The Epididymis

Spermatozoa in the unobstructed testis are not motile and are incapable of fertilizing ova. Spermatozoa become functional gametes only after they migrate through the epididymis and undergo an additional maturation process, thereby acquiring the capacities for both progressive motility and fertility. The function of the obstructed epididymis and its effects on maturation of spermatozoa may be very different from what is observed in the unobstructed state. Anatomically, the epididymis can be divided into three regions: the caput, the corpus, and the cauda epididymis. However, these anatomical divisions have been defined based on findings in animals, not in humans. The human epididymal epithelium is relatively homogeneous as viewed under the microscope, and grossly, the epididymis does not have the same distinct gross anatomical subdivisions that are easily seen in the rat, rabbit and other animals. Unfortunately, there is little information available regarding the functional diversity of these three regions of the human epididymis. The data that exist on human epididymal function are almost entirely derived from observations of men after relief of chronic epididymal obstruction.

In humans, the epididymis receives its blood supply from two sources: branches of the testicular as well as the deferential vessels that travel along the vas deferens. Clinically, the blood supply becomes important after a vasectomy with interruption of the vasal vessels. At the time of vasectomy reversal, the testicular side of the vas deferens is then supplied by branches off the testicular vessels, which travel either through collaterals, or along the entire length of the epididymis. Similarly, if division of the vasal vessels occurs accidentally during vasography, and a secondary obstruction to the vas deferens (and vasal vessels) is found and addressed in the groin, then the intervening segment of vas deferens can be totally devascularized. The viability of the vas should be considered prior to vaso-vasostomy in this situation.

Observations by Silber regarding the fertility of men who have undergone bilateral vasal anastomosis to the vasa efferentia, indicate that in the obstructed human male reproductive tract, some sperm may acquire motility and fertilizing ability without passing through the epididymis. Although not absolutely required for male fertility, the functional importance of the human epididymis is strongly supported by several other observations regarding men who have undergone intervention for the relief of chronic epididymal obstruction. In 1990, Silber reported fertility results for men who underwent vasoepididymostomy at the level of either the caput or corpus epididymis. For both groups of men, the patency rate was approximately 70%. For men who underwent anastomosis at the corpus level, 72% of men achieved pregnancy with their partners, whereas only 43% of men were able to impregnate their wives after vasoepididymostomy at the caput level. Therefore, the presence of a longer length of epididymis appears to promote fertility after relief of chronic obstruction. Our studies in men with congenital absence of the vas deferens or other surgically unreconstructable obstruction of the vas have indicated that the longer the segment of epididymis present, the greater the likelihood of pregnancy with sperm obtained from these men. Almost all of the information regarding human epididymal function are derived from observations of men who have undergone surgical reversal of long term reproductive tract obstruction. The results of several published observations of men with unobstructed reproductive tracts was compiled by Bedford. As indicated (in NY Acad Sci 541: 284-291, 1988), the fertility potential of sperm from the caput region of the obstructed human epididymis is minimal.

The exact fate of unejaculated epididymal spermatozoa in humans is also unknown. Some sperm have been documented to be phagocytosed by macrophages in the epididymis, and sperm have been detected in urine. However, the mechanisms by which quantitative clearance of sperm occurs from men during periods of sexual abstinence is unknown.

In normal, unobstructed systems, the majority of spermatozoa taken from the vasa efferentia and diluted in a physiologic solution are immotile or exhibit only weak tail movements. These observations in the unobstructed human epididymis are very different from what is seen after relief of chronic epididymal obstruction. Our observations confirm those of several investigators, that, although the motility of ejaculated spermatozoa is initially poor following vaso-epididymostomy at the level of the caput epididymis, sperm motility may improve greatly up to 1 1/2 years after vasoepididymostomy. These findings suggest that following vasoepididymostomy the caput epididymis or vas deferens may undergo compensatory adaption over time and support sperm motility maturation despite exposure of sperm to a shortened length of epididymis. In animal models, vasal obstruction causes changes in the intrinsic motility of fluid through the epididymal lumen. These changes are not completely reversed by technically successful vasovasostomy. It is not known what effect the apparent change in epididymal contractility may have on epididymal function.

