Systemic and intrafollicular components of follicle selection in mares.

The researchers examined follicle selection processes in mares, using them as a model due to their similarities with the human reproductive system. They determined that systemic and intrafollicular components together contribute to follicle selection, which is particularly evident when studying the processes associated with the growth and differentiation of follicles during the equine estrous cycle.

Study of Follicle Selection in Mares

• The choice of mares for the study was based on their similarity to women concerning the relative follicle diameters during specific events of the follicular wave and the easy accessibility of follicles for sampling and experimentation.
• A typical estrous cycle in mares lasts between 22 to 24 days, during which usually only one major follicular wave develops with a dominant follicle (DF) greater than 30mm. It’s termed the primary or ovulatory wave.
• There is an occasional occurrence of a major secondary wave early in the cycle, accounting for about 25% of the cycles.
• The focal point of this research relates to the specific changes and behaviors seen in follicular development throughout a mare’s estrous cycle, particularly around the onset of deviation identified typically around day 16 of the cycle.

Follicle Deviation Process

• The researchers found that follicle deviation begins a day before the diameter deviation becomes visible. The deviation process is marked by increased vascularization and specific hormonal changes within the dominant follicle.
• The alteration in hormone concentrations, notably of IGF1, estradiol, and inhibin-A, seems to be a crucial factor in setting the DF apart from the subordinate follicles (SF).
• Notably, the production of inhibin and estradiol suppresses follicle-stimulating hormone (FSH) in systemic circulation, with the production of these hormones stimulated by IGF1 and luteinizing hormone (LH).

Systemic and Intrafollicular Factors in Follicle Selection

• The different cycles and deviations within follicular development are dictated by both systemic elements, such as the suppression of FSH due to hormone secretion, and local internal elements, like the production of IGF1 that increases the sensitivity of DF to FSH.
• The researchers suggest that the ability of DF to react more to low FSH concentrations than the SF is pivotal in follicle selection.

Further Observations and Hypotheses

• In analyzing individual estrous waves, the study revealed that the two largest follicles switch ranks during the common growth phase in about 30% of primary waves.
• Experiments involving follicle ablation provided additional insights, showing that when one, two, or three of the largest follicles were removed at the beginning of the expected deviation, the next largest follicle assumed the role of the dominant follicle.
• This implies that various follicles have the potential for dominance, and it has led the researchers to propose that the deviation process essentially embodies the whole mechanism of follicle selection in mares.