The spermatozoa are formed within the seminiferous tubules of the testes in a complex process called spermatogenesis. The continuous maintenance of this process is enabled by androgens secreted mainly by testicular Leydig cells located in the interstitial tissue outside the seminiferous tubules. In addition, the epididymal maturation of spermatozoa is also an androgen-dependent process (Robaire & Hermo 1988).
Once formed within the seminiferous tubules, the immotile spermatozoa are released into luminal fluid and transported to the epididymis, where they gain the ability to move and fertilize the ovum (Yanagimachi 1994).
The testicular spermatozoa are transported passively to the rete testis, which is a branched reservoir of the openings of the seminiferous tubules. From the rete testis, the transport of spermatozoa to the epididymis takes place via the efferent ducts, whose number varies between studies and individuals (Stieve 1930, Holstein 1969, Jonté & Holstein 1987, Saitoh et al. 1990). The epithelium lining these ducts is columnar and consists of two cell types called ciliated and nonciliated cells (Robaire & Hermo 1988). Both cell types are capable of performing endocytosis, and ciliated cells also maintain the movement of luminal fluid and sperms. Nonciliated cells are mainly responsible for the absorption of water and ions.
The efferent ducts absorb most of the fluid discharged from the testis with spermatozoa, thus increasing the epididymal sperm concentration (Clulow et al. 1994).
The epididymis can be divided into three parts, called caput, corpus, and cauda. In the human epididymis, the caput is mainly filled by efferent ducts, which open to the epididymal duct near the border between the caput and corpus (Saitoh et al. 1990, Yeung et al. 1991). The human ductus epididymis contains four types of epithelial cells called principal cells, basal cells, apical mitochondria-rich cells (AMRC), and halo cells (Reid & Cleland 1957, Martan et al. 1964, Robaire & Hermo 1988, Palacios et al. 1991). Principal and basal cells are the main cell types, the former being involved in secretion and absorption, while the latter probably participate in detoxification processes or act as scavenger cells (Robaire & Hermo 1988, Veri et al. 1993, Yeung et al. 1994).
In mammals, the transit of spermatozoa through the epididymis usually takes 10-13 days, whereas in humans the estimated transit time is 2-6 days (Amann & Howards 1980, Johnson & Varner 1988, Robaire & Hermo 1988). The epididymal segment where most spermatozoa attain their full fertilizing capacity appears to be the proximal cauda. The spermatozoa from that region are capable of moving progressively, which is characteristic of spermatozoa preceding fertilization, and bind to zona-free hamster ova in vitro at a higher percentage than spermatozoa obtained from more proximal locations (Yanagimachi 1994, Turner 1995). To attain the capacity to fertilize, sperm undergoes many maturational changes during its transit in the epididymal duct (Yanagimachi 1994). These include, for instance, changes in plasma membrane lipids, proteins and glycosylation, alterations in the outer acrosomal membrane, gross morphological changes in acrosome in some species, and cross-linking of nuclear protamines and proteins of the outer dense fiber and fibrous sheath. Numerous studies have questioned whether the human epididymal and efferent ducts are necessary for sperm maturation. In these studies, sperms that have bypassed the epididymis partly or completely or have also passed the efferent ducts and rete testis have still been able to fertilize eggs (Schyosman & Bedford 1986, Silber et al. 1988, Silber 1988, 1989, Schyosman 1993). However, these studies have been conducted on subjects with an abnormal reproductive tract, and spermatozoa aspirated from these subjects are not comparable to sperm aspirated from the corresponding segment in normal subjects. In addition, after surgical bypassing of the proximal excurrent ducts, vas deferens may become able to substitute the bypassed segments. Thus, these studies may, misleadingly, underevaluate the importance of the human epididymis and efferent ducts in sperm maturation (Cooper 1993, Bedford 1994, Turner 1995, Jones 1999). Alternatively, the requirement for post-testicular sperm maturation is not so essential in men as in other mammals (Yanagimachi 1994, Jones 1999), or this maturation may be faster and require only a brief exposure to some part of the post-testicular tract (Bedford 1994, Turner 1995). The cauda epididymidis (and proximal ductus deferens) are the regions where spermatozoa are stored before ejaculation (Turner 1995, Jones 1999). When ejaculation occurs, the stored spermatozoa with the surrounding fluid are mixed with the alkaline secretions of the male accessory sex glands and deposited to the vagina.
Sperm maturation and transport in the male reproductive tract