Adverse manifestations and ameliorative strategies in ram semen preservation: a review

Ram spermatozoa are adversely affected during various stages of semen preservation. One of the major factors contributing to the disruption of cellular function in sperm during preservation is the formation of reactive oxygen species (ROS). ROS are required for maturation, capacitation and acrosome reaction but they also combine with several peroxidable cellular components and transform them into oxygen-based deleterious products. Spermatozoa do not possess any internal repair mechanism against cellular damage for which an environment is essential to protect them from ROS-induced damage. The use of antioxidants and chelating agents can increase the short-term storage ability of ram sperm in liquid state by reducing the oxidative stresses. Unlike some mammalian species, ram spermatozoa are susceptible to various stresses during freezing and thawing. Premature capacitation as a consequence of freezing and thawing curtails the life span of frozen-thawed spermatozoa. Addition of antioxidants generally does not maintain the level of antioxidant(s) following thawing of semen. The seminal plasma proteins (BSP) bind to capacitation factors, namely, high-density lipoproteins and glycosaminoglycans to promote sperm capacitation, indicating their beneficial role in sperm function and fertility. In contrast to their beneficial role in sperm function, BSP proteins may be detrimental to sperm storage as they also stimulate cholesterol and phospholipids efflux from the sperm membrane in a time and concentration dependent manner. Continuous exposure of sperm to seminal plasma that contains BSP proteins can render sperm very sensitive to storage in the liquid or the frozen state. Melatonin has a capacity to act as a ROS scavenger and antioxidant. Further studies are still required to determine if there is either a direct or indirect action of the pineal hormone on the spermatozoa. Inspite of good post-thaw sperm motility of frozen ram semen the cervical insemination using frozen-thawed semen results in unacceptably low fertility. The major factors limiting the successful conception are the inability of frozen-thawed ram sperm to transit the cervical canal, short duration of sperm survival and the complex anatomy of cervix. Laparoscope aided intrauterine AI using frozen-thawed semen invariably improves the fertilization rate over cervical insemination but the cost, skill, complicacy and minor surgery limits its greater utilization. Efforts are going on to develop a simple, cost-effective and non-invasive transcervical AI technique in order to penetrate the cervix and achieve the acceptable lambing with frozen semen. Computer-aided semen analysis (CASA) is an accurate technique for objective measurements of sperm kinematics and has been applied for short-term and long-term preservation of ram spermatozoa. The development of sperm sexing technique has added new dimensions on semen processing for the associated uses of the semen. The ability of sexed sperm samples to bring about conception in vivo has not been successful but lambs have been born after frozen semen has been thawed, sorted, refrozen, thawed and used following in vitro fertilization and transfer of resultant embryos to surrogate dams.