Idiopathic infertility: Survival and function of sperm in the female reproductive tract

Abstract

Fertilization is undoubtedly one of the most complex and tightly regulated biological events in life. Understanding both the cellular and molecular mechanisms that underlie successful fertilization has great implications toward the etiology of human infertility, and grasping the role and function of spermatozoa in this regard may present a potential in the field of idiopathic male infertility. Spermatozoa are deposited in the female reproductive tract during intercourse, where they have to gain both motility and fertilizing ability in a foreign and hostile environment. The spermatozoon’s ability to fertilize an oocyte is gained through a series of events, including capacitation and the acrosome reaction, which are initiated at very strategically specific points in the female reproductive tract, to ensure that fertilizing capability is maintained until the oocyte can be reached. The journey to the oocyte is an arduous one, and the spermatozoa must complete it independently and with very limited resources, as they are not directly connected to the blood supply of the female. In the female reproductive tract, spermatozoa encounter obstacles at practically every point they reach. However, evolution has provided the physiologically superior spermatozoa with the means to overcome these challenges. In the vagina, spermatozoa must face gravity, an acidic environment, as well as an influx of female immune cells, all of which is overcome by the coagulation of the seminal plasma in the anterior vaginal regions close to the cervical opening. Entrance to the cervix can only be gained around the time of ovulation, but once inside, the mature and vigorously motile spermatozoa are favored and protected from microbes in the deep cervical channels, as they make their way to the uterine cavity. In the uterus, spermatozoa are swept toward the fallopian tubes mainly by smooth muscle contractions of the uterine wall. The fallopian tubes provide a safe haven for spermatozoa, due to the absence of female immune cells and the existence of a functional sperm reservoir intended to maintain spermatozoa in a fertile state until the oocyte can be reached and fertilized.