We studied peculiarities of morphofunctional organization of the immune system in C57Bl/6g and CBA mice differing by their susceptibility to various types of infectious agents. The revealed differences in the structure of lymphoid organs, T lymphocyte subpopulation ratio and their differentiation into Th1/Th2 cells after mitogen stimulation drove us to a conclusion on genetically determined regularities in the development of the immune response in these animal strains.

Elimination of "excessive" myocytes and their structures during involution of the myometrium after the first and third pregnancies was realized by clasmocytosis (eliminating the greatest volume of myocyte cytoplasm fragments), apoptosis, and necrosis (equal percentage by volume). In contrast to the first pregnancy, involution after the third one was not over by day 10 because of inhibited elimination of functionally lost myocytes by necrosis and apoptosis mechanisms. Presumably, this was caused by slower hydrolysis of apoptotic bodies by macrophages. The concentration of macrophages in the myometrium on day 10 of the involution period in females after the third delivery was 4-fold higher than in intact mice and in females after the first delivery during the same period.

Myocyte hypertrophy and proliferation in the myometrium were observed in mice during the first pregnancy and involution of the myometrium was completed by day 10 postpartum. Parameters of all structures returned to values observed in intact uterus. During the third pregnancy, the processes of myometrium hypertrophy were similar to those during the first pregnancy, but the intensity of proliferation was higher against the background of poorer vascularization, which led to destruction of some myocytes. Postpartum involution of the myometrium in these mice was not completed by day 10 after delivery. We observed hypertrophy and proliferation of some myocytes followed by their destruction, decrease in vessel number, and many-fold decrease in collagen content in the myometrial interstitium. The number of fetuses in the litter decreased after multiple pregnancies.

Numerical densities of the nuclei were morhometrically evaluated in all myocytes and myocytes expressing nuclear estrogen- and progesterone-receptor complexes, which were revealed immunohistochemically with monoclonal antibodies in C57Bl/6 mice. It was shown that the above quantitative parameters of myometrial cells after the first pregnancy were similar to those in nonpregnant mice by day 10 after delivery. In the third pregnancy, especially developed after the second interrupted pregnancy, proliferation processes in the myometrium were not completed by postpartum day 10, but dramatically progressed. It was associated with a significant decrease in the fraction of myocytes carrying nuclear hormone-receptor complexes with estradiol and progesterone and their disturbed physiological relations in the myometrium during and after pregnancy probably due to dedifferentiation of a considerable part of myocytes.