The preparation of the female body to pregnancy is determined not only by sex hormones but also by many biologically active substances, among which serotonin plays an essential role. It has been established that during pregnancy, the amount of serotonin increases gradually in the placenta contrary to monoamine oxidase activity, which decreases. The disturbance of biogenic amines metabolism is quite common during pregnancy and is the one which links it to the pathogenesis of toxicosis. The aim of the study was studying of the organ topography, morphology and histophysiology of diffuse endocrine system cells of rat uterus during gestation and postnatal involution of the uterus. Materials and methods. The experiments were performed on laboratory white female rats with a body weight of 160-180g, maintained in the standard vivarium conditions. The experimental material was divided into 8 groups of 4-16 animals each. We studied the rat uterus in pregnancy (days 12-14) and after delivery (days 1, 3, 4, 6, 10 and 15 postnatal). Results and discussion. In the specimens with pregnant uterus, the fluorescent cells were characterized by a small sized granules and their smaller amount. In addition, the number of extracellular granules was much reduced. Thus, we can conclude that during pregnancy both the synthesis and the releasing of serotonin into the extracellular space decrease. In the endometrium on the first postnatal day, the fluorescence cells were detected in a very low amount with much reduced size. Their cytoplasm contained few weakly fluorescent granules. Consequently, we observed an increased amount of extracellular granules. The fluorescence intensity of these granules was much increased, which indicated the presence of a large amount of extracellular serotonin. We also observed that the cell volume increased, reaching on the 3rd day the point of 949,6±57,53 mkm³. At the same time, the amount of granules in cells increased. Moreover, in the endometrium enhanced the proliferation of DES cells, serotonin synthesis and accumulation processes. On the 4th postnatal day there was a considerable increase in cell population density, which consisted of 1721,66±127,92 cells per 1mm². The number of intracellular granules was also increased. On the 6th day after delivery, the cell population density decreased slightly and was 1578,1±65,43 cells per 1mm². Intracellular granules were diminished and fluorescence intensity was low. On the 10th postnatal day, the density of serotonin secreting cells was slightly increased in the endometrium comparatively with the 6th postnatal day, reaching 1702,88±41,83 cells per 1mm². The amount of intracellular granules also increased. The highest level of DES cell was on the 15th day after delivery, constituting 2267.1 ± 76.2 cells per 1 mm². Based on the obtained data, we can emphasize the role of serotonin in the uterine labor and in the postnatal uterine involution. These data correlates with the results obtained by the other authors. Once the serotonin is released into the uterine tissues, it is involved in triggering of uterine labor. Immediately after delivery, the serotonin is involved in the hemostasis process by constriction of the uterine vessels, subsequently in the postpartum uterus involution. Serotonin plays an important role in initiating massive degradation of collagen in the postnatal period. Conclusion. The lowest content of intracellular serotonin and serotonin secretion index was recorded in gestation. These facts show low intensity of synthetic and secretory processes. In postnatal involution of uterus, endometrial endocrine cells undergo the following phase changes. On the first day there was a severe decrease in the population of endometrial serotonin secreting cells as a result of serotonin secretion intensification in the delivery process. During the 4th day the serotonin secretion prevail its synthesis and intracellular accumulation. Starting with the 4th day increases synthesis and intracellular accumulation of serotonin with parallel diminution of serotonin secretion. The restoration of initial levels of serotonin takes place on the 15th postnatal day.
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