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Title:   Author  item
                                            Vallet, Jeff
  Submitted to: Journal of Applied Animal Research
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 1/15/2000
                                    Publication  Date: 3/20/2000
                                    Citation: Vallet, J.L. 2000. Fetal erythropoiesis and other factors which influence uterine capacity in swine. Journal of Applied Animal Research. 17:1-26.Interpretive Summary: Litter size in swine is currently limited by our knowledge of the factors influencing the ability of the uterus to carry fetuses to the end of pregnancy, a trait which has been named uterine capacity. However, recent work has provided clues to various factors which may play a role in uterine capacity. The physical  size of the uterus is one factor which may limit the number of fetuses that can be carried to term. The uterus also appears to have an influence on the growth rate of fetuses. Uterine factors that can limit the growth of fetuses may also allow more fetuses to be accommodated to term. The placenta provides a mechanism whereby nutrients from the sow are transferred to the fetus. Results suggest that placenta differ in their ability to perform this function, and this may have a significant impact on litter  size. Finally, the uterine crowding which results from attempting to increase litter size of swine causes anemia (low red blood cell numbers) to occur in developing fetuses. This anemia may influence the survival of the fetuses throughout pregnancy. Red blood cell development is reliant on specific nutrients provided by the uterus and improvements in the delivery of these nutrients to the fetus may alleviate the anemia associated with uterine crowding and thus increase litter size. Thus, uterine, placental and fetal factors all combine to make up the capacity of the uterus to carry fetuses. Improvements in the capacity of the uterus, combined with methods to increase the number of eggs shed, should result in increased litter size of swine.
                                Technical Abstract: Litter size in swine is influenced by ovulation rate, fertilization rate, early embryonic mortality and uterine capacity. Current knowledge provides methods to overcome the limitations imposed by the first three factors, but the factors influencing uterine capacity and methods to improve uterine capacity are only now  beginning to be understood. Uterine, placental and fetal factors all influence the number of fetuses that can be maintained to term. Genetic changes in the length of the uterus are correlated with genetic changes in uterine capacity. Uterine and conceptus factors are capable of modifying placental and fetal growth, resulting in smaller conceptuses that take up less uterine space and require fewer nutrients during gestation. There are also periods during development during which the conceptus is more sensitive to uterine  crowding. Dramatic changes in fetal erythropoiesis are among the changes that occur coincident with periods of conceptus loss. Fetal erythropoiesis is sensitive to intrauterine crowding, resulting in a population of small fetuses with impaired erythropoiesis. Current results suggest that improved delivery of uteroferrin and folate binding protein by the uterus may improve fetal erythropoiesis, and may therefore result in lower conceptus loss. In summary, uterine, placental and fetal factors all contribute to uterine capacity, which in turn, limits litter size.


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