In situ localization of mRNA for the interferon, ovine trophoblast protein-1, during early embryonic development of the sheep.

The embryonic interferon, ovine trophoblast protein-1 (oTP-1), is considered to be the major protein signal by which the developing ovine conceptus communicates its presence to the mother in order to provide extension of luteal progesterone secretion critical for the establishment of pregnancy. The objective of the present study was to examine the distribution of mRNA for oTP-1 in developing ovine embryos by using in situ hybridization. A total of 11 ovine embryos were collected on days 11, 13, 15, 17, and 23 of gestation (n = 1, 2, 3, 3, and 2, respectively) and were subjected to either immediate paraformaldehyde fixation or culture for 24 h followed by fixation. Fixed embryos were embedded in paraffin and oTP-1 mRNA levels determined by in situ hybridization with a 35S-labeled cDNA probe specific for the 3'-untranslated region of the oTP-1 mRNA. Controls included parallel hybridizations with a 35S-labeled gamma-actin cDNA to detect actin mRNA (positive control) and with 35S-labeled plasmid cDNA (negative control). Hybridization signals were detected by autoradiography and quantified by computer-assisted video image analysis. Ovine TP-1 mRNA levels in tissue were low but detectable on day 11, rose 6.5-fold to peak concentrations on day 13, and declined in a linear fashion through day 23. A low, detectable signal was present in portions of chorionic tissue on day 23. Messenger RNA was localized solely to the trophectoderm and did not appear in the extraembryonic endoderm, yolk sac, and embryonic disc. The relative hybridization signal for actin mRNA was approximately 12-fold lower than that for oTP-1 mRNA on day 13. However, by day 17 oTP-1 and actin mRNA hybridization signals were similar. In conclusion, oTP-1 mRNA is localized in the trophectoderm of the developing embryo, being produced between days 11 and 23 of gestation with peak amounts produced per cell at approximately day 13 of gestation.