Cessation of Transition-Phase Follicle Growth in the Guinea Pig by Follicle-Regulatory Protein1

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Abstract
The granulosa cell produces a protein inhibitor of aromatase activity (follicle-regulatory protein: FRP), which recently was purified to homogeneity. To determine the possible involvement of FRP in follicular maturation, we examined the size distribution of follicles and their morphological patterns as well as serum steroid levels after the systemic administration of FRP and/or gonadotropin to gunea-pigs, which have 5-6 days between luteolysis and ovulation in a 16-day cycle. FRP was partially purified from porcine follicular fluid by ammonium sulfate precipitation (0-35%). Dye Matrex Orange A Chromatography, dialysis, and lyophylization. To investigate the effect of pregnant mare''s serum (PMS) during the periovulatory period in follicular development, adult guinea-pigs underwent unilateral ovariectomy on Days 10, 12, and 14 of the estrous cycle (N=6 each). Guniea-pigs were injected twice daily with vehicle or PMS (5 IU) and 2 days thereafter the remaining ovaries were removed. Another group of guniea-pigs received, in addition, intraperitoneal injections of FRP (1 mg) each morning from Day 8 of estrus until they were killed. The resected ovaries were fixed, embedded in paraffin, serially sectioned (7 .mu.m), and stained with Azan for comparative study via light microscopy. All follicles > 400 .mu.m were classified by size, and the artertic pattern was determined by mural granulosa cell pyknosis and antral sloughing. The distribution of follicular of follicular size was not affected by hemicastration at Day 10, although the percentage of total atretic follicles decreased. In the PMS-treated group, there was a significant decrease in the number of viable follicles (700-899 .mu.m) after hemicastration. Also pronounced in follicles of this size was the lack of mid-atretic follicles. After injections of FRP for 3 or 5 days, the overall number of follicles was almost doubled as compared to the number found in the normal ovary. Additionally, there was a significant increase in the percentage of follicles that were recently atretic, although the total percentage of atretic follicles was unchanged. After hemicastration at Day 10 followed by FRP treatment for 2 days, the total percentage of atretic follicles in the remaining ovary decreased to 18% compared with 35% in the normal ovary, 46% in the hemicastrated plus PMS-treated groupo, and 38% in the hemicastrated and PMS- and FRP-treated group (all p < 0.01). Treating thew hemicastrated animal with PMS increased the percentage of atretic follicles in all groups. Thus, it can be concluded that 1) the atretic process is driven by gonadotropins in an individual follicle, 2) FRP directly increases atresia in the untreated ovary during the estrous cycle, and 3) hemicastration can "rescue" follicles from atresia in the remaining ovary.