Lipid and fatty acid analysis of fresh and frozen-thawed immature and in vitro matured bovine oocytes.

The lipid content and fatty acid composition of fresh immature and in vitro matured bovine oocytes cultured in media with or without serum, and also those of frozen-thawed immature oocytes were analysed. All oocytes were ranked (A or B) on the basis of their cytoplasmic quality. Fatty acid composition (mol %; w/w) in the total lipid fraction was analysed by gas chromatography. Triglyceride, total cholesterol, phospholipid (phosphocholine-containing phospholipid) and non-esterified fatty acid contents of immature and in vitro matured oocytes were determined using lipid analysis kits. Phosphocholine-containing phospholipid and non-esterified fatty acid contents were determined in frozen-thawed immature bovine oocytes. Palmitic acid was the most abundant fatty acid in immature oocytes (A: 35%, B: 36%), and in in vitro matured oocytes cultured in the medium containing serum (A: 36%, B: 35%) or polyvinyl alcohol (A: 33%, B: 36%). Oleic acid was the second most abundant fatty acid in all A ranked oocytes, whereas stearic acid was the second most abundant fatty acid in all B ranked oocytes. There were significant differences (P < 0.05) in linoleic and arachidonic acid fractions between A and B ranked immature oocytes. In vitro matured oocytes had significantly (P < 0.05) lower proportions of linoleic and arachidonic acids, and significantly (P < 0.01) lower contents of triglyceride and total cholesterol compared with those of immature oocytes. The fatty acid composition of in vitro matured oocytes cultured in medium containing fetal calf serum or polyvinyl alcohol was similar, but significant differences (P < 0.01) in triglyceride and the total cholesterol content were observed. There was a significant decrease (P < 0.05) in the arachidonic acid proportion in frozen-thawed immature oocytes compared with that in fresh immature oocytes. In addition, significant (P < 0.05) decreases in both phospholipid (15.8--10.6 pmol) and non-esterified fatty acid (11.0--4.1 pmol) were found in frozen--thawed immature oocytes. The results indicate that lipids are available for use as an energy source for maturation and that serum lipids are incorporated into the oocyte cytoplasm during in vitro maturation. The changes in the lipid content (mainly phospholipid) and fatty acid composition were also observed in frozen--thawed immature oocytes. The study indicates that the alteration of fatty acid composition in bovine oocytes might improve maturation and cryopreservation.