What is a dominant ovarian follicle?

Dominant Ovarian Follicle: Definition and Significance

A dominant ovarian follicle is a single selected follicle that outgrows other cohort follicles during the follicular phase, becomes responsive to luteinizing hormone (LH), and is destined for ovulation while suppressing the development of other follicles.

Characteristics of the Dominant Follicle

The dominant follicle has several key characteristics that distinguish it from other follicles:

• Size: Typically reaches 1.5-2.5 cm in diameter before ovulation 1
• Selection process: Emerges from a cohort of growing follicles during declining FSH levels
• Hormone responsiveness: Transitions from FSH-dependence to LH-dependence
• Survival advantage: Uniquely escapes atresia (programmed cell death) that affects all other follicles in the cohort
• Suppressive function: Inhibits the growth of other follicles through hormonal mechanisms

Physiological Development Process

The development of a dominant follicle follows a specific pattern:

1. Recruitment phase: Rising FSH levels stimulate the emergence of a cohort of follicles
2. Selection phase: As FSH levels decline, most follicles undergo atresia while one follicle becomes dominant
3. Dominance phase: The selected follicle continues growing despite declining FSH levels by:
   • Acquiring FSH independence
   • Developing LH receptors on granulosa cells
   • Enhancing its responsiveness to growth factors, particularly insulin-like growth factors (IGFs) 1
4. Maturation phase: The dominant follicle reaches full maturity and prepares for ovulation

Molecular Mechanisms of Dominance

Several molecular mechanisms contribute to dominant follicle selection:

• FSH threshold adaptation: The dominant follicle develops the ability to continue growing at lower FSH concentrations than other follicles 1
• Growth factor regulation: Enhanced IGF activity through regulation of IGF binding proteins and specific proteases 1
• Anti-Müllerian hormone (AMH): Levels decline as follicle size increases, becoming largely absent in dominant follicles 1
• Vascular development: Differential regulation of blood vessel formation may provide the dominant follicle with greater access to gonadotropins and nutrients 1

Clinical Significance

The dominant follicle concept has important clinical implications:

• In natural cycles: Only one follicle typically reaches dominance and ovulation
• In ovarian stimulation: Multiple follicles may achieve dominance, increasing the risk of multiple pregnancies
• Risk assessment: When more than two dominant follicles (>15 mm) are present during ovarian stimulation for IUI, the risk of multiple pregnancies increases significantly 2, 3
• Cycle management: IUI should be withheld when more than two dominant follicles >15 mm or more than five follicles >10 mm are present at the time of hCG injection or LH surge 2
• Fertility preservation: Understanding dominant follicle development is crucial for techniques like in vitro follicle culture and artificial ovary development 2

Practical Applications

The concept of dominant follicle selection has practical applications in:

• Timing of interventions: The optimal time for inducing ovulation is when the dominant follicle measures 0.9-1.9 cm in diameter 4
• In vitro maturation: Understanding dominant follicle selection helps determine optimal timing for oocyte retrieval to obtain competent oocytes 1
• Multiple pregnancy prevention: Monitoring follicle development during ovarian stimulation helps prevent high-order multiple pregnancies 3

Understanding the dominant follicle and its development is fundamental to reproductive medicine, particularly for timing interventions and managing risks in assisted reproductive technologies.