Chinese Taihu pigs are highly prolific; the Erhualian (ER), one of the Taihu pigs, is known for producing the highest recorded litter sizes in the world . Litter size is influenced by many factors, such as the boar, season, and nutrition. However, it has been demonstrated that these factors do not account for the prolificacy of Meishan pigs, which are another breed of Chinese Taihu pig . In addition, Meishan sows are little affected by the factors involved in stillbirth . Taihu pigs express a high level of maternal heterosis in litter size when used in crosses with Western pig breeds [4, 5]. Studies have indicated that the large litter sizes of Meishan pigs are due to genes acting in the dam [6, 7]. The ER sows can give birth to more than 15 piglets per litter, even when the coefficient of inbreeding is as high as 0.25 . These findings indicate that the desirable alleles related to litter size are preponderant in Taihu sows.
Embryonic loss is one of the major barriers to large litter size [2, 9]. It is estimated that approximately 20-30% of embryonic death occurs during gestation days (GD) 11–12 . The embryonic survival rate does not differ among pig breeds until GD11, but it is elevated on GD12 in Meishan pigs when compared with Landrace × Large Yorkshire (LL) pigs [7, 11]. At this stage, the blastocysts undergo dramatic morphological changes, developing from an 11–50 mm tubular structure into a 100 mm filamentous structure. The rapid changes in shape and size caused by the elongation of porcine blastocysts are not a result of cellular hyperplasis but cellular rearrangements and remodeling of the trophectoderm . These changes coincide with the synthesis and release of maternal-fetal recognition signals (estrogen) and cytokines required for the establishment of pregnancy [13–15]. Porcine conceptuses initiate the secretion of estrogen on GD10-15 , although Meishan embryos are smaller and contain fewer cells when they initiate steroidogenesis and begin to elongate . Meishan conceptuses also secrete less estrogen into the uterine luminal fluid and elongate to a reduced length  and diameter [17, 19] when compared with Large White conceptuses.
The level of estrogen in porcine uterine flush samples is determined primarily by the amount of estrogen secreted by the embryos . The estrogen level in the uterine lumen will have multiple effects on the embryonic survival rate. Firstly, the estrogen level may affect placental weight and survival of the conceptus. When Meishan gilts were treated with estrogen on GD12 or GD13, placental weights were increased significantly (P < 0.05); litter size was not affected significantly (P > 0.05) but it tended to decrease . However, others have shown that placental weights are negatively correlated with litter size (P < 0.05)  and uterine capacity at GD105  (P < 0.01) in Western breeds. The non-significant result in the former study  may have been a consequence of smaller sample size. Secondly, embryonic estrogen, as an embryo-maternal recognition signal, can change uterine secretion of histotroph . The lower amount in Meishan embryos may cause a more gradual change of the gravid uteri, which decreases the negative impact that faster-developing embryos could have on their slower-developing littermate embryos [25, 26].
Endometrial synthesis of prostaglandins (PG) is essential for the establishment and maintenance of pregnancy in pigs [27, 28]. During maternal recognition of pregnancy around GD12, PGF2α, which is synthesized mainly by the endometrium , has a luteolytic effect, while PGE2 can antagonize this effect [29, 30]. The secretion of PGF2α is redirected from the uterine venous drainage (endocrine) during luteolysis to the uterine lumen (exocrine) at the time of maternal recognition of pregnancy. Studies have shown that the PGE2:PGF2α ratio is crucial for the regulation of the estrous cycle, and the establishment and maintenance of pregnancy [31, 32]. The sum of PGE2 and PGF2α and their ratio were higher in Meishan sows than that in Large White pigs .
On GD12, the placenta (trophectoderm) has not yet formed, the conceptus is free-floating and not attached to the endometrium [10, 12], hence embryonic growth and development is dependent on histotroph in the uterine lumen. Histotroph includes hormones, growth factors, and transport proteins . The uterine histotroph is synthesized and secreted primarily by the epithelia of the maternal uteri during early pregnancy . Experiments have demonstrated that embryonic growth and development are affected by the environment of the uterine lumen [18, 36]. In the present study, we detected the differentially expressed genes in the endometrium of ER and LL pigs on GD12 by digital gene expression profiling (DGE) using an Illumina Genome Analyzer platform. This work will be helpful for understanding the molecular basis of different prolificacy between Chinese Taihu and Western pigs.