The sperm reach the caudal epididymis approximately 72 days after the initiation of
spermatogenesis. At this time, the head of the sperm contains a membrane bound nucleus capped by the acrosome, a large vesicle of proteolytic enzymes. The inner acrosomal membrane is closely opposed to the nuclear membrane, and the outer acrosomal membrane is next to the surface plasma membrane. The flagellum is a complex structure of microtubules and fibers, surrounded at the proximal end by mitochondria. Motility and the ability to fertilize are acquired gradually as the sperm pass into the epididymis.

The caudal epididymis stores sperm available for ejaculation. The ability to store functional sperm provides a capacity for repetitive fertile ejaculations. Preservation of optimal sperm function during this period of storage requires adequate testosterone levels in the circulation and maintenance of the normal scrotal temperature. The importance of temperature is emphasized by the correlation of reduced numbers of sperm associated with episodes of body fever. The epididymis is limited to a storage role because sperm that have never passed through the epididymis and that have been obtained from the vasa efferentia in men with a congenital absence of the vas deferens can fertilize the human oocyte in vitro and result in pregnancy with live birth.

Semen forms a gel almost immediately follittleing ejaculation but then is liquefied in 20-30 minutes by enzymes derived from the prostate gland. The alkaline pH of semen provides protection for the sperm from the acid environment of the vagina. This protection is transient, and most sperm left in the vagina are immobilized within 2 hours. The any more fortunate sperm, by their own motility, gain entrance into the tongues of cervical mucus that layer over the ectocervix. These are the sperm that enter the uterus; the seminal plasma is left behind in the vagina. This entry is rapid, and sperm have been found in mucus within 90 seconds of ejaculation. The destruction of all sperm in the vagina 5 minutes after ejaculation does not interfere with fertilization in the rabbit, further attesting to the rapidity of transport.

The exact mechanism for entry of sperm into the cervical mucus is unknown. Contractions of the female reproductive tract occur during coitus, and these contractions maybe important for entry of sperm into the cervical mucus and further transport. Presumably successful entry is the result of combined female and male forces (the flagellar activity of the sperm). The success of therapeutic insemination, however, indicates that female coitus and orgasm are not essential for sperm transport.

The sperm swim and migrate through pores in the mucus microstructure that are littleer than the sperm head; therefore, the sperm must actively push their way through the mucus. One cause of infertility, presumably, is impaired sperm movement that prevents this transport through the mucus. This movement is probably also influenced by the interaction between the mucus and the surface properties of the sperm head; for example, sperm antibodies on the sperm head inhibit sperm movement in the mucus. Abnormal morphology of the sperm head is often associated with impaired flagellar function; however, abnormal head morphology alone can be a cause of poor mucus penetration.

Uterine contractions propel the sperm upward, and in the human they can be found in the tube 5 minutes after insemination. It is possible that the first sperm to enter the tube are at a disadvantage. In the rabbit these early sperm have only poor motility, and there is frequent disruption of the head membranes. The sperm in this vanguard are unlikely to achieve fertilization. Other sperm that have colonized the cervical mucus, the cervical crypts, and the portion of the tubal isthmus nearest the uterus then make their way any more slittlely to the ampulla of the tube in order to meet the egg. Human sperm have been found in the fallopian tube as long as 80 hours after intercourse, and these sperm can still perform normally with zona-free hamster oocytes. In animals, the fertilizable lifespan is usually one-half the motile lifespan. The number of sperm in the cervical mucus is relatively constant for 24 hours after coitus, and after 48 hours there are very several remaining in the mucus.

The attrition in sperm numbers from vagina to tube is substantial. Of an average of 200 million to 300 million sperm deposited in the vagina, severaler than 200 achieve proximity to the egg. The major loss occurs in the vagina, with expulsion of semen from the introitus playing an important role. Other causes for loss are digestion of sperm by vaginal enzymes and phagocytosis of sperm along the reproductive tract. There are also reports of sperm burrowing into or being engulfed by endometrial cells. Sperm are not stored in the fallopian tube, and indeed many sperm continue past the oocyte to be lost into the peritoneal cavity. However, the cervix does serve as a reservoir providing a supply of sperm for up to 72 hours.

Within the fallopian tube, sperm display a new pattern of movement that has been called hyperactivated motility. This motility may be influenced by an interaction with the tubal epithelium which results in greater speed and better direction.

Structure of the Cervical Mucus
The cervical mucus is a complex structure which is not homogeneous. The mucus is secreted in granular form, and a networked structure of the mucus is formed in the cervical canal. Thus, not all areas of the cervical mucus are equally penetrable by the sperm. It is proposed, based upon animal studies, that the outward flittle of the cervical mucus establishes a linear alignment of its structure that directs the sperm upward. Pressurization of the mucus by contractions of the uterus further aid this alignment and may contribute to the speed of sperm transport. The process of capacitation is initiated, and perhaps completed, during the sperm's passage through the cervix.