Function of the Epididymis in Eutherian Mammals

Embryologically, the epididymis derives from the mesonephric duct (Wolffian Duct) which has lost its original urinary function and now serves to convey the gametes from the testis to the vas deferens (Romer 1960). page 24 However, it is not a simple connecting tube, for it serves both to concentrate the spermatozoa and to enable them to complete the process of maturation which starts in the seminiferous epithelium. That spermatozoa do not acquire fertility until they have passed through the epididymis was first shown by Young (1929 a,b, 1931) and Young and Simeone (1930), in work on the guinea-pig. Since those early experiments, a variety of studies has shown this to be true for a number of laboratory and domesticated species, and presumably for all eutherian mammals. The significance of the epididymis is thus well established (Hamilton 1972; Bedford 1975).

The acquisition of fertility during epididymal maturation is not precisely related to any particular change in sperm structure, but rather to physiological and biochemical changes - particularly in membrane structure and composition -which presumably render the spermatozoa capable of effecting fertilization. Some morphological changes do occur, however, notably contraction of the acrosome and distal migration of the cytoplasmic droplet; and these changes are accompanied by increases in specific gravity (Lindahl and Kihlström 1952; Lavon et al. 1966). This general pattern is now fairly well established, at least for the domestic and laboratory species (Glover 1974; Bedford 1975; Prasad and Rajalakshmi 1977), and it appears to be under the control of the epididymis, which is in turn dependent upon an adequate level of androgens (Hamilton 1972; Glover 1974; Prasad and Rajalakshmi 1977).

The epididymis is usually described, in gross anatomical terms, in relation to its attachment to the testis; thus we have the caput, corpus and cauda epididymidis; or 'head', 'body' and 'tail'. Unfortunately, there are considerable differences between species in the cytology of these regions, and it seems unlikely that the gross anatomy of the epididymis is related to function in any but the most general terms. Glover and Nicander (1971) proposed a uniform classification for the eutherian epididymis, based upon histology and a study of gross maturational changes in spermatozoa. They described three distinct regions: the Initial Segment, characterized by a high degree of fluid resorption from the testicular exudate; the Middle Segment, where sperm concentration continues and maturation is completed; and finally a Terminal Segment, where the mature spermatozoa are stored in an inactive state before ejaculation. In most species, the Terminal Segment corresponds to the cauda epididymidis, but there may be considerable variation in the positioning of the other regions in terms of gross anatomy. The situation is especially complicated when we consider species which have page 25 abdominally situated testes, in which the Terminal Segment may be at a considerable distance from the testis, close to the surface of the body; for example, in the elephant Loxodonta africana and rock hyrax Heterohyrax brucei (Glover 1973).

The spermatozoa in the Terminal Segment, although maintained in an immobile state, are nevertheless highly sensitive to temperature. In the rabbit Oryctolagus cuniculus, for example, subjecting the testis and epididymis to body temperature by means of artificial cryptorchidism rapidly inhibits the fertility of sperm in the cauda epididymidis (Cummins and Glover 1970). It seems likely that the need to maintain a sperm store at a temperature below that of the body cavity may have been a powerful influence behind the development of a thermo-regulatory scrotum (Glover 1973, 1974; Bedford 1977), and this intriguing possibility will be dealt with later.