Structural and viral association comparisons of bovine zonae pellucidae from follicular oocytes, Day-7 embryos and Day-7 degenerated ova

Bovine zonae pellucidae (ZP) from follicular oocytes and from embryos and degenerated ova collected on Day 7 from superovulated cows were examined by scanning electron microscopy, by dimensional measurement, and by total protein determination. The number of plaque-forming units (PFU) of infectious bovine rhinotracheitis virus (IBRV) that were associated with ZP-intact embryos/ova from each of the 3 sources after in vitro exposure was also determined.

Scanning electron microscopy revealed that the surfaces of Day-7 embryos and degenerated ova were smoother than those of follicular oocytes. Mean dimensional measurements of the diameter/thickness of the ZP from follicular oocytes, Day-7 embryos, and degenerated Day-7 ova were 156.7 μm/12.3μm, 161.3μm/12.6μm, and 158.9μm/12.8μm, respectively. The mean total protein per ZP of follicular oocytes, embryos, and degenerated ova was 0.331 μg, 0.349 μg, and 0.254 μg, respectively. Considerable variability existed within groups, but significantly greater quantities of IBRV were associated with follicular oocytes (mean PFU/oocyte = 68.1) than with Day-7 embryos (mean PFU/embryo = 43.0; P<0.05) or with Day-7 ova (mean PFU/ovum = 31.9; P<0.01).

The reliability of using an assay for IBRV associated with nontransferable ova/embryos as an indicator of the presence or absence of the virus in transferable embryos from the same collection (Day 7) was supported. Although structural differences between the ZPs of follicular oocytes and Day-7 embryos were observed in this study, further investigation is needed to determine if there are differences in the protective function of the respective ZPs.