Frozen sperm quality with reference to reactive oxygen species: A review

Reactive oxygen species are highly reactive intermediates of normal cell metabolism. Their generation is well controlled and serves physiological purposes, such as energy generation, defense mechanisms and signal transduction. Counterparts are several low molecular weight and enzymatic antioxidants. An imbalance of pro-and antioxidants in favour of the former is termed as oxidative stress. In the male genital tract, ROS are produced by spermatozoa, granulocytes and macrophages. It severely impairs sperm function like motility or fertilization or post fertilization developments. It plays physiological role like involves in the process of capacitation and acrosomal reaction. It influences thiol-oxidation stages of DNA compaction, and several other physiological functions. Despite several physiological useful roles, oxidative stress can arise as a consequence of the excessive production of ROS and an impaired antioxidant defense system, leading to several sperm pathology. It causes arrest of sperm motility, oocyte penetration, blocks sperm-oocyte fusion, damage of DNA, inhibit mitochondria1 functions, and synthesis of DNA. RNA, proteins and also affect spermatozoa axoneme. Excess ROS production has a multidimentional effect on spermatozoa, by altering membrane lipid compositions to different cellular structural functions and does not leave even nuclear components which is very essential for life propagation. Majority of the reports are confined to human spermatozoa barring only a few on spermatozoa of domestic animals. Among those covers bull and ram spermatozoa. Cryodamnges induced by ROS are also equally anticipated in spermatozoa of domestic species. But the exact mechanism of generation of ROS during the process of freezing and thawing is not very well known. Further, effects of ROS on sperm morphology and fertility (including its role in sperm-egg interaction during fertilization) are still on the way of research in domestic animals. The extent of damages incurred by such process in relation to post thaw survival of sperm and fertility are also not fully understood, therefore, need further investigation. If these basic mechanisms of damages are understood, then we will be in a position to specifically proceed ahead towards resolving the problems of such damages using suitable preventive measures. So there is scope to carry out work in this area particularly in domestic species, buffalo of Indian sub continental interest. Reports are also available to mention about ROS production during ctyopreservation as well as its contributory role for bringing cryo-capacitation (capacitation like changes) during freezing of semen. It is not known how free radical generation leads to capacitation. Oxidative stress could also be asociated with impaired antioxidant protection. possibly related to dietary deficiencies, age or genetic factors. There is ample scope to understand the process of ROS generation and to study its effects on spermatozoa in domestic species. But we are more concerned to the effects which are unavoidable while processing and freezing of semen. We need to reply ourselves whether ROS generated during freezing of bovine semen have similar effects as in human spermatozoa. If so then, our approach should be on to prevent the pathological aspects of ROS generation which obviously causes a lot of damage to spermatozoa. Additions of exogenous antioxidants principles in semen extenders would be the area of future research as an applied angle. To begin with some work has already started in this direction but some more attempts with a comprehensive nature of work is still needed.