Transfer of a uterine lipocalin from the endometrium of the mare to the developing equine conceptus

One of the major, progesterone-dependent proteins secreted into the uterine lumen of the mare is a 19-kDa lipocalin (P19). It associates strongly with the embryonic capsule that envelops the young horse conceptus in early gestation, suggesting that it may be involved in sustaining early development. However, it was not known whether the protein was transported through the capsule and/or trophoblast layer and into the yolk sac cavity. To address this question, polyclonal antisera were raised against a C-terminal peptide (based on the deduced amino acid sequence of P19) and a recombinant-derived P19 fusion protein. The antiserum raised against the C-terminal peptide recognized P19 on Western blots of denatured uterine secretions (subjected to SDS-PAGE), but it did not bind to the protein in tissue sections. However, the antiserum raised against the recombinant-derived fusion protein recognized P19 both on Western blots and in histological sections. Western blot analysis of tissues and fluids collected from early-pregnant mares demonstrated significant quantities of P19 in the endometrium and uterine secretions and in the embryonic capsule, the chorion, and the yolk sac fluid, showing that the protein is transferred through to the developing embryo. Concentrations of immunoreactive P19 declined during gestation so that, by Day 30, it had virtually disappeared from both maternal and fetal tissues and fluids. Immunohistochemical staining of endometrial biopsies collected during early pregnancy localized P19 to the glandular and luminal epithelia and to the lumina of the endometrial glands. The capsule and the trophoblast layer of the chorion from early (Days 16-17) horse conceptuses also stained positively with localization of P19 to the apical surface of the trophoblast cells. There was no detectable staining either in or on the embryonic disc. The presence of P19 in both the trophoblast layer and the yolk sac fluid suggests that P19 passes into the yolk sac fluid via trophoblast cells.