Retinol-binding protein in the baboon (Papio anubis) uterus: immunohistochemical characterization and gene expression

Retinols and retinoic acid (vitamin A) are essential for embryonic development; they are transported in circulation bound to retinol-binding protein (RBP). RBP has been shown to be synthesized in extrahepatic sites, i.e., the progestational uterus and concepti of pig, sheep, and cows. This study was undertaken to determine whether RBP is also synthesized by the primate reproductive tract. Uterine tissues were obtained from mature female baboons during the follicular and luteal stages of the menstrual cycle, after steroid (estrogen +/- progesterone) treatment of ovariectomized baboons and between Days 18 and 32 of pregnancy. Portions of the uterine tissues were either fixed in Bouin's fixative and embedded in paraffin, subjected to explant culture, or immediately frozen in liquid nitrogen for RNA isolation. Our studies indicate that RBP is present in the uterine glandular epithelium of the baboon during the late luteal stages of the menstrual cycle. Steroid hormone treatment of ovariectomized female baboons confirmed that both transcription and translation of uterine RBP is under progestational control. When uterine and placental tissues from pregnant baboons were analyzed between Days 18 and 32 of pregnancy, it was apparent that immunoreactive RBP increased in the mid and basal glands initially, but by Day 32 all of the glands stained positively. In contrast, the mRNA levels peaked at Day 25, and RBP message was more highly expressed in the basalis region of the uterus compared to the functionalis. Immunoreactivity for RBP was higher at the implantation site compared to the nonimplantation site. Neither RBP protein nor message was detectable in the baboon conceptus/placenta.