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After mating, inseminated spermatozoa are transported to the oviduct. They attach to and interact with oviductal epithelial cells (OEC). To investigate sperm-OEC interactions, we used chlortetracycline to study the capacitation status of boar spermatozoa in coculture with homologous OEC and cells of nonreproductive origin (LLC-PK1, porcine kidney epithelial cell line). Boar spermatozoa were cocultured with OEC and LLC-PK1 cells for 15, 60, 120, or 240 min. The proportion of capacitated spermatozoa in coculture with the isthmic and ampullar cells increased significantly (p < 0.05) during incubation. However, most spermatozoa in coculture with LLC-PK1 cells or blank (medium only) remained uncapacitated. In addition, preferential binding of uncapacitated, capacitated, or acrosome-reacted boar spermatozoa to OEC and the other cell type was investigated. Our approach was to vary the proportions of uncapacitated, capacitated, or acrosome-reacted boar spermatozoa in suspension using long preincubation and lysophosphatidylcholine treatment of semen prior to a very short incubation with OEC or LLC-PK1 cells. The results showed that the majority of spermatozoa that were bound to OEC or LLC-PK1 cells were uncapacitated and that a significant relationship existed between the relative proportion of uncapacitated spermatozoa in the control samples and those bound to LLC-PK1 cells (r2 = 0.43, p < 0.005). However, there was no correlation between the proportion of uncapacitated spermatozoa in the control samples and the proportion of those bound to isthmic or ampullar cells. In conclusion, the results clearly demonstrated the specific nature of the sperm-OEC interaction in the porcine species. This interaction is initiated by uncapacitated spermatozoa binding to OEC and is continued by the induction of capacitation in cocultured spermatozoa.

In mammals, the physiological interaction between spermatozoa and oviductal epithelia involves intimate and specific contact between the two cell types. Spermatozoa may undergo stringent selection processes within the female reproductive tract before they meet and fertilize oocytes. The physiological basis of the sperm selection process is largely unknown. Here we tested the hypothesis that the oviduct has a recognition system for spermatozoa that can detect the arrival of spermatozoa in the oviduct after insemination, resulting in alterations of the oviductal transcriptome. We initially performed a global screening of the oviductal transcriptome in mice 1) at the time of estrus (mating) and 2) 6 h after mating. Transcriptional alterations in the oviduct after mating were attributed to the presence of spermatozoa in the oviduct after mating and also to changes in the hormonal environment as female mice underwent the transition from estrus to diestrus. To distinguish these possibilities, female mice were then mated with T145H mutant mice, which because of spermatogenic arrest, produce seminal plasma but no spermatozoa. Focusing on two molecules that in the first experiment were upregulated after mating, it was found that adrenomedullin and prostaglandin endoperoxidase synthase 2 transcripts were upregulated in the oviducts of mice only after mating with fertile males; those mated with T145H infertile males showed significantly less response. These results indicate that it is the arrival of spermatozoa in the oviduct that activates one or more signal transduction pathways and leads to changes in the oviductal transcriptome profiles.

Stringent selection mechanisms, in both internal and external fertilisation systems, reject all but a significant minority of the spermatozoa released at ejaculation. Sperm competition theory provides circumstantial evidence that the selection process involves mechanisms by which the quality of the fertilising spermatozoon is controlled, thereby ensuring that females and their offspring receive high quality genetic material. In this review we examine some of these selection processes to see whether they could be exploited for the improvement of laboratory tests of sperm quality. Such tests are not only required for clinical and agricultural purposes, but are increasingly needed in fields such as reproductive and environmental toxicology where the species requirement is much broader. Despite many years of research, sperm quality assessment methods continue to provide imprecise data about fertility; here we suggest that this may be a consequence of using tests that focus on the spermatozoa that would normally be unable to fertilise under natural conditions. To achieve fertilisation a spermatozoon must be capable of responding appropriately to external signalling stimuli; those involving protein kinase-regulated flagellar function seem especially influential in governing effects ranging from non-Mendelian inheritance in mammals to sperm chemotaxis in sea urchins. Examination of the elicited responses reveals considerable heterogeneity in all species. Here we propose that this level of heterogeneity is meaningful both in terms of understanding how spermatozoa from some individuals possess fertility advantages over spermatozoa from their rivals in sperm competition, and in that the heterogeneity should be exploitable in the development of more accurate laboratory tests.

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This study investigated two hypotheses: 1) that consistent between-boar variation in frozen semen quality exists and is genetically determined, and 2) molecular markers linked to genes controlling semen freezability can be identified using amplified restriction fragment length polymorphism (AFLP) technology. Five ejaculates were collected from each of 129 boars. Semen was diluted into a commercial freezing buffer (700 mOsm/kg, 3% glycerol) and five straws (0.5 ml) per ejaculate were cryopreserved (to -5 degrees C at 6 degrees C/min, then -5 degrees C to -80 degrees C at 40 degrees C/min). Semen was assessed for percentage of motile cells, motility characteristics (computer-aided semen analysis; CASA), plasma membrane integrity (SYBR-14 positive), and acrosome integrity (positive for fluorescein-labeled peanut agglutinin; PNA). Consistent between-boar variability was detected for postthaw sperm motility (P < 0.01), membrane integrity (P < 0.01), acrosome integrity (P < 0.01), and all CASA characteristics (P < 0.05). There was no significant difference between ejaculates (P > 0.05) or straws (P > 0.05) for any viability assessment. Multivariate pattern analysis of the viability data set highlighted three groups of boars producing spermatozoa with poor, average, and good postthaw recovery (42, 63, and 24 boars, respectively). DNA from Large White boars (n = 22) previously classified as good and poor freezers was screened for AFLP markers. Twenty-eight polymerase chain reaction primer combinations generated 2182 restriction fragment bands, of which 421 were polymorphic. Sixteen candidate genetic markers (P < 0.005) were identified by comparing the AFLP profile with semen freezability using logistic regression analysis. These findings support the hypothesis that there is a genetic basis for variation in postthaw semen quality between individuals, and that AFLP technology may be able to identify molecular markers linked to genes influencing this variation.

SUMMARY When spermatozoa from two or more boars are mixed and females inseminated,the resulting litters are often skewed in favour of one male but there is currently no satisfactory physiological explanation for this effect. However,to reach the oocytes, the spermatozoa must enter the oviduct where they are exposed to factors that modulate their activity. They either become sequestered within the oviductal sperm reservoir or bypass the reservoir and proceed towards the oocytes. The oviduct may therefore hold the key to mammalian sperm selection, thereby explaining why laboratory tests of sperm function, performed on whole ejaculates, are unable to account for the boar-specific skewing effects. We have previously shown that boar sperm motility is highly stimulated by bicarbonate, a naturally abundant component of oviductal fluid. Using motility-based sperm subpopulation analysis, we show here that the relative sizes of bicarbonate-responsive and unresponsive sperm subpopulations vary between individual boars. Proteins derived from oviduct epithelial plasma membranes suppress the activation response and modify sperm movement trajectories in a subpopulation-specific and dose-dependent manner. The suppression response varies between boars and some spermatozoa remain unsuppressed in the presence of oviductal proteins. When boars are ranked according to their susceptibility to bicarbonate-induced stimulation, rankings differ depending upon the presence or absence of oviductal proteins. The suppression response is not caused by inhibition of bicarbonate uptake; on the contrary this is enhanced by oviductal proteins. We suggest that the boar-specific and sperm subpopulation-specific interactions between sperm motility activation and suppression responses are likely to result in sperm selection before the spermatozoa meet the oocytes.