Prognostic markers for competent human spermatozoa: fertilizing capacity and contribution to the embryo.

ABSTRACT

Management of male infertility largely depends on our understanding of cellular and molecular aspects of spermatogenesis as well as sperm function. Apart from standardized and comprehensive semen analysis, prognostic markers estimating the fertilizing capacity of either ejaculated spermatozoa or testicular spermatids are required. While there is general agreement that correct replacement of DNA-binding histones by protamines represents a prerequisite for achieving competent spermatozoa, especially in intracytoplasmic sperm injection (ICSI) where natural selection mechanisms are bypassed, the function of a variety of transcripts within the spermatozoa's cytoplasm and of remaining highly acetylated histones is still a matter of debate. Hence, this review brings the up-to-date research on mammalian spermatozoal chromatin composition into focus, which is discussed in conjunction with the paternal role on epigenetic reprogramming of the zygote following fertilization. As paternal transcripts have been demonstrated to be transmitted to the oocyte, it is now accepted that they represent more than solely remnants of previous transcriptional activity. Acetylation of histones, normaly a characteristic of transcriptional activity, was for a long time a miracle, as spermatozoa are transcriptionally inactive cells, but is now suggested to represent epigenetic marks that are transmitted to the oocyte and play an important role in the regulation of early gene expression in the developing embryo.