Cryopreservation of cat semen in straws: comparison of five different freezing rates

The first pregnancies in domestic cats were obtained using semen frozen in pellets (Platz et al. 1978). Other freezing methods, vials (Lengwinant and Blottner 1994) or straws (Pope et al. 1991; Hay and Goodrowe 1993), have also been used. Pelleted freezing has often been the standard method (Howard 1986). Opinions about the freezing method are discordant; the best method for Pope et al. (1991) was using straws; in fact, the post-thaw motility and the percentage of normal acrosomes were of 44 +/- 4 and 62 +/- 3%, respectively, with straws and 11 +/- 3 and 26 +/- 4%, respectively, with pellets. According to Wood et al. (1993), there are no differences between the two methods, with a motility of 66.2% and a percentage of normal acrosomes of 28.6% for the pellet method and a motility of 67.0% and a percentage of normal acrosomes of 27.4% for the straw container. However, these two authors used two different freezing protocols. A high concentration of glycerol (i.e. 8%, vol/vol) damages cat semen (Zambelli 1994; Nelson et al. 1999), because of his toxicity to spermatozoa (Graham 1996); while a concentration of 4% is suggested (Zambelli 1994). Fast green FCF Bengal pink staining is often used to evaluate the acrosomal morphology (Wood et al. 1993; Zambelli et al. 1993). As there are no studies on the influence of freezing rate on motility and on acrosomal morphology, the aim of this study was to test five freezing rates in order to verify which is the best for the cryopreservation of cat semen in straws.