Dynamics of the sperm nucleus

Abstract

Sperm DNA and the associated nuclear matrix are completely enclosed within the nuclear envelope and are not directly exposed to the cytoplasm. In contrast, the maternal genome is not enclosed within a nuclear envelope and is directly exposed to the cytoplasm. Despite being protected by a rigid enclosure, sperm chromatin DNA strands are likely to be damaged during the long process of spermatogenesis, in which apoptosis also causes DNA fragmentation. Further, sperm DNA fragmentation is also induced during the long journey of the sperm to reach the oocyte (Aitken and Krausz 2001). Such DNA fragmentation may lead to poorfertilization and embryonic development, reduced rates of implantation and successful pregnancy, and recurrent pregnancy losses (Chohan et al. 2006).

During spermatogenesis, the sperm chromatin is essentially remodeled, and it condenses. The genomic DNA is transformed from a histone-associated nucleosome to a protamine-rich toroid (doughnut-like) structure (Fig. 7.1). Although these doughnut-like structures are highly resistant to external agents such as environmental toxins, genetic mutagens, oxidative stress, and smoking, the nuclear DNA is still considerably susceptible to oxidative stress (Aitken and Baker 2004; Bennetts and Aitken 2005; Lewis and Aitken 2005). Chromatin modification during the epididymal maturation of the sperm is important to ensure that the nucleus (genome) develops resistance to such external agents. During the entry of the sperm into the oocyte, the nuclear envelope and nuclear matrix proteins are gradually degraded by the action of the reducing agent glutathione, which is abundant in the ooplasm.

It is interesting for researchers to evaluate the status of sperm DNA and to investigate the factors that induce DNA damage. In this regard, I describe the basic findings on the sperm nucleus and then proceed to discuss recently developed techniques for evaluating the degree of DNA damage in sperm.