Secretory and morphological changes in the baboon (Papio anubis) uterus and placenta during early pregnancy

The study was undertaken to characterize changes in uterine endometrial secretory activity and morphology in the baboon between Days 18 and 32 of pregnancy. Proteins synthesized by the implantation site endometrium (RI) and non-implantation site endometrium (RIII) and developing placenta were analyzed by two-dimensional PAGE and fluorography. The electrophoretic mobility and localization of chorionic gonadotropin (CG) in the baboon placenta, and insulin-like growth factor binding protein-1 (IGFBP-1) and retinol binding protein (RBP) in the baboon uterus were confirmed by Western blot analysis and immunocytochemistry. Placental protein synthesis was primarily characterized by the presence of radiolabeled CG at Days 18 and 25, which decreased by Day 32. Peak CG synthetic activity, immunolocalized to the syncytiotrophoblast, occurred during the formation of tertiary villi. Also associated with the development of mature villi was a corresponding increase in a basic protein (M(r) 23,000). The overall secretory pattern of the baboon endometrium was similar to that observed during the midluteal stage; however, the synthetic activity was enhanced during pregnancy. Radiolabeled and immunoreactive IGFBP-1 and RBP were detectable in the Day 32 functionalis and basalis, respectively, and their synthesis was higher in RI compared to RIII. The immunolocalization of IGFBP-1 in RI appeared to be directly related to the degree of decidualization. In addition, a novel group of low molecular-weight (M(r) 27,000-30,000) isoelectrovariant proteins was secreted predominantly by the basalis at Days 18 and 25 of pregnancy and disappeared by Day 32 in all but one animal. The synthesis of this group of proteins did not show regional specificity. These studies indicate that the baboon endometrium undergoes morphological and secretory changes during pregnancy that are associated with the development of a functional placenta and decidualized endometrium. Furthermore, these studies suggest that the conceptus/placenta may mediate specific changes in IGFBP-1 and RBP expression at the implantation site.