The epididymis, cytoplasmic droplets and male fertility

Abstract

The potential of spermatozoa to become motile during post-testicular maturation, and the relationship between the cytoplasmic droplet and fertilizing capacity are reviewed. Post-testicular maturation of spermatozoa involves the autonomous induction of motility, which can occur in vivo in testes with occluded excurrent ducts and in vitro in testicular explants, and artefactual changes in morphology that appear to occur in the testis in vitro. Both modifications may reflect time-dependent oxidation of disulphide bonds of head and tail proteins. Regulatory volume decrease (RVD), which counters sperm swelling at ejaculation, is discussed in relation to loss of cytoplasmic droplets and consequences for fertility. It is postulated that: (i) fertile males possess spermatozoa with sufficient osmolytes to drive RVD at ejaculation, permitting the droplet to round up and pinch off without membrane rupture; and (ii) infertile males possess spermatozoa with insufficient osmolytes so that RVD is inadequate, the droplet swells and the resulting flagellar angulation prevents droplet loss. Droplet retention at ejaculation is a harbinger of infertility caused by failure of the spermatozoon to negotiate the uterotubal junction or mucous and reach the egg. In this hypothesis, the epididymis regulates fertility indirectly by the extent of osmolyte provision to spermatozoa, which influences RVD and therefore droplet loss. Man is an exception, because ejaculated human spermatozoa retain their droplets. This may reflect their short midpiece, approximating head length, permitting a swollen droplet to extend along the entire midpiece; this not only obviates droplet migration and flagellar angulation but also hampers droplet loss. Keywords: epididymis; fertility; infertility; sperm maturation INTRODUCTION During passage of mammalian spermatozoa through the epididymal duct, the functionally incompetent germ cell produced by the testis is matured and stored. In this time (around 1-2 weeks in most species), the spermatozoon undergoes many changes that prepare it for achieving the diverse tasks required of it. At ejaculation, when it is rapidly expelled from the epididymis through the vas deferens to the world outside, the spermatozoon undergoes another phase in which it encounters fluids of the male accessory sex glands, escapes from them in the vagina, cervix or uterus, depending on species, and interacts with the oviductal lining before fertilizing the female gamete. Many mechanisms control the timing of these events, all of which require adequate responses by the fertilizing spermatozoon, and several of these have their origins in the epididymis. This paper addresses a few controversial, novel or still unanswered topics related to the maturation, volume regulation and morphology of spermatozoa.

Flagellar ion channels of sperm: similarities and differences between species

“…This arrangement provides spermatozoa with a ridged structure to which membrane proteins can be tethered to the fibrous sheath [36] to ensure their strict compartmentalization [37]. Many sperm cells possess a single region of the sperm plasma membrane that is loosely attached to the rigid intracellular structures, a so-called cytoplasmic droplet (CD) [38,39]. The CD represents a remnant of the spermatid's cytoplasm and can be located around midpiece, or even more distally on the principal piece.…”

“…The CD represents a remnant of the spermatid's cytoplasm and can be located around midpiece, or even more distally on the principal piece. The CD is shed from spermatozoa in certain species upon ejaculation; however, a small number of cells retains it [38,40]. Whether the presence of the CD is an indication of an immature spermatozoon is still questionable, a significant portion of fertile human sperm cells retain this structure.…”

“…Whether the presence of the CD is an indication of an immature spermatozoon is still questionable, a significant portion of fertile human sperm cells retain this structure. The CD has been proposed to serve as a reservoir for adaption to osmotic stress upon exposure to various environments [38,[40][41][42]. It does however, serve another technical, but useful purpose: the CD allows for formation of tight, gigaohm contact (gigaseal) between sperm membrane and recording electrode which has made sperm patch clamp studies possible [16,43].…”

Cytometric solutions in veterinary andrology: Developments, advantages, and limitations

“…Infertility can be ascribed either to inadequate functioning of the sperm cell or to the male's failure to produce enough morphologically normal spermatozoa [as a consequence of disrupted spermatogenesis: e.g. (25,26)]. In this regard, one needs to distinguish between in vivo and in vitro situations (27).…”

“…There are internal changes in enzyme activity and the ability to express motility develops (although the sperm remain essentially immotile within the epididymis until after ejaculation). Another important feature is that during epididymal passage, the spermatozoa are exposed to a varying environment, characterized by high concentration of osmolytes (26,32). Finally, the mature spermatozoa accumulate in the cauda epididymidis where they are stored stable and immotile under hyperosmotic conditions until ejaculation (33).…”