Morphology of the oocyte-follicular connection in the mare

The present study characterised the oocyte-follicular connection (i.e., ooc yte fixation site) in Graafian follicles of the mare morphologically. Antra l follicles were dissected in tote from ovaries obtained from oestrous, dio estrous 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 relativ e dimensions and histological aspects of the cumulus oophorus were investig ated. For ultrastructural analysis of the junctional regions, cumulus-oocyt e complexes were recovered in vivo by transvaginal ultrasound-guided follic le 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 densi ty of the blood vessels surrounding the oocyte fixation site were correlate d to each other, but independent of follicular size and cyclic stage. The h istological appearance of the cumulus oophorus varied, especially in respec t to the compactness, and loosening of the cumulus cell population was obse rved in several medium-sized follicles from dioestrous mares. Loosening of the cumulus cell population was apparently associated with decreased interd igitation between adjacent corona radiata cells. It can be concluded that t he fixation site of the equine cumulus oophorus complex represents a firm c ellular anchorage between follicular wall and oocyte. Furthermore, the loca tion of the cumulus oophorus complex in relation to the ovulation fossa and characteristics of the surrounding blood vessels is independant of follicu lar size and cyclic stage.