Uterine function in Meishan pigs.

The Meishan pig provides a biological model with the genetic capacity to express a high prolificacy. This prolificacy can be partially attributed to a higher ovulation rate and a higher rate of prenatal survival at a given ovulation rate throughout gestation than in European breeds. Both early embryonic survival (factors inherent to the ovum and uterus, which occur before day 25 of gestation) and uterine capacity (factors inherent to uterine limitation, which occur from 30 days of gestation to parturition) may contribute to prenatal survival. Crossbreeding studies show that the prolificacy of Meishan pigs is primarily of maternal origin, but not whether the effects occur via the ovum, uterus or systemic factors. Although there have been few comparative studies on uterine function for prolific pigs, experiments relating to three general areas have been reported. During neonatal uterine development, endometrial gland development occurs shortly after birth in Meishan and European breeds. Tissue culture for 24 h showed that secretion of at least three uterine proteins (one identified as retinol-binding protein) increased in clear temporal association with development of the endometrial gland. The secretion of two additional proteins appears to change in a breed-specific manner. Although regulatory roles for uterine proteins remain to be defined, biochemical events associated with neonatal endometrial gland development may affect subsequent adult uterine function. During the time of maternal recognition of pregnancy or early gestation, reports suggest that uterine secretions are enhanced in Meishan pigs, which may partially explain their increased prolificacy. However, other studies show smaller embryo and placental weights at day 30 of gestation, suggesting that prolificacy of Meishan females is related to a maternal ability to control embryo/fetal and placental growth. During the later fetal period (uterine capacity) in European pigs, endometrial response to a crowded uterine environment is minimal, but the conceptus responds to a crowded uterine environment by altering placental function; similar experiments using Meishan pigs are in progress. Once the regulatory determinants of uterine function are understood, improvements can possibly be made in the reproductive efficiency of all swine.