It was hypothesized the lower fertility of repeat-breeder (RB) Holstein cows is associated with oocyte quality and this negative effect is enhanced during summer heat stress (HS). During the summer and the winter, heifers (H; n=36 and 34, respectively), peak-lactation (PL; n=37 and 32, respectively), and RB (n=36 and 31, respectively) Holstein cows were subjected to ovum retrieval to assess oocyte recovery, in vitro embryonic developmental rates, and blastocyst quality [terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive cells and total cell number]. The environmental temperature and humidity, respiration rate, and cutaneous and rectal temperatures were recorded in both seasons. The summer HS increased the respiration rate and the rectal temperature of PL and RB cows, and increased the cutaneous temperature and lowered the in vitro embryo production of Holstein cows and heifers. Although cleavage rate was similar among groups [H=51.7% ± 4.5 (n=375), PL=37.9% ± 5.1 (n=390), RB=41.9% ± 4.5 (n=666)], blastocyst rate was compromised by HS, especially in RB cows [H=30.3% ± 4.8 (n=244) vs. 23.3% ± 6.4 (n=150), PL=22.0% ± 4.7 (n=191) vs. 14.6% ± 7.6 (n=103), RB=22.5% ± 5.4 (n=413) vs. 7.9% ± 4.3 (n=177)]. Moreover, the fragmentation rate of RB blastocysts was enhanced during the summer, compared with winter [4.9% ± 0.7 (n=14) vs. 2.2% ± 0.2 (n=78)] and other groups [H=2.5% ± 0.7 (n=13), and PL=2.7% ± 0.6 (n=14)] suggesting that the association of RB fertility problems and summer HS may potentially impair oocyte quality. Our findings provide evidence of a greater sensitivity of RB oocytes to summer HS.

The introduction and widespread application of vitrification are one of the most important achievements in human assisted reproduction techniques (ART) of the past decade despite controversy and unclarified issues, mostly related to concerns about disease transmission. Guidance documents published by US Food and Drug Administration, which focused on the safety of tissue/organ donations during Zika virus spread in 2016, as well as some reports of virus, bacteria, and fungi survival to cryogenic temperatures, highlighted the need for a review of the way how potentially infectious material is handled and stored in ART-related procedures. It was experimentally demonstrated that cross-contamination between liquid nitrogen (LN₂) and embryos may occur when infectious agents are present in LN₂ and oocytes/embryos are not protected by a hermetically sealed device. Thus, this review summarizes pertinent data and opinions regarding the potential hazard of infectious transmission through cryopreserved and banked reproductive cells and tissues in LN₂. Special attention is given to the survival of pathogens in LN₂, the risk of cross-contamination, vitrification methods, sterility of LN₂, and the risks associated with the use of straws, cryovials, and storage dewars.