Relationship between follicle size, oocyte chromatin compaction, and acquisition of meiotic and developmental competences in Nellore ( Bos indicus ) cattle

This study aimed to investigate the relationship between oocyte meiotic and developmental competences and antral follicle diameter on random days of estrous in naturally cycling zebu females (Nellore - a tropical Bos indicus breed). Immature oocytes recovered from early antral follicles (diameter of 0.5 to 2 mm) exhibited a higher percentage of intact transzonal projections, but lower viability, smaller diameter and a higher proportion of chromatin decondensation when compared to those recovered from mid- (diameter > 2 to 6 mm) and late antral follicles (diameter > 6 mm), demonstrating that oocytes recovered from early antral follicles lack meiotic and developmental competence. Immature oocytes recovered from mid- or late antral follicles were homogeneous in terms of morphology and size, in addition to exhibiting a high proportion of a pattern of chromatin configuration compatible with high developmental competence, which may help explain the better oocyte quality and improved performance of zebu donors in IVP programs. At the end of the in vitro maturation culture, the percentage of oocytes that completed meiotic maturation was higher when they were recovered from late antral follicles compared to mid antral follicles, although the rate of embryonic development to the blastocyst stage was similar. Our results demonstrated a relationship between the acquisition of competences during the final stages of oocyte growth and morphofunctional events, including the chromatin structure remodeling, but further research is needed to better characterize the functional differences in ovarian physiology between bovine breeds that may impact the oocyte developmental potential.