Introduction

Since spermatozoa were first observed in the human ejaculate by Leeuwenhoek in the late seventeenth century, it has been believed that a threshold number of motile and morphologically normal spermatozoa is required to initiate a pregnancy. More recently it has been determined that the functional competence of these sperm is somewhat poorly reflected by the conventional semen analysis. Therefore, much work has been done to develop functional tests which may more accurately reflect fertility status. The sperm penetration assay (SPA) or zona-free hamster egg test is one such test which is designed to evaluate sperm functional competence or fertilizing potential. It is based on the phenomenon that a hamster egg, when rendered zona-free by proteolytic digestion of the zona pellucida, can be penetrated by spermatozoa from another species, such as the human. The cross-species ‘fertilization’ assay is correlated with true fertility and thus has diagnostic potential in human fertility problems. Much has been written about this assay since the presentation of our first human–hamster experiments in 1976 (Yanagimachi et al., 1976). The purpose of this chapter is to focus on a few aspects of the assay, including a brief review of SPA methodology, comparison of the SPA with other semen tests, validity of the SPA in prediction of in vivo pregnancy, and predictive potential of the SPA for the success of in vitro fertilization (IVF).

SPA methodology

The original SPA methodology (Yanagimachi et al., 1976) used washed sperm preincubated in BWW (Biggers, Whitten and Whittinghams's medium) for 2–7 hours.