Semen composition differs amongst mammalian species with respect to volume and sperm concentration, and these differences are related to the site of insemination
Ž .
Hunter, 1988 . Biochemical constituents of seminal plasma, such as prostaglandins can stimulate smooth muscle activity of the female reproductive tract and thereby assist the
Ž distribution of semen or spermatozoa within the tract Hunter, 1975; Drobnis and
. Overstreet, 1992; Harper, 1994 . The mechanical stimulus of mating may also enhance
Ž .
visceral contractions and sperm distribution Overstreet and Katz, 1990 . The direct effects of seminal plasma on the female tract may be localized. For ruminants and
Ž .
primates, cervical mucus forms a complete barrier to seminal plasma Katz et al., 1989 . The passage of seminal plasma into the oviducts is blocked by the UTJ in the rat
Ž .
Ž Carballada and Esponda, 1997 but this is apparently not the case in the horse Mann et
. al., 1956 . In the pig, biochemical evidence of seminal plasma was not detected in the
Ž .
oviducts following mating Mann et al., 1956 ; however, radiolabelled tracers of
different molecular weights will enter the oviducts when combined with sperm-free Ž
. seminal plasma and artificially inseminated Einarsson et al., 1980 , and this transport
Ž .
across the UTJ is rapid Viring et al., 1980 .
3. Sperm distribution
Sperm distribution within the female tract has classically been described as occurring in phases, which are defined by their relationship in time to the event of insemination
and the relative contribution of passive movement caused by visceral contractions of the Ž
. female genital tract vs. active sperm migration Overstreet and Cooper, 1978a,b . The
rapid transport phase is a pericoital event, characterized by the presence of sperm in the Ž
oviducts within minutes of mating or artificial insemination Overstreet and Cooper, .
1978a . This rate of transport is much faster than sperm swimming speeds; conse- quently, it is attributed to muscular contractility of the female tract and attendant
Ž .
changes in intraluminal pressures see Drobnis and Overstreet, 1992; Harper, 1994 . Rapidly transported spermatozoa do not contribute to the fertilizing population in the
Ž .
oviduct, because they are moribund, dead or disrupted Overstreet and Cooper, 1978a . The rapid transport phase is followed by a prolonged phase of sperm migration
Ž .
Overstreet and Cooper, 1978b , during which the distribution of spermatozoa within the female tract continues and sperm reservoirs are established. It is during this phase that
competent spermatozoa will arrive in the oviducts. This journey takes 1–2 h in the pig Ž
. Ž
. Hunter, 1988 , 1.5–6 h in rabbits Overstreet and Cooper, 1978b and 6–8 h in sheep
Ž .
and cattle Hunter, 1988 . The recovery of highly motile sperm from the mare’s oviduct Ž
. at 4 h after insemination Scott et al., 1995 and the knowledge that uterine lavage prior
Ž .
to this interval adversely affects conception rates Brinsko et al., 1990, 1991 support the hypothesis that this journey requires 4 h in the horse.
3.1. Sperm motility and sperm transport Sperm motility, which is regarded as a manifestation of sperm functional competence,
results from the propagation of principal and reverse bends of the flagellum. The
contribution of sperm motility to the distribution of spermatozoa along the female tract is most evident during the sustained phase of sperm transport. Within the female tract,
sperm motility is modulated by the dynamic forces imposed on the flagellum by spatial constraints, epithelial surface characteristics, and the rheological characteristics of fluid
Ž .
secretions Katz et al., 1989 . Motility is needed for spermatozoa to colonize and cross Ž
. Ž
. the cervix Cooper et al., 1979 and cervical mucus Overstreet and Katz, 1990 , and
may also be requisite to traversing the UTJ. Using an elegant preparation for in vitro Ž
. observation, Gaddum-Rosse 1981 found that motile rat spermatozoa emerged from the
cut oviductal end of the UTJ of excised tracts, whereas non-motile spermatozoa andror uterine fluid did not, despite the presence of smooth muscle contractions. Spermatozoa
Ž .
appear to ascend the tract by moving along epithelial surfaces Katz et al., 1989 . This migration has been visualized directly through the transparent wall of excised oviducts
Ž .
Ž .
in the hamster Katz and Yanagimachi, 1980
and mouse Suarez, 1987 , and is
suggested by the appearance of the flagellar curvature and epithelial orientation of Ž
. equine spermatozoa fixed in situ at the UTJ Fig. 1; Scott et al., 2000 . Nilsson and
Ž .
