have completed a long and arduous journey, during which they have surmounted the formidable anatomical barriers within the female tract and have undergone the physio-
logical changes that are required to initiate and complete fertilization. The journey begins at insemination and proceeds with a variable timetable that appears to be closely
regulated by the female. Sperm distribution and function in the female are influenced by the site of semen deposition, seminal characteristics, the anatomy of the female genital
tract, and the microenvironment of the lumen. The duration of sperm transport depends on the interval between insemination and ovulation and the functional lifespan of
spermatozoa in the female tract. Accordingly, inter-species variation in sperm transport biology likely reflects the diversity of social behaviors and mating strategies that have
evolved in the animal kingdom.
This review provides a general overview of the natural history of mammalian spermatozoa following insemination, with special emphasis on sperm–epithelium inter-
action as an integral feature of sperm function in vivo. For a more detailed examination of sperm transport, the reader should consult one or more of the many comprehensive
Ž reviews that are available in the literature e.g., Hunter, 1975; Overstreet, 1983; Hunter,
1988; Overstreet and Katz, 1990; Drobnis and Overstreet, 1992; Harper, 1994, Yanagi- .
machi, 1994 .
2. Copulatory behavior and insemination
In most domestic animal species, sperm arrival in the female genital tract is temporally associated with the timing of ovulation, either because ovulation is induced
Ž .
by coitus e.g., cat, rabbit, and llama , because ovulation occurs during estrus when the Ž
. female is receptive to coitus, e.g., dog, horse, sheep, pig, mouse, rat and hamster , or
Ž .
because ovulation occurs shortly after estrus e.g., cattle . Amongst mammals, the most profound example of a prolonged interval between coitus and ovulation is the hibernat-
Ž .
ing bat, in which this interval may exceed 6–7 months Racey et al., 1987 . The site of semen deposition defines the anatomical barriers that spermatozoa will
encounter during transit to the oviducts. These barriers restrict sperm passage, establish a gradient in sperm numbers along the tract, and may function as sperm reservoirs
Ž .
Overstreet and Cooper, 1978b; Hunter, 1988 . The establishment of a sperm gradient Ž
. appears to reduce the risk of polyspermy see Hunter, 1988 . For species with uterine
Ž .
deposition of semen e.g., dog, horse, pig, llama, laboratory rodents , the uterotubal Ž
. junction UTJ is the primary physical barrier to the oviducts, and a sharp gradient in
Ž .
sperm numbers occurs cranial to this barrier Hunter, 1988 . For species with vaginal Ž
. insemination e.g., ruminants, primates, rabbit, hare , the initial barrier is the cervix, and
the UTJ serves to further restrict sperm access to the oviducts. In ruminants and primates, the cervical canal is filled with mucus, and the biophysical characteristics of
this secretion are affected by the endocrine status of the female. Cervical mucus may
Ž .
block sperm passage luteal phase, progesterone dominance , or, under estrogen domi- Ž
. nance such as in the periovulatory period , the more hydrated mucus matrix forms
Ž .
channels through which spermatozoa can migrate see Harper, 1994 .
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
