The role of the conceptus in determining placental efficiency and the capacity of the uterus

摘要

Uterine capacity has been used to describe simply the number of conceptuses that can be gestated until the term of gestation. Furthermore, there has been widespread acceptance of the concept that ovulation rate and uterine capacity are the two major determinants of litter size. Placental efficiency has emerged in the last decade as a novel component of litter size and may impact both postnatal survival and growth. Grossly, placental efficiency has been described as the weight of the neonate (or foetus) divided by the weight of the placenta. Although numerous reports have indicated the strong positive relationship between foetal weight and placental weight, variation in placental weight does not completely describe foetal weight. It has been proposedthat the residuals from the line describing the relationship between foetal weight and placental weight reflect variation in placental efficiency. It has been argued that this measure provides some indication of the capacity of the placenta to maintaingrowth of the foetus throughout gestation. It also predominantly reflects variation in placental weight and not variation in foetal weight. In numerous reports the negative correlation between placental weight and placental efficiency has been describedincluding as early as day 25 of gestation. The Meishan breed exhibits a very high placental efficiency (nearly three to four times that of controls) and this appears to play a role in this breed's prolificacy. Tremendous variation in placental efficiencyexists both within and among litters of commercially relevant breeds and lines. A significant amount of the variation in placental efficiency can be described by the density or number of blood vessels in the placenta and to a lesser extent the adjacentendometrium. Litde to nothing is known about how specific transport mechanisms may influence placental efficiency and this is an area of research ripe for investigation.