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The purpose of the study was to determine and compare the secretion of RBP by bovine spherical, elongating and filamentous conceptuses, and to identify the cellular location of RBP in developing conceptuses by immmunocytochemistry. Bovine conceptuses were removed from the uterus between days 13 and 22 of pregnancy. Events of early bovine embryonic development were observed. The conceptuses underwent a transformation from a spherical to a filamentous morphology during the periattachment period of placentation. Isolated conceptuses were cultured in a modified minimum essential medium in the presence of radiolabeled amino acids. Presence of retinol-binding protein (RBP) in culture medium was determined by immunoprecipitation using bovine placental anti-RBP serum. Presence of immunoreactive RBP in detectable quantities in spherical blastocyst (day 13) culture medium was evident. Increased amounts of RBP were clearly detected in cultures on days 14 and 15, the time of elongating conceptuses. RBP was abundant in cultures on day 22, when the conceptuses were filamentous. Cellular sources of RBP in day 15 and 22 conceptuses were determined by immunocytochemistry with anti-RBP serum. Strong immunoreactive RBP was localized in trophectoderm of day 15 conceptuses, and in epithelial cells lining the chorion and allantois of day 22 conceptuses. RBP originating from the conceptus may serve to transport retinol locally from the uterus to embryonic tissues. Secretion of RBP by spherical blastocysts (day 13) suggests that retinol is needed for initiating the early events of embryonic development. Increased secretion of RBP on days 14 and 15, the time of elongating conceptuses, was coincident with the timing of rapid trophoblast hyperplasia and remodeling. Greater secretion of RBP by the filamentous conceptus (day 22) coincided with the timing of differentiation of the chorion and allantois as well as growth of the embryo proper. Production of RBP by the developing conceptuses may be indicative of an important role for conceptus RBP in the transport, availability and metabolism of retinol during early pregnancy.

Secretion and Immunolocalization of Retinol-Binding Protein in Bovine Conceptuses during Periattachment Periods of Early Pregnancy.