72 SUPERIOR PREGNANCY RATES WITH THE TRANSFER OF OPEN PULLED STRAW-VITRIFIED V. CONVENTIONALLY CRYOPRESERVED EMBRYOS IN GOATS

Abstract

Two separate studies were conducted in the course of which goat embryos were cryopreserved either by the open pulled straw (OPS) procedure of (Vajta et al. 1998 Mol. Reprod. Dev. 51, 53–58) or by conventional slow freezing. Embryos of each doe were assigned to both methods. A total of 94 morphologically intact blastocysts, collected by transcervical flushing from superovulated pluriparous Boer goat does, were involved. For OPS-vitrification the method of Vajta et al. (1998) was slightly modified (El-Gayar et al. 2008 Cryobiology 57, 191–194). It involves exposure of the embryos to a medium containing 10% dimethyl-sulfoxide (Me₂SO) + 10% ethylene glycol (EG) for 1 min, followed by 20 s in medium containing 20% Me₂SO + 20% EG. After aspiration into thinly drawn-out straws, the straws were plunged directly into liquid nitrogen. Conventional freezing comprises equilibration in M2 medium containing 1.4 M glycerol and cooling in 0.25-mL straws at a rate of 0.3°C min^–1 as described in (Nowshari and Holtz 1993 J. Anim. Sci. 71, 3403–3408). Embryos of both treatment groups were transferred to synchronized recipients (2/recipient) within 2 h after thawing. The surrogate mothers were diagnosed for pregnancy 5 weeks later and, if gravid, permitted to carry to term. Differences between means were tested for significance by the chi-square test. All embryos were recovered after thawing. Within the 2-h period from thawing to transfer, OPS-vitrified embryos had the appearance of flawless, expanded blastocysts, whereas conventionally frozen–thawed embryos had not quite reached full re-expansion. Of 25 recipients receiving OPS-vitrified blastocysts, 22 (88%) kidded with average litter size of 1.5. Of 22 recipients receiving conventionally frozen–thawed blastocysts, 10 (45%) kidded with average litter size of 1.8. Embryo survival in vitrified blastocysts amounted to 67%, and in conventionally frozen–thawed blastocysts 42%. The difference in kidding rate between the two methods was statistically significant (P < 0.01). Regardless of the cryopreservation method all kids born were viable, their birth weight being within the range typical for the population. The results indicate that, for the cryopreservation of caprine blastocysts, OPS vitrification is a more suitable technique than conventional freezing and should be routinely employed.