Due to the fact that sperm cells accumulate on surfaces26 they enter the microchannels mostly from the?upper and lower walls of the chambers (Fig.?1b). up to 40%. We propose that Dorzolamide HCL this is caused by the combination of cell-cell collisions in the corners of the microchannel and the existence of morphologically different spermatozoa: slightly asymmetric cells with trajectories curved left and the symmetric ones, with trajectories curved right. Our findings suggest that cell-cell interactions in highly folded environment of mammalian reproductive tract are important for spermatozoa swimming behavior and play role in selection of highly motile cells. optical coherence tomography17. Female reproduction tract has a complex 3-dimentional structure with large surface-to-volume ratio covered with cilia2,18,19. Between the secondary folds of mucosa of the tract, sperm can be stored for many hours waiting for ovulation20. Recent experiments on spermatozoa of various types of mammals and bacteria, as well as numerical simulations show that hydrodynamic interaction of cells with a solid boundary leads to their surface accumulation21C25. In most cases, the flagellum-beating pattern near a surface remains three-dimensional with helical structure26. Despite the fact that for all human sperm cells, the direction of flagellum beating plane rotation seen from head-on is counter clockwise, there are two kinematically distinct swimming states differing in the cells turning direction against the fluid flow due to rheotactic behavior27. The cell turning direction strongly correlates with the angle between the midpiece and the head of the cell. Moreover, cells located at a distance of less than 1 m from the surface can switch into a two-dimensional slither swimming mode, which is distinguished by the planar flagellum beating in the boundary plane28. For human sperm, such a swimming mode can only occur if viscosity of the medium is more than 20 mPa*s. In the presence Dorzolamide HCL of multi-scale surfaces, cells can concentrate near structures with lower dimensions. In rectangular microchannels, they concentrate in the corners due to interaction with the walls29,30. By designing channels of a particular geometry, it is possible to control the swimming direction hence arranging cell flows5. The ability of spermatozoa to move along the surface boundaries, as well as against the fluid flow, correlates with a low level of DNA fragmentation31,32. Nowadays, this phenomena is being used for development of microfluidic devices for cell selection for IVF or ICSI procedure33C35, Dorzolamide HCL as Dorzolamide HCL it has been shown that low DNA fragmentation and high swimming velocity of sperm cells correlate with success of IVF and ICSI36,37. Recent studies show that selection of motile spermatozoa in the sorting microfluidic devices displays significant advantages compared to traditional methods of swim-up and centrifugation in a density gradient38C40. Due to the fact that millions of cells are injected during coitus into a female reproduction tract, spermatozoa rarely swim alone. Thus, it is important to study cell-cell interactions and their influence on the navigation process. The fluid velocity field created by a sperm cell in bulk or near a wall is similar to that generated by a bacteria E.coli and can be described by the force dipole approximation with a good accuracy41,42. Superposition of fluid flows induced by two cells cause hydrodynamic dipole-dipole interaction between them. At high concentrations, this leads to synchronization of the flagella beating and the emergence of collective behavior43C45. In case of sea urchins spermatozoa, cell-cell interaction leads to formation of self-organized vortex arrays46, and in case of ram spermatozoa and E.coli C to formation of self-organized direct flows in microchannels47. Nevertheless, it is not completely known how the presence of different types of kinematic swimming modes of the cells, characterized by different types of trajectories and flagella beating patterns, affects cell-cell interactions and navigation in a female reproduction tract. Understanding the influence of cells kinematic states on collective behavior is difficult due to high concentration of cells when such behavior starts playing significant role48,49. In this work, we used sperm accumulation in the corners of rectangular microchannels29 to study cell-cell interactions. The total concentration of cells in the channels was relatively low, thus it was possible to track individual cells on the lower wall (the wall closer to the objective of an inverted microscope) and divide them into two kinematic states, which are cells turning left and cells turning right if observed from a direction normal to the wall. Ctgf Our results show that cells moving in opposite directions organize into four-lane flows with similar structure, which has been universally observed in all microchannels connecting sperm-filled chambers. This self-organisation strongly depends on.