Immunohistochemical localization of placental lactogen in binucleate cells of bovine placentomes.

Abstract

Bovine placental lactogen (bPL) has been isolated from bovine trophoblast and characterized as a 32 K mol wt protein which exists in three different forms which differ in their isoelectric point values and their amino acid compositions. Two of the three forms have been shown to have both bovine GH (bGH)- and bovine PRL (bPRL)-like activities equal on a molar basis to bGH and BPRL in radioreceptor assays. It has been postulated that, in sheep, PL is delivered to the maternal circulation by the migration of fetal binucleate cells from the trophoblast across the fetal-maternal boundary into the uterine epithelium. To determine whether an analogous situation exists in the cow, antibodies to bPL were used to localize bPL in bovine placentomes and to measure its concentration in fetal and maternal sera. For cytology, bPL was localized on sections of placentomes from midgestation and term bovine placentas using an indirect immunoperoxidase technique. Stained binucleate cells were demonstrated throughout the trophoblast, often in close association with the microvillous boundary which separates the trophoblast from the maternal epithelium. In cross-sections of fetal villi, binucleate cells with cytoplasmic processes extending into and through the uterine epithelium were immunostained as well as cells within the plane of the uterine epithelium in close approximation or apposition to the maternal basement membrane. RIA demonstrated bPL to be present in maternal sera in concentrations of 1-2 ng/ml and in fetal sera at 5-12 ng/ml. These data are consistent with the hypothesis that binucleate cell migration accomplishes the delivery of bPL to the maternal circulation.