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Evidence for the presence of luteinizing hormone–chorionic gonadotrophin receptors in the pig umbilical cord

Pig umbilical cord, like that of humans, contains two arteries and a vein surrounded by Wharton's jelly with amnion covering the exterior surface. The aim of the present study was to investigate whether LH–hCG receptors are present in the pig umbilical cord, using light microscope immunohistochemistry, semiquantitative autoradiography, western blotting and reverse transcription–polymerase chain reaction. Umbilical cords were collected on days 48, 71 and 103 of fetal life (n = 6). Monoclonal and polyclonal anti-LH receptor antibodies were used to study receptor distribution. Immunoreactivity was observed in the umbilical blood vessels, the epithelium of umbilical amnion and cells in the Wharton's jelly. No differences in LH–hCG receptor distribution related to the sex of the fetus, period of fetal life or section of the umbilical cord were observed. Strong immunostaining was observed in umbilical vein and in umbilical arteries. However, in the arteries, the tunica media expressed weaker receptor immunostaining than did the tunica intima and tunica adventitia. No immunoactivity was detected in non-target tissue (skeletal muscle) but LH receptors were immunostained in the pig ovary. Topical autoradiography showed that vein and arteries in the umbilical cord bind¹²⁵I-labelled hCG, which was highly diminished after co-incubation with an excess of unlabelled hCG. The binding of¹²⁵I-labelled hCG to the Wharton's jelly and epithelial amnion was less intense than it was to vessels. Gonadotrophin binding sites were not present in the skeletal muscle. The pig umbilical arteries, vein and Wharton's jelly contained a 75 kDa immunoreactive LH–hCG receptor protein similar to that found in corpora lutea. Southern blot analysis of reverse transcription–polymerase chain reaction products, performed to enhance the sensitivity and specificity of LH receptor transcripts determination in umbilical cord tissues, revealed that the expected fragments of 740 and 470 bp were present in the arteries, vein, Wharton's jelly and corpora lutea (positive control). An additional product of 670 bp was found in the corpora lutea and arteries of umbilical cord, but not in the vein and Wharton's jelly. This is probably the first reported evidence of the presence of LH–hCG receptors in the umbilical cord of a non-human female mammal.