Comparison of Equine and Bovine Oocyte-Cumulus Morphology within the Ovarian Follicle1

Abstract

While oocytes are easily aspirated intact from follicles in the cow, oocytes are difficult to obtain by follicle aspiration in the horse and suffer cumulus damage during this procedure. To determine the histological basis for this difference, the morphology of the oocyte-cumulus complex was examined in 55 equine follicles collected from 29 pairs of ovaries and in 25 bovine follicles from five pairs of ovaries. The equine follicles had a significantly thinner cumulus layer between the oocyte and the follicle antrum (p < 0.001 for small follicles, p = 0.03 for medium and large follicles combined) and a significantly thinner cumulus depth between the oocyte and the fol-licular basal lamina (p < 0.001). The cumulus hillock of 94% of equine follicles created an obtuse angle with the surface of the mural granulosa, whereas only 36% of the bovine cumulus hillocks formed an obtuse angle (p < 0.0001). Ninety-two percent of the bovine cumuli and 48% of the equine cumuli showed gaps among the cumulus cells (p < 0.0001). An edematous, vacuolated, relatively acellular pad within the theca interna layer beneath the area of cumulus attachment was present in 37 (67%) of the equine follicles and in none of the bovine follicles. In the equine follicles, the thecal pad was present in 94% of viable follicles, 76% of primary atretic follicles, 57% of secondary atretic follicles, and 23% of tertiary atretic follicles. In transmission electron microscopy, long processes covered by basal lamina could be seen to project into the thecal pad from the overlying cumulus cells. The closer and broader attachment of the equine cumulus to the follicle wall, the greater integrity of the cumulus, and the extension of cumulus cell processes into the thecal pad may prevent the equine oocyte-cumulus complex from being aspirated intact from the follicle.