The influence of uterine function on embryonic and fetal survival

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 uterinesecreted 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 en