The assembly of a zona pellucida binding protein complex in sperm.

The surface of the sperm cell, after its production in the testis is heterogeneous and subjected to continuous surface remodelling during its voyage through the male and female genital tracts to reach the oocyte. This remodelling will result in a competent sperm that is suitably equipped to bind to the zona pellucida and to follow the processes required for fertilization. During in vitro fertilization (IVF) studies, some aspects of these surface remodelling are mimicked and optimized to achieve good fertilization rates in the reproductive clinic. Although, the IVF experiments with sperm provided knowledge about how sperm motility activation is achieved and gave insights into the extensive sperm surface remodelling that is needed to acquire the high zona binding affinity, it is far from clear how relevant these processes are, for the competent sperm that actually fertilizes the oocyte under in vivo conditions. Moreover, sperm surface remodelling, especially by the female genital tract, is only poorly understood. The aim of this study was to combine the knowledge we have about the sperm surface from in vitro and biochemical approaches on one hand and, on the other hand, to attempt to overview the molecular mechanisms involved in sperm surface remodelling at different places within the male and female genital tract. The relevance for this to predict and identify the proteins involved in sperm zona binding in vivo is discussed.