Damage to the oocyte during follicular growth has been
demonstrated in experiments to evaluate seasonal variation
in the competence of oocytes (harvested at slaughter or via
ultrasound-guided aspiration) to be fertilized in vitro and
develop into blastocysts. In these experiments, which used
lactating Holstein cows, the proportion of oocytes that
became blastocysts after in vitro fertilization10–12 or chemical
activation13 was lower in summer than winter (see
Figure 64.1 for an example).
The seasonal effects on blastocyst formation largely represented
decreased competence of cleaved embryos to
develop rather than competence of the oocyte to cleave.
One reason is that heat stress reduces accumulation of
molecules
in the oocyte important for early embryonic
development. Actually, there are differences in transcript
abundance between embryos produced by in vitro fertilization
of oocytes collected in summer versus winter.14 Also,
perhaps, oocytes in the summer are more likely to undergo
abnormal chromosomal segregation so that subsequent
embryos are chromosomally abnormal. Occurrence of chromosomal
abnormalities can affect the rate of embryonic
development.15