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1 May 2004 Critical Role of Insulin-Like Growth Factor System in Follicle Selection and Dominance in Mares
O. J. Ginther, E. L. Gastal, M. O. Gastal, M. A. Beg
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The role of the insulin-like growth factor (IGF) system in the deviation in growth rates among follicles (follicle selection) was studied in mares using an IGF binding protein (BP) to reduce the follicular-fluid concentrations of IGFs. The future dominant follicle (F1) was treated by intrafollicular injection at the expected beginning of deviation (F1 ⩾ 20 mm; Day 0). The experimental groups were control (no injection, n = 8), vehicle (injection of vehicle; n = 6), and BP (injection of 250 μg of recombinant human IGFBP-3; n = 6). A sample of follicular fluid was taken from F1 on Day 1 in all groups. Compared with the control group, IGFBP-3 reduced (P < 0.05) the follicular-fluid concentration of free IGF-1 by 90%; lowered (P < 0.05) the concentrations of estradiol, activin-A, inhibin-A, and vascular endothelial growth factor; and increased (P < 0.05) the concentration of androstenedione. The diameter of F1 decreased and the diameter of F2 increased after Day 0 in the BP group, compared with the control and vehicle groups. A greater (P < 0.05) increase in circulating concentrations of FSH between Days 0 and 1 occurred in the BP group than in the other groups and accounted for the increased growth of F2. Dominance and ovulation from F1 occurred from fewer (P < 0.03) mares in the BP group (1 of 6) than from the control and vehicle groups combined (11 of 14); the remaining mares in the BP group ovulated from F2. Results indicated that the IGF system has a critical intrafollicular role in the differential changes in concentrations of follicular-fluid factors between the future dominant and subordinate follicles, leading to the development of follicle dominance (selection) and ovulation in mares.

O. J. Ginther, E. L. Gastal, M. O. Gastal, and M. A. Beg "Critical Role of Insulin-Like Growth Factor System in Follicle Selection and Dominance in Mares," Biology of Reproduction 70(5), 1374-1379, (1 May 2004).
Received: 3 December 2003; Accepted: 1 January 2004; Published: 1 May 2004
follicular development
growth factors
steroid hormones
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