Implications of boar sperm kinematics and rheotaxis for fertility after preservation
Introduction
Artificial insemination (AI) is the predominant procedure for breeding commercial sows worldwide. The mostly used semen form is liquid-stored or extended, which involves diluting the ejaculate in an aqueous solution to increase the volume and preserve the functional lifespan of the spermatozoa for 3–7 d [1]. Typically, it is stored for 3–5 d at 15 to 17 °C, and yields farrowing rates (FR) of 80–90% with 13–14 total pigs born, although there is considerable variation among countries and farms [2]. Due to reduced fertility, frozen-thawed boar semen is broadly restricted to specific uses, such as preservation of valuable genetic material as well as long-distance transport of sperm [3].
In contrast to the human species, in which the WHO laboratory manual for the examination and processing of human semen [4] standardizes the procedures for the examination of semen, and there are reference values for semen characteristics [5]; in the porcine species there are no such guidelines that can be used as a reference. For this reason, and to ensure minimum quality of the AI doses produced, stakeholders in different countries have implemented standardised quality control and quality assurance systems in semen processing programmes.
Traditionally, AI was performed in sows, following manual heat detection, by cervical semen deposition (CAI) with 2–3 billion spermatozoa in an 80–100 mL dose with two inseminations on each day of oestrus. However, this AI method is gradually being replaced by new strategies which aim to deposit the semen closer to the site of fertilization while using a lower volume and number of cells (reviewed by Soriano-Úbeda et al., 2013 [6]). Among them, the post-cervical insemination method (PCAI), also known as intrauterine, involves deposition of the spermatozoa in the uterine body, after the cervix and just before the uterine bifurcation, and reduces the number of spermatozoa used to one billion while maintaining the fertilization results obtained with CAI [7]. Due to its simplicity and the numerous advantages that it provides at the production level, PCAI is gradually replacing traditional CAI, particularly in countries with intensive pig production industries [8].
Lowering the sperm number in each dose and/or reducing the number of inseminations per female through the use of a single, fixed-time insemination after ovulation induction, allows more doses to be obtained from each ejaculate and thus greater selection pressure for economically or socially important traits and the wider dissemination of elite genes to commercial farms. Strategies which include a reduction in the number of spermatozoa or the number of inseminations risks a reduction in pregnancy rate (PR) as well as litter size (LS) unless only the highest quality semen is used. Understanding the factors that contribute to successful pregnancy establishment is essential prior to modifying current procedures. This review focuses on boar semen quality, specifically on sperm motility, kinematics, and rheotaxis, and where data are available, how these relate to sow fertility.
Section snippets
Quality control in boar studs
The production of extended pig semen involves different steps that could risk semen quality such as boar management, bacterial contamination, semen collection, processing and storage [9]. Furthermore, variation in fertility between boars as well as ejaculates of individual boars are well described, and in an effort to minimise this variation, boar studs internationally implement standardised quality control and quality assurance systems. To name but a few, in the UK the Agriculture and
In vitro prediction of in vivo fertility
The use of in vitro assays to predict the in vivo fertility of a semen sample (or ejaculate) has been a long-term objective of researchers and industry alike across many species [11,12], the basis of which is sperm motility assessed subjectively under phase contrast microscopy. For this form of assessment, fertility reductions in liquid boar semen are most apparent when motility falls below 60% [13] and thus motility is best used as a negative biomarker. While the motility of spermatozoa in
Computer assisted sperm analysis
Despite the aforementioned limitations of sperm motility, its assessment is the backbone of quality control in boar studs. Over the last two decades, standardised CASA methods have been adopted, which have transformed the way many studs process and evaluate semen [20,21]. More recently, CASA systems have added morphometric and morphological parameters, performed on wet mounts and without the need for staining of spermatozoa. While first reported forty years ago by Dott and Foster [22], modern
Sperm rheotaxis
While chemotaxis and even thermotaxis may be important sperm guidance mechanisms within the oviduct, sperm rheotaxis has been proposed as a major determinant of sperm guidance over long distances in the mammalian female reproductive tract [53]. Positive rheotaxis is the ability of motile cells to orientate and swim against the flow of fluid and using in vitro microfluidic models has now been described in human [54], mouse [53], bull [55] as well as stallion and ram [56] spermatozoa. Rheotaxis
The importance of sperm motility in the establishment of sperm reservoirs in the sow
Given that the site of semen deposition in pigs is similar in conventional AI and natural mating, it is somewhat surprising that AI with one to three billion spermatozoa can result in similar fertility to natural mating, when a boar deposits 30 to 60 billion spermatozoa in 200–300 mL of seminal plasma [67]. This indicates that the necessary elements maintaining fertility are retained when sows are artificially inseminated, such as the number of motile and functional spermatozoa, inseminate
Future avenues of investigation and conclusions
Efforts to standardise the procedures for the examination of boar ejaculates, providing reference values for semen characteristics, have the overall objective to establish minimum semen quality requirements to meet agreed standards. On the contrary, such standardisation is difficult to find among centres employing the CASA technique for sperm motility assessments under field conditions. The automation of motility assessments within boar studs is inevitable and their integration into the
Declaration of interest
The authors have no vested interests to declare.
Acknowledgements
The authors would like to acknowledge the contribution of Marjolaine Lemoine, Jean-Philippe Perrier and Colin Byrne for the generation and analysis of the data in Fig. 1.
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