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In humans cervical ripening is an inflammatory reaction accompanied by an infiltration of white blood cells and the remodelling of the extracellular matrix. Similar changes occur in guinea-pigs during cervical ripening. Nitric oxide (NO) is thought to be important in the maintenance of pregnancy because it is synthesized by the uterus and inhibits contractility. Previous studies in rats also demonstrated that an NO generating system is present in the cervix and is up-regulated during labour. We studied the local effect of the NO donor sodium nitroprusside (SNP) on cervical ripening in guinea-pigs during advanced pregnancy. SNP (5 mg/injection) was administered into the cervical canal in 0.2 ml phosphate-buffered saline containing 3% hydroxycellulose twice a day either for 1 [on day 42 post coitum (p.c.)] or 2 consecutive days (days 42-43 p.c.; term day 67 + 3 p.c.). The effects were assessed 24 h after treatment by both extensibility measurements (force resistance to incremental stretch) and morphological evaluation (light and electron microscopy after in-situ fixation). The control animals were treated with the vehicle. In another experiment, the guinea-pigs were subcutaneously (s.c.) treated on day 43 p.c. with either the progesterone antagonist onapristone (3 and 10 mg/animal s.c.) or with prostaglandin E2 (PGE2) (1 and 3 mg/animal s.c.) and the PGE2 analogue sulprostone (0.03 and 0.1 mg/animal s.c.). The cervical extensibility was measured 24 h later. One-day SNP treatment tended to reduce cervical resistance, but not significantly, whereas 2 day treatment with SNP led to a significant increase in cervical extensiblity (P < 0.05) with little effect on cervical dilatation (indirect evidence of the absence of uterine contractions). The effects on cervical resistance were comparable to those achieved with 10 mg onapristone and high-dose prostaglandins (PG)s (3 mg PGE2 and 0.1 mg sulprostone) treatment. An electron microscope study of the SNP-treated animals revealed a dissolution of collagen fibres, stromal oedema, arterial dilatation, and the infiltration of macrophages, lymphocytes and granulocytes. Numerous mast cells were also present. The morphological effects of SNP were similar to those observed during normal cervical ripening at term. We conclude that the local application of a NO donor effectively induces cervical ripening without inducing labour in pregnant guinea-pigs.

Expression of connexins, the proteins which comprise gap junction channels, is regulated by ovarian hormones in the female reproductive tract of rodents. In order to determine if these hormones also affect connexin expression in the human uterus, the distribution patterns of different connexins (cx26, cx32, cx43) were investigated by immunohistochemistry in human endometrial tissue collected throughout the menstrual cycle. During the early proliferative phase of the cycle extremely low staining for connexin 43 was observed and connexin 26 antigens could not be detected. An increase in the amount of connexin 43 in stromal cells and of connexin 26 in glandular and luminal epithelial cells was seen from days 11-15 of the cycle. Following ovulation, the expression of both connexins was suppressed and was completely abolished in the late secretory phase. Weak staining for connexin 32 was found mainly in the late proliferative and the early secretory phase and was restricted to the basal membrane region of the glandular cells. These results suggest that the different connexins could represent cell biological markers for the proliferation and differentiation of the human endometrium throughout the menstrual cycle.

Endogenous nitric oxide (NO) can be synthesized by endothelial cells and can act as a potent vasodilator. We investigated endothelial nitric oxide synthase (eNOS), one of the three different enzymes responsible for the synthesis of NO by immunohistochemical methods throughout the menstrual cycle on 34 endometrial samples and compared its detection with the von Willebrand Factor (vWF) as a reliable marker molecule of the endothelium on serial sections. Immunoreactivity for eNOS was clearly localized in various types of arterial and venous endothelial cells as well as in capillaries. In addition, in some samples there was a positive staining in endometrial glandular epithelium. There was no staining in endometrial fibroblasts or in myometrial smooth muscle cells. Whereas the endothelium was constantly stained by the monoclonal antibody against vWF, eNOS was not always expressed in the endothelial lining of the vessels during the menstrual cycle. The number of vessels positively stained for eNOS increased gradually during the proliferation phase and most of the vessels were positive in the early secretory phase. These results suggest that its markedly increased expression during the early secretory phase of the menstrual cycle indicates a physiological significance.

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