Increased litter size and within-litter uniformity in birth weight would improve pig reproductive efficiency. This study compared the location and gene and protein expression of secreted phosphoprotein 1 in placental and uterine tissues supplying a normally sized and the smallest fetus carried by hyperprolific Large White and Meishan gilts on Days 41-42 of pregnancy. Immunohistochemistry and in situ hybridization showed that the protein and gene encoding secreted phosphoprotein 1 were located in the glandular and luminal epithelium of the endometrium and in the placenta. Secreted phosphoprotein 1 protein levels were higher in glandular epithelium, luminal epithelium, and placenta from Meishan gilts compared to corresponding tissues from hyperprolific Large White gilts. Reverse transcription quantitative PCR demonstrated secreted phosphoprotein 1 mRNA levels were higher in endometrium, but not placenta, from Meishan compared to hyperprolific Large White gilts. In hyperprolific Large White gilts, secreted phosphoprotein 1 protein levels were higher in glandular epithelium and placenta surrounding small fetuses than corresponding tissues supplying normal-sized fetuses. Similarly, in Meishan gilts, secreted phosphoprotein 1 protein levels were higher in luminal epithelium surrounding small compared to normal-sized fetuses. Within hyperprolific Large White, but not Meishan, gilts secreted phosphoprotein 1 mRNA was higher in endometrium surrounding the normal-sized fetus than the control fetus. The contradictory relationship between fetal size and secreted phosphoprotein 1 protein and mRNA in the hyperprolific Large White is intriguing and may reflect breed differences in posttranslational modification. The striking breed differences in secreted phospoprotein 1 expression suggest that SPP1 may be associated with placental efficiency.
Keywords: fetus; pig; pregnancy; secreted phosphoprotein 1.