Studies on laboratory animals describe many changes that occur to sperm during epididymal transit. These include a change in net sperm surface charge, addition and alteration of membrane proteins, alterations in sperm lectin-binding properties, changes in immunoreactivity and iodination characteristics, acquisition of an increased capacity for glycolysis, modification of adenylate cyclase activity, alterations in cellular phospholipid and phospholipid-like fatty acid content and an increased ability to adhere to the zona pellucida of the egg. Whether similar modifications occur in human spermatozoa during epididymal migration is unknown. Biochemical changes observed in human spermatozoa during epididymal transit involve the formation of disulfide bonds within the sperm nucleus and tail and the oxidation of sperm membrane sulfhydryl groups. These changes are thought to provide improved structural integrity to the sperm membrane. The changes in structural integrity of sperm may be necessary for the development of progressive motility and successful penetration of eggs.

Protein secretion and the storage capacity of the epididymis has been shown to be profoundly affected by changes in epididymal temperature in animals. Some have even postulated that the driving force for evolution of the scrotal location of testes is to have the epididymis maintained at a temperature below that of body core temperature, in the scrotum. Whether the functions of the human epididymis are similarly affected by body temperature is unknown. The potential influence of temperature on epididymal function in man may be an important consideration in explaining the relationship between varicocele and male infertility. If temperature significantly affects human epididymal function, then it could explain improvements in semen parameters that may occur less than three months (one full cycle of spermatogenesis) after varicocelectomy.

The human spermatozoon is approximately 60 µm in length. Normal forms have an oval sperm head, 4.5 µm long and 3 µm wide, consisting principally of a nucleus, which contains the highly compacted chromatin material, and the acrosome, a membrane-bound organelle that covers 40-70% of the surface of the sperm head and contains the enzymes required for penetration of the outer vestments of the egg prior to fertilization. The middle piece of the spermatozoon is a highly organized segment consisting of helically arranged mitochondria surrounding a set of outer dense fibers and the characteristic 9 + 2 microtubular structure of the sperm axoneme. The mitochondria contain the enzymes required for oxidative metabolism and the production of adenosine triphosphate (ATP), the primary energy source for the cell. Absence of some components of the microtubules is associated with immotile cilia in the sinus and pulmonary tracts, with resultant pulmonary and sinus infections. The association of sperm immotility with upper respiratory infections (Young's syndrome) due to ciliary dysfunction or absent dynein cross-arms between the microtubules (Kartagener's syndrome) resulting in bronchiectasis and nonmotile sperm have been well documented. Other syndromes of ciliary dysfunction have recently been described in children with recurrent sinusitis (ciliary dyskinesia). It is yet to be demonstrated whether the boys affected with the childhood manifestations of ciliary dyskinesia will suffer from infertility problems after puberty. The highly specialized structure and physiology of the spermatozoon is marvelously suited for its single purpose: to carry genetic material to the egg during reproduction.

The rate of transport of fluid through the vas deferens is not known in the human. Just prior to ejaculation, the testes are brought up close to the abdomen and fluid is rapidly transported through the vas deferens toward the region of the ejaculatory ducts and subsequently into the prostatic urethra. After ejaculation, intravasal fluid is transported back toward the epididymis and occasionally into the seminal vesicles as well. The retrograde transport of sperm to the seminal vesicles has been documented by videoradiography during ejaculation after vasography. The return of sperm to the seminal vesicles after ejaculation may help explain the prolonged presence of sperm in the ejaculate for some men after vasectomy.

The ejaculatory ducts enter the prostatic urethra just lateral to the verumontanum. In the case of obstruction of the ejaculatory ducts, resection of the floor of the prostate should be performed just lateral to the midline and superior to the verumontanum. Vasography can be performed prior to transurethral resection (TUR) of the ejaculatory ducts with placement of methylene blue in the vasography fluid. This allows the surgeon to cystoscopically identify the lack of flow of dye and confirm relief of obstruction of the ejaculatory ducts during TUR.

The ejaculate consists of components from different accessory organs. Each of these organs and the characteristics of the fluid that they produce is listed below. These characteristics can be used clinically to evaluate ejaculatory dysfunction.