Jj. Dufour et al., The effect of a GnRH agonist on follicular dynamics and response to FSH stimulation in prepubertal calves, REPROD NUTR, 39(1), 1999, pp. 133-144
Endocrine control of follicular growth was determined by observing the left
ovary of prepubertal calves previously treated with a patent GnRH agonist
for 13 days. The ovarian response to hormonal stimulation was determined us
ing the right ovaries of the same animals. Three-month-old crossbred calves
were assigned to one of the two following treatment groups: 1) saline cont
rol for 13 days, with purified porcine FSH for the last 3 days (n = 5); and
2) GnRHa for 13 days, with purified porcine FSH for the final 3 days (n =
5). The left ovaries were removed from all calves after 10 days, and the ri
ght ovaries were removed at the end of treatment. Plasma concentrations of
FSH, LH and oestradiol-17 beta were followed up during the GnRHa and pFSH t
reatments. The maximum macroscopic diameter of the Fl follicle, as determin
ed by daily ultrasonography, did not differ between GnRHa-treated calves (f
rom 6.6 to 10.4 mm) and the saline control calves (from 6.7 to 10.3 mm). Hi
stological analysis of the ovaries showed that the number of follicles > 0.
40 mm in diameter varied greatly for calves of the two groups (from 11 to 2
20 at 10 days). GnRHa significantly increased the mean number of follicles
(total and nonatretic) of size class > 5.4 mm as compared to saline control
calves (P < 0.05). The FSH treatment significantly increased the mean numb
er of follicles 3.00-5.4 and > 5.4 mm in diameter (P < 0.05), with no chang
e in the number of follicles smaller than 3.00 mm. The rate of atresia of l
arge follicles (3.01-5.40 mm) was significantly reduced by purified porcine
FSH treatment in both groups (P < 0.05). In no case did the GnRHa induce o
vulation or luteinization of follicles. The LH and FSH concentrations incre
ased transiently after GnRHa treatment on the first day, but afterwards, bo
th hormones increased to only one sixth of what was observed after the init
ial GnRHa injection treatment. This increase in LH and FSH was observed 1 h
after GnRHa treatment on each consecutive day of the experiment and were s
ignificantly different in the control group (0 h versus 1 h versus 2 h x sa
line control versus GnRH agonists groups; P < 0.01) During the superovulato
ry treatment, FSH concentrations peaked at around 0.70 ng.mL(-1) in both sa
line- and GnRHa-treated groups on the first day but on the last day of suro
vulatory treatment, FSH concentrations were higher in GnRHa agonist-treated
calves than in the control calves (day 11 versus day 12 versus day 13 x sa
line control versus GnRH agonist treatment groups; P < 0.01). LH profiles w
ere unchanged by surovulatory treatment. Concentrations of oestradiol-17 be
ta increased significantly over the three days (P < 0.001) of the superovul
atory treatments in both groups (P < 0.01). These results indicate that GnR
H agonist treatment allows recruited antral follicles to pursue their growt
h during the early selection process via sustained FSH and LH secretion all
owing more than a single large follicle to maintain their growth without go
ing to atresia. (C) Inra/Elsevier, Paris.