132 DIFFERENCES IN RESPIRATION RATES BETWEEN IN VIVO- AND IN VITRO-PRODUCED BOVINE EMBRYOS

Abstract

In vitro-produced (IVP) bovine embryos differ (e.g. morphology and physiology) from their in vivo counterparts. Oxygen consumption is an indicator of the overall metabolic activity of a single embryo. Therefore, the aim of this study was to determine and compare respiration rates of in vivo- and in vitro-produced bovine day 7 embryos. Diameters of these two embryo types were also compared. In vivo embryos (n = 28) were recovered from 8 superovulated Holstein Frisian cows on day 7 following AI, while IVP embryos (n = 160; Holm et al. 1999 Theriogenology 52, 683-700) were used on day 7 after fertilization. Embryos were measured (outer diameter) and morphologically evaluated (Quality 1 to 4, IETS Manual, 1998). Only transferable in vivo embryos were used (i.e. excluding Quality 4). Respiration rates were measured on each embryo by Nanorespirometer technology (Lopes et al. 2005 Reprod. Fertil. Develop. 17, 151). Data were analyzed using Proc Mixed, and values are presented as mean ± SEM. Values with different superscripts differ significantly (P < 0.05). The average respiration rates were 0.82 ± 0.06^a nL/h for in vivo vs. 1.37 ± 0.06^b nL/h for IVP embryos. The average respiration rates for the different morphological qualities were as follows (nL/h, numbers in brackets): IVP: 2.1 ± 0.08^a (38), 1.37 ± 0.07^b (55), 1.08 ± 0.07^c (48) and 0.62 ± 0.11^d (19) for Quality 1, 2, 3, and 4, respectively. In vivo: 1.17 ± 0.21^b,c,e (6), 0.80 ± 0.15^c,d,e (12), and 0.64 ± 0.16^d,f (10) for Quality 1, 2, and 3, respectively. The average diameter (mm) of in vivo and IVP embryos was 0.157 ± 0.002^a and 0.176 ± 0.002^b, respectively. Respiration rates were directly related to embryo diameter; larger embryos were associated with higher respiration rates (y = 17.55 ± 1.32 nL/h × mm, n = 188). Respiration rates of in vivo embryos were significantly lower than those of IVP embryos, regardless of quality. This difference could reflect an effect of the culture conditions on IVP embryos because media components affect embryo metabolism. Moreover, the different ages (day 7 for IVP vs. approximately Day 6.5 for in vivo embryos, because in vivo embryos are less than 7 days after fertilization at recovery) and stages (IVP: up to expanded blastocyst stage; in vivo: morula or early blastocyst stage) could have influenced the results and also partly explain the smaller diameter of the in vivo embryos. Finally, respiration rates decreased proportionately to the morphological quality within embryo type, indicating that morphological differences are reflected at the physiological level. In conclusion, this study further outlines metabolic differences between in vivo and IVP bovine embryos. Whether such differences are a manifestation of metabolic stress associated to the separation from the natural environment or reflect suboptimal culture conditions is yet to be determined.

ASL is supported by FCT, Portugal.