The profitability of dairy farming directly depends on the intensification of reproduction indicators, which requires an increase in the percentage of calves per hundred cows and optimisation of milk productivity. Given the need to improve reproductive efficiency, the aim of the study was to identify the morphological and physiological changes in the ovaries and endometrium of cows during the oestrous cycle for the further development of evidence-based methods for correcting sexual cyclicity. Analysis of histological samples revealed a number of important changes. During the oestrus phase, the endometrium shows maximum swelling and blood filling of the arterioles, while the cells of the functional layer become decidual-like due to the active growth of stromal cells. Numerous uterine glands have a convoluted shape and enlarged cavities filled with secretions, while the epithelial cells of the glands become multinucleated with vacuolisation due to the accumulation of glycogen. Primary follicles with cubic-shaped follicular cells are recorded in the ovaries, indicating active replication and remodelling processes, as cubic cells divide faster than flat ones. During proestrus, the covering epithelium of the endometrium becomes wavy, and the uterine glands, although numerous, are located next to a small number of stromal cells, do not contain secretions and have a folded shape. Vascular changes include hyperplasia of the intima of arterioles with narrowing of the lumen and obliteration of thrombotic masses, which is the beginning of the desolation of vascular territories and remodelling of the connective tissue matrix. At this stage, the ovaries contain tertiary follicles without signs of atresia and corpus luteum in the stage of resorption. The metestrus phase is characterised by the formation of large, almost completely structured corpus luteum with large lutein cell nuclei, while the corpus luteum from previous cycles show signs of degradation. During metestrus, the endometrium simultaneously shows signs of hormone-dependent desquamation and active cell proliferation. The practical value of the study lies in the creation of a reliable morphological basis that reveals in detail the mechanism of hormone-dependent changes for the development of evidence-based diagnostic protocols and methods for correcting hormonal status and sexual cyclicity
histological changes in the endometrium; ovarian follicles; ovarian reserve; corpus luteum; physiology of the sexual cycle
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