Transforming growth factor beta 1 expression in the endometrium of the mare during placentation

In situ hybridization, Northern blotting, and immunohistochemical techniques were used to study the expression of transforming growth factor beta 1 (TGF beta 1) in the endometrium of the mare during the first 150 days of pregnancy (term = 330-340 days). In situ hybridization using an oligonucleotide (45mer) probe, based on a homologous region within all known mammalian TGF beta 1 DNA sequences, demonstrated TGF beta 1 mRNA accumulation in the glandular and lumenal epithelial cells of the endometrium from day 33 onwards which corresponds to the time of implantation (day 33-45). Expression in the endometrium remained at a high level to the end of the sampling period (day 150). There was also marked expression of TGF beta 1 in the mononuclear cells accumulated around the periphery of the specialized trophoblast cells of the endometrial cup within the endometrium and in the mononuclear cells accumulated in the endometrial stroma of mares carrying failing donkey-in-horse pregnancies created by embryo transfer. The sense (control) oligonucleotide probe exhibited no hybridization to any tissue at any stage. Northern blot analysis demonstrated that the oligonucleotide probe and a porcine TGF beta 1 cDNA clone hybridized to a single 2.5 kb transcript in horse endometrial and lymphocyte RNA, thus validating the oligonucleotide probe for detection of horse TGF beta 1 mRNA. Furthermore, both probes demonstrated an increased signal in the pregnant endometrium from day 33 onwards, thereby confirming the in situ hybridization results. Immunostaining with a specific anti-bovine TGF beta 1 serum also showed increasing TGF beta 1 accumulation in endometrial epithelia during pregnancy, and localization of the protein in endometrial stroma and in the trophoblast layer of the placenta after day 60 of pregnancy. These results show that TGF beta 1 expression increases in the maternal endometrium of the mare at the time of implantation and that it may play a role in regulating endometrial and/or trophoblast growth and differentiation during placentation in this species. It may also influence fetal development (via placental transfer) at a later stage of gestation.