Developmental competence of bovine oocytes: follicular status and ultrasound image characteristics

Abstract

This thesis investigates the relationship between the physiological phase of follicles in cattle, their echotexture attributes and the developmental competence of the oocytes that they contain. This study also investigates the local influence of ovarian structures (CL and dominant follicle) on developmental competence of oocytes and follicular echotexture. The drive behind this work was to improve the understanding of ovarian function in this species for the purpose of refining the techniques of follicle manipulation, and to determine ultrasound image characteristics useful to the development of diagnostic and prognostic tools for use in cattle and other species, including humans. After stimulating the emergence of a new ovarian follicular wave in cows, images of dominant and subordinate follicles were digitalized at Days 2, 3, 5 and 7 of the follicular wave. Cumulus oocyte complexes from the same ovaries were collected from subordinate follicles ≥3 mm and underwent in vitro embryo production to the blastocyst stage. Image analysis revealed differences in echotexture between dominant and subordinate follicles among Days 2 to 7 of the follicular wave. Follicles at Day 7 of the wave displayed consistently lower (P < 0.05) values of image attributes in peripheral antrum, follicular wall and perifollicular stroma. Oocytes collected on this day displayed low developmental competence. The proportion of oocytes that developed to the blastocyst stage was higher (P<0.0) in COC collected on Day 5 after wave emergence than on any other day analysed. All follicles displayed a consistent pattern of variation in echotexture among follicular phases. Data did not support the hypothesis of a local effect of the CL or dominant follicle on oocyte competence, and no variation of echotexture attributes related to the presence of ovarian structures were detected by image analysis. Our results demonstrate a positive relationship between early follicular regression and oocyte competence. Moreover, changes in follicular image attributes were consistent with changes in follicular status. We conclude that echotexture analysis of ultrasound images could provide, if developed further, a very useful non-invasive, safe and easy to use diagnostic tool in assisted reproduction.