Ultrasound image characteristics of ovarian follicles in relation to oocyte competence and follicular status in cattle

Abstract

Assessment of the quality of the female gamete has become paramount for in vitro procedures. There is a need to identify reliable indicators of oocyte competence and develop a simple, non-invasive method to assess competence. The aim of this study was to investigate the relationships among ultrasonographic attributes of a follicle, its stage of development and the competence of the oocyte that it contains. We tested the hypotheses that follicular echotexture characteristics are related to: (1) the phase of development of the follicle, (2) the presence of the corpus luteum (CL) and/or the dominant follicle in the ovary, and (3) developmental competence of cumulus oocyte complexes (COC) from the same ovary. Crossbred beef cows ( n=143), age 4–14 years, were given a luteolytic dose of dinoprost to cause ovulation. Ultrasound-guided ablation of all follicles ≥4 mm was done 8 days later to induce new follicular wave emergence during a luteal phase. Ultrasonographic images of dominant follicles and the three largest subordinate follicles ( n=402 follicles; 84 cows) were acquired on Days 2, 3, 5 or 7 of the follicular wave (Day 0: wave emergence), i.e. growing, early-static, late static, and regressing phases of subordinate follicle development, respectively. From a subset of these animals ( n=33), ovaries were collected within 30 min of slaughter and COC from subordinate follicles ≥3 mm underwent in vitro maturation, fertilization and culture to the blastocyst stage. Image analysis revealed differences in echotexture between dominant and subordinate follicles among Days 2–7 of the follicular wave. Images of dominant and subordinate follicles at Day 7 of the wave displayed consistently lower grey-scale values ( P<0.05) in the peripheral antrum, follicular wall and perifollicular stroma than all other days. Follicle images 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 follicular echotexture. Echotexture values of the perifollicular stroma were lower in ovaries that did not produce embryos compared to ovaries that produced embryos. Our results showed that the changes in follicular image attributes are consistent with changes in follicular status. The sensitivity of the technique is not yet sufficient for use in a diagnostic setting, but results provide rationale for further development of image analysis as a tool for evaluating oocyte competence in situ.