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Abstract
A total of 64 ewes was used to determine if the changes in superovulatory yields related
to the ovarian status at the start of superovulatory treatment are due to differences
in the population of gonadotrophin-responsive follicles, alterations in the processes
of ovulation or transport of embryos from oviduct to uterus and/or developmental competence
of the oocyte/embryo. Ovarian status at the start of a superovulatory FSH step-down
treatment, administered coincidentally with a progestagen, was assessed by ultrasonography.
On Day 4 after progestagen withdrawal, embryos were recovered from oviduct and their
viability was determined by assessing development in vitro culture (IVC) until the
hatched blastocyst stage. In all the ewes, the ovulation rate was related positively
to the number of 2-3 mm follicles at first FSH injection (P<0.005). However, the total
number of embryos and their viability were related to the more limited category of
3 mm follicles (P<0.05), whereas a higher degeneration rate was related to the number
of 2mm follicles. The presence of a corpus luteum (CL) at the start of superovulatory
treatment exerted a protective effect on embryonic viability, decreasing the degeneration
of embryos. On the other hand, the presence of a dominant follicle at first FSH dose
affected the mean size of the pool of follicles responding to the superovulation treatment,
because ovulation arose from 3 to 5 mm follicles in absence of large follicles (P<0.05),
but from 2 to 3 mm follicles when large follicles were present (P<0.005), indicating
atresia in medium sized follicles in the presence of a large follicle.