Insulin-Like Growth Factor-I Expression during Early Conceptus Development in the Pig1

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Abstract
Porcine conceptuses (embryo and associated membranes) in utero undergo developmental morphological transformations coincident with structural and biochemical changes in the uterine endometrium during early gestation. To elucidate a possible role for insulin-like growth factor-I (IGF-I) in these events, porcine endometrial (Days 8, 10, 11, 12, 14, and 30) and conceptus (Days 12, 14, and 16) tissues were characterized for the presence of IGF-I peptide and mRNAs. The corresponding uterine luminal fluids (ULF) at these stages of pregnancy were also analyzed for immunoreactive IGF-I concentration. ULF IGF-I was lowest on Day 8, highest on Day 12, and declined by Day 14. In contrast, endometrial tissue IGF-I content remained constant during this period. Conceptus tissues contained less IGF-I than endometrial tissues; however, conceptus IGF-I values were maximum on Day 12 coincident with peak values for ULF IGF-I. Dot-blot hybridization revealed temporal variation in steady-state levels of IGF-I mRNAs in endometrium. Highest levels of endometrial IGF-I mRNA were detected on Day 12 and were about 4-fold greater than on Day 30 of pregnancy. IGF-I mRNA expression in conceptus tissues on Days 12, 14, and 16 was the same and was significantly less than that in endometrium on Day 12. These results demonstrate the temporal variation of IGF-I mRNA abundance in uterine endometrium and of immunoreactive IGF-I in ULF and in conceptus tissues, with the developmental processes occurring in the conceptuses at early pregnancy. Thus IGF-I may act with other peptide growth factors present in ULF, through autocrine or paracrine mechanisms, to mediate uterine and conceptus growth and differentiation.