ABSTRACT The secretion rate of growth factors and the delivery rate of nutrients by the uterus to the conceptus affects the growth rate, development, and survival of the conceptus. For most growth factors and nutrients, passage into the uterus is not simply controlled by diffusion. Many growth factors are products of uterine tissue. Transport of some nutrients is aided by specific transporter molecules on the uterine endometrial epithelial cell, whereas others (e.g., retinol, iron, and folate) are incorporated into uterine-secreted proteins (e.g., retinol binding protein, uteroferrin, and folate binding protein). The rate of production of these proteins during pregnancy profoundly affects pregnancy outcome. Uterine gland knockout experiments in sheep demonstrate that pregnancy fails in the absence of uterine glands, the source of many uterine-secreted proteins. In pigs, both global and specific effects of uterine products on aspects of conceptus development can influence litter size. The provision of growth factors and nutrients by the uterus plays a role in entraining conceptus development, so that the uterine environment and the developmental stage of the conceptus match. This uterine-dependent control of conceptus development influences pregnancy success. For example, the uterus of the Meishan pig secretes less protein prior to elongation, which slows conceptus development, results in smaller placentas and smaller fetuses, and allows for greater litter size. Furthermore, in Occidental pig breeds, an earlier rise in progesterone at the beginning of pregnancy accelerates the onset of protein secretion, increases estrogen secretion by the conceptus, increases the size of the fetus in later pregnancy, and decreases litter size. Studies of fetal erythropoiesis also indicate that specific uterine products (uteroferrin and folate binding protein) are required for this important aspect of fetal development and that greater litter size is associated with improved erythropoiesis. Thus, manipulation of uterine function can modify conceptus development and affect pregnancy success in domestic livestock.
Key Words: Embryo, Fetus, Mortality, Perinatal, Progesterone, Stillbirth
Ö2002 American Society of Animal Science. All rights reserved.
J. Anim. Sci. 80(E. Suppl. 2):E115-E125
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