Reinius 1969 noted that mouse spermatozoa at the ostium of the colliculus tubarius ‘‘almost climb on the surrounding microvilli of the tightly apposed luminal surfaces and
follow the longitudinal ridges of the junctura’’ to enter the oviduct, and suggested that a narrow lumen at the UTJ would effectively channel spermatozoa in an adovarian
Ž .
direction Nilsson and Reinius, 1969, p. 79 . Spermatozoa that move progressively forward are more likely to penetrate the
Ž .
microstructure of cervical mucus see Katz et al., 1989 or successfully cross the UTJ Ž
. Gaddum-Rosse, 1981 . However, transport across the UTJ is significantly impaired for
uterine spermatozoa that display the vigorous but non-progressive motility of hyperacti- Ž
vation, whether this motility is induced by in vitro capacitation hamster; Shalgi et al., .
Ž 1992 or preincubation of epididymal spermatozoa with calcium mouse; Olds-Clarke
. and Wivell, 1992 prior to artificial insemination. Furthermore, in horses, even if the
insemination doses are balanced for number of progressively motile spermatozoa, fewer spermatozoa from subfertile stallions reach the oviducts of normal mares compared with
Fig. 1. Scanning electron micrograph of equine spermatozoa fixed in situ at the UTJ of a mare, 4 h after preovulatory insemination. The shape of the flagella and orientation of these spermatozoa with the epithelium
Ž .
gives the appearance of active migration from Scott et al., 1999; with permission .
Ž .
fertile stallion sperm numbers Scott et al., 1995 . Similarly, subfertile mares have fewer spermatozoa in their lower oviducts than fertile mares despite receiving equal numbers
Ž .
of progressively motile fertile stallion spermatozoa at insemination Scott et al., 1995 . Thus, motility per se does not assure successful sperm migration.
The defect may be at the level of sperm–epithelium interaction. Recent studies with transgenic mice have demonstrated that altering proteins on the sperm surface can
compromise the migration of spermatozoa that otherwise display normal motility. Ž
. Fertilin and the testis isozyme of angiotensin converting enzyme ACE are distinct
sperm surface proteins. Uterine spermatozoa recovered from female mice mated with Ž
. mutant males lacking either the gene for fertilin beta Cho et al., 1998 or the gene for
Ž .
testis ACE Hagaman et al., 1998 show normal motility characteristics, capacitation, and acrosome reaction in vitro, but have impaired transport to the oviducts in vivo. The
results of these independent studies strongly suggest that sperm surface proteins are important functional elements in the process of sperm transport and stress the signifi-
cance of sperm–epithelium interaction as a key component of normal sperm migration. Support for this hypothesis is derived from the knowledge that mouse spermatozoa
normally adhere intermittently to the epithelium of the uterus, colliculus, UTJ and
Ž .
isthmus during transit through the female tract Suarez, 1987 . 3.2. Sperm morphology and sperm transport
Morphologically abnormal spermatozoa may also be functionally incompetent. Motile spermatozoa that are morphologically abnormal may be unable to swim through cervical
mucus. Exclusion of abnormal spermatozoa at the cervix may be due to inferior motility Ž
. andror cell surface abnormalities see Katz et al., 1989 . In cattle, abnormal spermato-
Ž zoa remaining at oviductal isthmus and UTJ 12 h after intrauterine insemination i.e.,
. bypassing the cervical barrier
had a significantly reduced proportion of knobbed acrosomes than the inseminate, indicating compromised transport in the anterior genital
Ž .
tract for spermatozoa with this defect Mitchell et al., 1985 . Also, few spermatozoa with abnormal heads were recovered from the oviduct or UTJ of heifers 2 h after
intrauterine insemination with semen having a post-thaw motility of 50 and a Ž
. proportion of spermatozoa with abnormal heads exceeding 40 Larsson, 1988 . The
UTJ appears to block the passage of most mouse spermatozoa that have severe head Ž
. abnormalities Krzanowska, 1974 . In mares, more than 90 of spermatozoa visualized
in situ at the UTJ using scanning electron microscopy are morphologically normal even when the inseminate contains high numbers of spermatozoa with major morphological
Ž .
defects Scott et al., 2000 . Those results led the authors to suggest that most morpholog- ically abnormal equine spermatozoa either do not reach the UTJ or do not develop
Ž normal associations with the epithelium during transit through the tract Scott et al.,
. 2000 . Collectively, these observations in a wide variety of species support the notion
that a functional interaction between competent spermatozoa and the luminal fluids and epithelial surfaces of the female tract, particularly at the sites of sperm restriction,
ultimately promotes the selection of a physiologically normal population of spermatozoa during transit.
4. Sperm reservoirs
