Granulated bovine oocytes represent a lipid rich phenotype with enhanced developmental competence

Immature bovine cumulus-oocytes complexes (COCs) with several layers of tightly packed cumulus cells and a homogeneous, darkish ooplasm are considered to be of good quality, whereas COCs exhibiting cytoplasmatic granulation are frequently observed but routinely discarded. This study compared the protein and lipid landscape, as well as the developmental competence, of immature cattle oocytes with either granulated or homogeneous ooplasm but similarly compact cumulus investment. Nile Red staining revealed increased lipid droplet accumulation, greater heterogeneity in droplet size, and formation of droplet clusters in granulated oocytes. Lipidomics via ESI-IT MS corroborated these findings by discovering a significantly higher content of phosphatidylcholines and a trend toward increased levels of free fatty acids in granulated compared to homogeneous oocytes. Protein profiling via LC-MS/MS was performed to characterize the molecular features of granulated oocytes in more detail. In total, 2136 proteins were detected, with only minor differences between the proteomes of granulated and homogenous oocytes. All ten proteins classified as differentially expressed displayed a lower abundance in granulated oocytes - seven were directly associated with peroxisomal lipid metabolism and peroxisome formation, mitochondrial function, and autophagy initiation. Protein interaction analysis revealed methyl-branched fatty acid metabolic process and fatty acid alpha-oxidation as enriched pathways with the highest strength. Granulated oocytes displayed significantly higher cleavage and blastocyst formation rates following in vitro embryo production than homogeneous oocytes, indicating superior developmental competence. Granulated oocytes thus represent a distinct, lipid‑rich phenotype with configured peroxisomal and mitochondrial pathways that, contrary to current selection practice, can enhance early embryonic development.