DEVELOPMENT OF THE PERINEUM STRUCTURES IN THE EMBRYONIC AND PREFETAL PERIODS

Traditionally, it is believed that the division of the cloaca into the ventral genitourinary pert and the dorsal - ureteric part occurs at the expense of the urorectal septum during the fourth week of intrauterine development. The separation of the cloaca finally ends during the sixth week of fetal development, with the fusion of the urorectal septum with the cloaca membrane. During the seventh week, both departments become separate entities. However, a significant cohort of authors question the traditional view of the mechanisms of cloacal separation.Purpose to establish and visualize the mechanism of cloaca division; to resolve the controversy regarding the role of the urorectal septum in the separation of the cloaca; to reveal the beginnings of future organ complexes of the male perineum.Material and method. Series of sections of 15 embryo preparations (4-7 weeks of gestational age; 4.5-13.5 mm. PCL) and 15 preparations of pre-fetus (7-9 weeks of gestational age; 14.0-35.0 mm. PCL) were studied by the methods of morphological research: microscopy, morphometry. In order to more accurately visualize and understand the processes of formation of the structures of the male perineum, the method of three-dimensional visualization was used.The results of own research. We found a significant number of mitotic divisions in the dorsal and lateral parts of the mesodermal tissue around the cranial part of the cloaca, but the absence of mitotic divisions was observed in the cells of the mesodermal tissue of the urogenital septum. The urorectal septum becomes visible after the cloacal folds separate the cloaca from the hindgut, distal to the junction between the mesonephric ducts and the cloaca. We established that from the 36th day of intrauterine development, an abdominal cavity is formed, which is located inside the urorectal septum and extends in the caudal direction distally from the point of entry of the mesonephric ducts to the middle of the distance between the entrance of the mesonephric ducts and the caudal top of the urorectal septum. On the 41st day of intrauterine development, parts of the mesonephric ducts, which are located between the cloaca and the ureteric buds, gradually disappear. Thus, both ureters and mesonephric ducts have a common entrance to the genitourinary sinus. Taking into account our observations, it should be noted that the division of the cloaca is a direct consequence of the growth and increase in the volume of the mesenchymal tissue that surrounds the cloaca and, especially in the later stages, its ventral part - the genitourinary sinus. During the 6-9th week of intrauterine development, the genitourinary sinus and urinary bladder are formed along with the demarcation of the mesonephric ducts and ureters.Conclusions. We identified three main structures that indirectly affect the division of the cloaca: after the formation of the urorectal septum, a decrease in the frequency of mitotic divisions of the latter is noted, which indicates its temporary and initial role in the division of the cloaca. Disproportionate growth and increase in the volume of mesenchymal tissue of the ventral part and dorsal part of the cloaca significantly affects the change in its shape and its division. The urorectal septum is not a single, separate structure and extends laterally. Finally, role of the abdominal cavity in the distribution of the cloaca is uncertain.