In the present study it was observed that progressive motility, viability, membrane integrity (HOST +ve), acrosome integrity (Table 1) and total abnormalities (Table 2) declined significantly (P<0.01) at 24 h of cryopreservation as compared to fresh semen. These results are in conformity to the observations of various researchers who concluded that cryopreservation causes adverse effects on the spermatozoa manifested as a depression in viability rate, structural integrity, depressed motility and conception rates
(Chen et al., 2015; Amidi et al., 2016; Ahmed et al., 2019; Dewry et al., 2020). This sharp reduction could be due to cryopreservation induced extensive biophysical damage as a consequence of supercooling, extracellular ice crystals formation, concentration of solutes such as sugars, salts and proteins (
Lemma, 2011) and ultimately dehydration
(Andrabi, 2007) causing the ultrastructural damage to membranes and destabilizing them; predisposing sperms to gross morphologic defects, such as missing and abnormal acrosomes. The higher level of ROS generation during the process of freezing was also reported to damage normal spermatozoa by inducing lipid peroxidation and DNA damage
(Moustafa et al., 2004; Aitken et al., 2016) and also deteriorate its quality by compromising membrane integrity, chromatin integrity and blocking oxidative metabolism
(Lone et al., 2016). ROS attack reduces intracellular ATP concentration that leads to decreased sperm motility and viability; axonemal damage and increased sperm morphological abnormalities (
Bansal and Bilaspuri, 2011). Moreover, in the present study, there was a non-significant reduction in the above mentioned sperm parameters observed at 1 and 2 month intervals. However, sperm viability reduced significantly up to 1 month of cryopreservation. This minute reduction in quality during the storage period could be due to the excess generation of ROS and RNS. Actually, neither of these reactive oxidants is toxic
in vivo as these are regulated to a physiological concentration by the homeostasis process or by the complex antioxidant system. While during
in vitro conditions the sperm competency is challenged due to higher production levels of ROS as compared to their detoxification (
Du Plessis et al., 2008;
Balamurugan et al., 2018). This hypothesis was in agreement with
Pacher et al., (2007) and
Radi (2013) where they reported that the detrimental effects of NO and O
2- is mediated by formation of more noxious peroxynitrite. In consonance with the present study, Bansal and Bilaspuri (2011) detailed the excessive production of these reactive oxidants in semen can cause insufficient axonemal phosphorylation and ATP depletion which ultimately decreases sperm motility and viability. Moreover, polyunsaturated fatty acids (PUFAs) rich sperm plasma membrane is very susceptible to the attack of ROS due to binding of PUFAs to oxygen
(Gadea et al., 2013) and RNS due to abstraction of hydrogen by NO, that causes a cascade of reactions, resulting in the production of free radicles triggering lipid peroxidation
(Dhindsa et al., 2004; Semenova et al., 2005; Makker et al., 2009). The scrambling in phospholipid configuration is associated with misconfiguration of protein and lipids; morphological changes in the organization, fluidity, permeability of the sperm membranes and disruption of protein-lipid interactions (
Gadella and Harrison, 2002) which disrupt sperm morphology and physiological activity
(Casas et al., 2010; Flores et al., 2010; Sood et al., 2020).
The capacitation status and level of seminal biomarkers have been presented in Table 3 and Fig 1 respectively. The present study revealed significant loss of non-capacitated, acrosome intact (F pattern) sperms during cryopreservation in comparison to fresh semen. Percent capacitated sperms (B pattern) were significantly (p<0.01) greater in cryopreserved semen (37.13±0.64) as compared to fresh ejaculate (26.02±0.60) which is in consonance with previous findings. After cryopreservation, surviving bull and human sperm contain more intracellular calcium than earlier, reûecting impaired membrane selective permeability mechanisms
(Longobardi et al., 2017; Vignesh et al., 2020). The increased sperm calcium levels and reactive oxidants levels are thought to trigger an intracellular signaling cascade that has recently been associated with capacitation (
Rodriguez and Beconi, 2009). This cryo-capacitation like change in sperms is induced due to the loss of membrane cholesterol during cryopreservation
(Neild et al., 2003; Rajoriya et al., 2016). Thus it can be concluded that sperm membrane structural changes like efflux of cholesterol trigger the capacitation events, leading to an increase in the fluidity of plasma membrane, bicarbonate (HCO
3-), Ca
2+, intracellular pH and cAMP levels (
Therien, 1995). A significant (p<0.05) increase in acrosome reacted spermatozoa was observed in cryopreserved semen (7.52±0.47) as compared to fresh samples in the present study. A body of evidence indicates that seminal plasma ameliorates cryo-injuries and possibly extends sperm longevity by somewhat delaying the undesired onset of capacitation and acrosome responsiveness
(Vadnais et al., 2005; Vadnais and Roberts, 2007).In the present study, the mean levels of NO (μmol/L) in seminal plasma of neat semen stored at -20°C was 2.31±0.27, while the value in the extended seminal plasma after cryopreservation was 2.37±0.31. These results suggest that there is a non-significant increase in the production of reactive species, particularly NO during cryopreservation, which is in consonance with
Ugur et al., (2019). Cryopreservation decreases the antioxidant defenses of whole semen characterized by the loss of superoxide dismutase activity in human and bull semen
(Dewry et al., 2015; Aliakbari et al., 2016; Najafi et al., 2018) when compared to fresh semen. The total antioxidant capacity (Units/ml) in seminal plasma of neat semen (0.73±0.04) stored at -20°C was found to be higher in comparison to extended seminal plasma (0.44±0.03) obtained after cryopreservation and the possible reason behind this is the presence of oxygen and nitrogen free radicals which enhances its utilization and further reducing its quantity. Besides this, the reduction in the seminal antioxidant profile during cryopreservation further lowers the natural antioxidant capacity in semen
(Lone et al., 2018). Thus seminal antioxidant defense becomes incompetent to maintain equilibrium between the production and detoxification of ROS, leading to oxidative stress during the freezing process (
Dowling and Simmons, 2008) causing a reduction in the semen quality. The lipid peroxidation status (Units/ml) in seminal plasma of neat semen (1.11±0.16) stored at -20°C was found to be higher in comparison to extended seminal plasma (0.65±0.03) of the cryopreserved semen. Most of the lipid peroxidation occurs during phase transition of cryopreservation, however, due to the presence of antioxidants in egg yolk extender, reduction in lipid peroxidation during freezing in comparison to the storage of seminal plasma at -20°C was observed in our study. An earlier study indicated two fold increase in MDA production in frozen as compared to freshly ejaculated spermatozoa
(Karan et al., 2018).