Morphology of the oocyte-follicular connection in the mare

The present study characterised the oocyte-follicular connection (i.e., oocyte fixation site) in Graafian follicles of the mare morphologically. Antral follicles were dissected in toto from ovaries obtained from oestrous, dioestrous and transitional mares after slaughter. The location of the cumulus oophorus complex in relation to the ovulation fossa, the width and density of the blood vessels surrounding the cumulus oophorus complex, the relative dimensions and histological aspects of the cumulus oophorus were investigated. For ultrastructural analysis of the junctional regions, cumulus-oocyte complexes were recovered in vivo by transvaginal ultrasound-guided follicle aspiration. The location of the oocyte fixation site was independent of mare, follicular size and stage of the oestrous cycle. In 82% of follicles, the oocytes were embedded in a broad based cell mount. The width and density of the blood vessels surrounding the oocyte fixation site were correlated to each other, but independent of follicular size and cyclic stage. The histological appearance of the cumulus oophorus varied, especially in respect to the compactness, and loosening of the cumulus cell population was observed in several medium-sized follicles from dioestrous mares. Loosening of the cumulus cell population was apparently associated with decreased interdigitation between adjacent corona radiata cells. It can be concluded that the fixation site of the equine cumulus oophorus complex represents a firm cellular anchorage between follicular wall and oocyte. Furthermore, the location of the cumulus oophorus complex in relation to the ovulation fossa and characteristics of the surrounding blood vessels is independent of follicular size and cyclic stage.