Abstract
BackgroundAlthough spermatozoa with normal morphology are assumed to have uniform fertilization ability, recent data show that even normal spermatozoa have considerable variation in their head shape which is associated with differences in fertilization ability. Appropriate quantitative indicators for good sperm morphology, however, remain unidentified.MethodsTherefore, in an effort to identify such an indicator, we compared the nuclear contour of normal mouse spermatozoa by quantitative multivariate analysis using elliptic Fourier descriptors combined with principal component analysis. The spermatozoa were obtained from different strains and collection sites which have been shown to be associated with different fertilization abilities.ResultsWe found that the head was 5.7% thinner in spermatozoa from the B6D2F1 (BDF1) strain, known to have a higher fertilization rate, than in those from the C57BL/6N (B6N) strain, which has a lower fertilization rate. Moreover, zona-penetrated spermatozoa in the perivitelline space consistently had 5.4% thinner heads than those isolated from the epididymis before ejaculation. The aspect ratio, which represents the sperm head thinness, uniquely distinguished these sperm populations, confirming its validity as a morphological indicator.DiscussionBecause aspect ratio has also been shown to characterize human spermatozoa, this unique morphometric indicator might be applicable to compare normal spermatozoa among multiple patients, which will greatly facilitate and enhance current reproductive technologies.
Highlights
Male infertility causes half of infertility cases in humans (Irvine, 1998)
Elliptic Fourier descriptor (EFD) and Principal component analysis (PCA) revealed sperm head aspect ratio as unique fertility indicator In order to compare zona penetrated spermatozoa and ejaculated sperm, we focused on contour of the nucleus, which does not change by spontaneous acrosome reaction
We quantified the variation in head morphology of normal spermatozoa, as optimally separated into multiple principal components (PCs; Fig. S6), among B6N (n = 170, Fig. 2A) and BDF1 spermatozoa (n = 163, Fig. 2B)
Summary
Male infertility causes half of infertility cases in humans (Irvine, 1998). To assess male infertility, features of the population of spermatozoa in the semen that positively correlate with fertilization ability, including the ratio of spermatozoa showing normal morphology and the number of motile spermatozoa (World Health Organization, 2010), have been used (Jouannet et al, 1988; Toner et al, 1995; Eggert-Kruse et al, 1996; Coetzee, Kruger & Lombard, 1998; Menkveld et al, 1990; Van Waart et al, 2001; Garrett et al, 2003; Liu, Garrett & Baker, 2003). Selection of good sperm morphology is beneficial when injecting a single spermatozoon into an ovum in the process known as intracytoplasmic sperm injection (ICSI) (Palermo et al, 1992) to resolve male infertility. It is unclear whether good sperm can be identified by indicators of infertility or whether they are represented by other morphologies, because normal sperm indicators that are quantitatively correlated with the success of fertilization have not been well studied. The aspect ratio, which represents the sperm head thinness, uniquely distinguished these sperm populations, confirming its validity as a morphological indicator. Because aspect ratio has been shown to characterize human spermatozoa, this unique morphometric indicator might be applicable to compare normal spermatozoa among multiple patients, which will greatly facilitate and enhance current reproductive technologies
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