Assessing human sperm morphology: top models, underdogs or biometrics?

Abstract

The assessment of the percentage of spermatozoa having an 'ideal' morphology using so-called strict method is the method recommended in the latest edition of the World Health Organization (WHO) laboratory manual for semen analysis. This recommendation is a result of the statistical association between 'ideal' sperm morphology and fertility, and of the current general belief that sperm morphology assessment should be used primarily as a fertility tool. The notion of an 'ideal' sperm morphology has persisted despite the very low percentage of such spermatozoa in the semen of fertile men, a subject of intense controversy. The detailed categorization of each abnormal spermatozoon has thus, for a long time, been considered optional and partially redundant, an idea which is reflected in the earlier editions of the WHO manual. However, several recent studies have shown the importance of carefully assessing abnormal sperm morphology for use in the diagnosis of infertility, to determine fertility prognosis, and for basic or public health studies. One approach, which combines videomicroscopy and computer vision, and is the only approach able to assess the continuum of sperm biometrics, has been used successfully in several recent clinical, basic and toxicology studies. In summary, the visual assessment of detailed sperm morphology-including the categorization of anomalies allowing arithmetically derived indices of teratozoospermia-and the more modern computer-based approaches, although often considered to be redundant, are in fact complementary. The choice of the most appropriate method depends on the field of investigation (clinical, research, toxicology) and the problem being addressed. Each approach has advantages as well as certain limitations, which will be discussed briefly herein.

Figure 1

Unadjusted probability of pregnancy per month of attempt at pregnancy, based on normal morphology using World Health Organization (WHO) strict method (borderline = abnormal [A]), modified David's criteria (borderline = normal [B]), and multiple anomalies index (MAI [C]) in a population of 942 fertile European couples. Reproduced from Slama et al. . In this study, thresholds were found by both approaches: 19% with the WHO method and 39% with the method of David; there was no threshold for MAI.