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Sialic acid (Sia) is a major constituent of both the sperm glycocalyx and female reproductive mucosal surface and is involved in regulating sperm migration, uterotubal reservoir formation and oocyte binding. Siglecs (sialic acid-binding immunoglobulin - like lectins) commonly found on immune cells, bind to Sia in a linkage- and sugar-specific manner and often mediate cell-to-cell interactions and signalling. Proteomic and transcriptomic analysis of human and bovine sperm have listed Siglecs, but to date, their presence and/or localisation on sperm has not been studied. Therefore, the aim of this study was to characterise the presence of Siglecs on the surface of bovine, human and ovine sperm using both immunostaining and Western blotting. Siglec 1, 2, 5, 6, 10 and 14 were identified and displayed both species- and regional-specific expression on sperm. Almost universal expression across Siglecs and species was evident in the sperm neck and midpiece region while variable expression among Siglecs, similar among species, was detected in the head and tail regions of the sperm. The possible role for these proteins on sperm is discussed.

The commercial applicability of bovine artificial insemination (AI) depends on the effectiveness of diluents for maintaining sperm fertility. Challenges faced by the AI industry due to recent advances in assisted reproduction, and the limitations inherent in using fresh and frozen-thawed sperm for AI, could be overcome with the development of better semen diluents. Research into the different microenvironments of bovine sperm as they progress towards maturity, capacitation and fertilisation is revealing various mechanisms that could be exploited to improve the formulation of semen diluents. These are reviewed here. A rationale for a more detailed investigation of bovine cervical mucus for factors that may allow further progress towards this goal are also discussed.Keywords: bovine sperm, semen diluents, artificial insemination ImplicationsArtificial insemination is used for almost 20% of breeding cows worldwide. There is a need for semen extenders that enhance and prolong semen fertility beyond that which is possible using existing commercial formulations for cryopreservation or liquid semen storage. This could allow the use of reduced numbers of sperm per insemination, enhance the use of bulls of high genetic merit, and facilitate the greater application of fresh and sex-sorted semen. Here we review current information on the microenvironments of sperm as they mature and migrate. We also discuss how this knowledge can be used in the design of next-generation semen diluents. IntroductionArtificial insemination (AI) has facilitated improvements to the genetic merit of dairy herds worldwide through provision of greater access to sires with favourable genetic traits, such as those relating to milk production and health. AI also allows easier herd management, reduces the risk of venereal disease, and improves farm safety. The semen extenders employed to successfully maintain the fertility of sperm are central to the widespread and commercial use of AI and these have been extensively reviewed (Vishwanath and Shannon, 2000;Vishwanath, 2003). However, current drawbacks to storage of sperm in a cryopreserved or fresh (liquid) state highlight the need for improvements to semen diluents.Cryopreservation of sperm facilitates its storage for indefinite periods. It is an indispensable technology to the AI industry internationally with 95% of AI worldwide carried out using frozen-thawed sperm (Thibier and Wagner, 2002). However, higher concentrations of sperm must be used per insemination dose to achieve non-return rates (NRRs) comparable to AI with fresh sperm (Vishwanath and Shannon, 2000). Sperm damage occurs during sperm cryopreservation as a result of mechanical stress to the plasma membrane and the generation of excess reactive oxygen species (ROS) (Bailey et al., 2000). This cryodamage is likely to account for the reduced viability of frozen-thawed compared with fresh sperm. However, cryocapacitation, a phenomenon of sperm cryopreservation (Bailey et al., 2000), could account in part for reduced fertility of frozen-tha...