Mechanisms mediating nutritional effects on embryonic survival in pigs.

The inconsistency of data from experiments designed to show nutritional effects on embryonic survival is perplexing. However, a number of experimental models have provided some insight into the mechanisms that potentially mediate interactions between nutrition, metabolic state and embryonic survival. The developing ovarian follicle provides the maturational environment for the oocyte, and differences in follicular maturation are associated with differences in the ability of these follicles to support oocyte maturation. In turn, the rate of oocyte maturation and the maturational state of the oocyte immediately before ovulation may contribute to differences in embryonic survival. Therefore, evidence that nutritionally induced differences in metabolic state affect follicular development and the maturational state of the oocyte in the late follicular phase may constitute a mechanism by which nutrition affects the very early stages of conceptus development. Once ovulation occurs, the process of fertilization and early cleavage occurs in the environment of the oviduct. Nutritional state might affect the secretory and motile activity of the oviduct both directly, by influencing the physiology of the oviductal cells, or indirectly by affecting the secretion of key regulatory hormones. Thus evidence for nutritionally dependent effects on plasma progesterone concentrations in early pregnancy and associations with differences in embryonic survival may be partly mediated at the oviductal stage of development. Nutritional effects on circulating progesterone concentrations may also affect the uterine environment. However, the metabolic state of the gilt or sow, or specific nutrients in the diet, may directly affect the integrity of the endometrium and thus affect embryonic survival at this stage of development.