Attaining dominance in vivo: distinguishing dominant from challenger follicles in humans.

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Abstract
This study provides the first report of local suppression of follicle growth in vivo by the dominant follicle in humans. Analyses were focused on antral follicles > or =6 mm in diameter, classified as dominant (ovulatory) and challenger (large, non-ovulatory) follicles. We examined antral follicle characteristics, interactions and location in the ovary in the follicular phase of natural, ovulatory cycles in humans. Individual antral follicles (> or =2 mm) were identified and monitored using a system, developed by the authors, incorporating mapping and computer modelling from ultrasound imaging of the ovary. Results showed that antral follicle development was not restricted to a particular location in the ovary. The apparent randomization of location may provide a developmental advantage for follicles. The population of challenger follicles peaked at mid follicular phase, suggesting a period when the ovarian environment was supportive of larger antral follicle development. As ovulation date approached, however, challenger follicles did not attain the visible characteristics of dominance: mid-level echogenicity, large size, smooth antral wall, round shape. Furthermore, dominant follicles subsequently appeared to reduce the number of neighbouring challenger follicles. The results strongly suggest that a paracrine secretion of the dominant follicle was the source of this localized effect. This process may provide increased nutrient level and space at a critical growth stage for dominant follicles.

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