On the epididymis and its role in the development of the fertile ejaculate.

A variety of mammalian species have been used in studies of epididymal function. The knowledge gained from these laboratory and domestic species has been applied to the human as well, but the anatomy and biology of the human epididymis is not commonly investigated directly. Nonpathological specimens from men within their reproductive years are understandably difficult to obtain, and even when such epididymides are obtained, they often lack the proper preservation for good biological studies. Because the human epididymis is not often addressed directly, the present review initially focuses on that organ before going on to more general comments. In nature, the distal portion of the human epididymis disappears into the epididymal fat, making it appear that the epididymis has a minimal cauda region. In fact, the still-coiled human epididymis is 10-12 cm long before becoming the convoluted vas, and the convoluted vas extends for approximately another 7-8 cm. Nevertheless, the human epididymis does not have a bulbous cauda for sperm storage, as do the caudae of most other species, including the rat (Fig. 1A). Whereas the cauda of the rat epididymis contains approximately 60% of the sperm in that epididymis (Amann, 1981) the ‘tail’ of the human epididymis (Fig. 1B) contains at least 50% of the sperm present in that epididymis (Amann and Howards, 1980; Johnson and Varner, 1988). Because the demarcation of the tail or cauda of the epididymis has been unclear in most human studies, it seems likely that much of this sperm storage capacity is in what some would call the proximal convoluted vas deferens (Fig. 1B). In fact, it may be that the bulbous cauda seen in most species is simply extended into what is called the proximal convoluted vas in the human. This makes the human cauda