within a litter underlies much of the losses of conceptuses that are otherwise genetically normal, but which are lagging in their development; however, the underlying mechanisms remain unclear. Here we hypothesize that oestrogenic compounds derived from more advanced conceptuses or provided prematurely, either by injection or in the diet, trigger a massive increase in uterine expression and secretion of retinol-binding protein laden with retinol. We propose that less developed, smaller conceptuses are least able to contend with the sudden exposure to this potential teratogen at a time when they are particularly susceptible to imbalance in retinol supply. Hence, even though their growth proceeds for a few days, their developmental potential is irrevocably compromised.