Wallace J M, Bourke D A, Aitken R P
Rowett Research Institute, Bucksburn, Aberdeen, UK.
J Reprod Fertil Suppl. 1999;54:385-99.
Inappropriate maternal nutrient intake at key developmental timepoints during ovine pregnancy has a profound influence on the outcome of pregnancy and aspects of postnatal productivity. However, the responses to alterations in maternal nutrition in adult sheep are often highly variable and inconsistent between studies. The growing adolescent sheep provides a new, robust and nutritionally sensitive paradigm with which to study the causes, consequences and reversibility of prenatal growth restriction. Overnourishing the adolescent dam to promote rapid maternal growth throughout pregnancy results in a major restriction in placental mass, and leads to a significant decrease in birthweight relative to moderately fed, normally growing adolescents of equivalent gynaecological age. Maternal insulin and IGF-I concentrations are increased from an early stage of gestation in overnourished adolescent dams and these hormones ensure that the anabolic drive required to promote maternal tissue synthesis is initiated at a time when the nutrient requirements of the gravid uterus are low. The major restriction in fetal growth in rapidly growing dams occurs irrespective of high concentrations of essential nutrients in the maternal circulation and suggests that the small size or altered metabolic and transport capacity of the placenta is the primary constraint to fetal growth. The decrease in placental weight in the overnourished animals reflects a significant reduction in both fetal cotyledon number and mean cotyledon weight. The role of nutritionally mediated alterations in progesterone and the components of the IGF system in this early pregnancy placental phenomenon are being investigated. Nutritional switch-over studies have demonstrated that reducing maternal nutrient intake at the end of the first third of pregnancy can stimulate placental growth and enhance pregnancy outcome, but increasing nutrient intake at this time has a deleterious effect on placental development and fetal growth.
绵羊孕期关键发育时间点母体营养摄入不当会对妊娠结局和产后生产性能产生深远影响。然而，成年绵羊对母体营养变化的反应在不同研究中往往高度可变且不一致。生长中的青春期绵羊提供了一个新的、强健且对营养敏感的模型，用于研究产前生长受限的原因、后果及可逆性。在整个孕期过度喂养青春期母羊以促进其快速生长，会导致胎盘质量大幅受限，并使出生体重相对于适度喂养、正常生长且妇科年龄相当的青春期母羊显著降低。在过度喂养的青春期母羊中，母体胰岛素和IGF-I浓度从妊娠早期就开始升高，这些激素确保在妊娠子宫营养需求较低时启动促进母体组织合成所需的合成代谢驱动力。快速生长的母羊中胎儿生长的主要受限情况无论母体循环中必需营养素浓度多高都会发生，这表明胎盘体积小或代谢及转运能力改变是胎儿生长的主要限制因素。过度喂养动物的胎盘重量下降反映出胎儿子叶数量和平均子叶重量均显著减少。目前正在研究营养介导的孕酮和IGF系统成分变化在这种早期妊娠胎盘现象中的作用。营养转换研究表明，在妊娠前三分之一期末减少母体营养摄入可刺激胎盘生长并改善妊娠结局，但此时增加营养摄入会对胎盘发育和胎儿生长产生有害影响。
