Characterization of morphology and angiogenesis in follicles of mares during spring transition and the breeding season.

The research focuses on studying and comparing the structure and blood flow of the ovarian structures in horses during the seasonal fertility changeover in spring. The research discovered that these ‘transitional’ follicles were less developed and lacked the level of blood vessels that typical, pre-ovulatory follicles have, potentially due, in part, to lower levels of the hormone LH.

Introduction and Objectives

• The study was designed to analyze the formation and richness of blood vessels (angiogenesis) in the ovarian structures termed ‘follicles’ in mares as they transition from the non-reproductive winter season to the active breeding period in the spring.
• Focus was put on these ‘transitional’ follicles which reach a similar size to the normal preovulatory follicles but do not actually lead to ovulation.
• Another intention was to understand how these follicles develop when compared with preovulatory follicles.

Methodology and Findings

• The research technique involved analyzing the morphological structure by directly looking at the follicle and also examining its structure under a microscope.
• The study found that the wall of these transitional follicles was visibly less supplied with blood vessels than regular, preovulatory follicles.
• A microscopic level investigation also noted that the ‘theca’, a particular layer of the follicle, was only poorly developed in these transitioning follicles.
• This was further confirmed with immunostaining for factor VIII (a marker for vascular endothelial cells), which identified that there were fewer blood vessels in the theca of the transitional follicles.
• Also, the levels of cell division were significantly less in the transitional follicles compared with the preovulatory follicles as seen through immunostaining for Ki67 (a marker for proliferating cells).

Observable Differences and Causes

• The investigation also highlighted the difference in the presence of the vascular endothelial growth factor (VEGF). While it was abundant in the theca of the regular follicles, it was scant in the transitional follicles.
• In conclusion, these findings suggest that the less developed state and lower vascularity of the theca layer in transitional follicles could be due to the lower levels of LH and potentially other growth-promoting hormones.
• Such conditions are likely to be the primary reason behind the inability of transitional follicles to produce the necessary steroids, thus remaining unable to support ovulation.