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In this study concentration and composition of non-esterified fatty acids (NEFA) in follicular fluid (FF) of high-yielding dairy cows were determined during the period of negative energy balance (NEB) early post partum. NEFA were then added during in vitro maturation at concentrations measured previously in FF to evaluate their effect on the oocyte's developmental competence. At 16 and 44 days post partum, FF of the dominant follicle and blood were collected from nine high-yielding dairy cows. Samples were analysed for NEFA concentration and composition. NEFA concentrations in FF (0.2 -0.6 mmol/l) during NEB remained 6 40% lower compared with serum (0.4 -1.2 mmol/l). The NEFA composition differed significantly between serum and FF with oleic acid (OA), palmitic acid (PA) and stearic acid (SA) being the predominant fatty acids in FF. Based on these results, 5115 oocytes were matured for 24 h in serum-free media with or without (negative control) the addition of 0.200 mmol/l OA, 0.133 mmol/l PA or 0.067 mmol/l SA dissolved in ethanol or ethanol alone (positive control). Matured oocytes were fertilized and cultured for 7 days in SOF medium. Addition of PA or SA during oocyte maturation had negative effects on maturation, fertilization and cleavage rate and blastocyst yield. More (late) apoptotic cumulus cells were observed in cumulus -oocyte complexes matured in the presence of SA or PA. Ethanol or OA had no effect. These in vitro results suggest that NEB may hamper fertility of high-yielding dairy cows through increased NEFA concentrations in FF affecting oocyte quality. Reproduction (2005) 130 485-495

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-Cystic ovarian follicles (COF) are an important ovarian dysfunction and a major cause of reproductive failure in dairy cattle. Due to the complexity of the disorder and the heterogeneity of the clinical signs, a clear definition is lacking. A follicle becomes cystic when it fails to ovulate and persists on the ovary. Despite an abundance of literature on the subject, the exact pathogenesis of COF is unclear. It is generally accepted that disruption of the hypothalamo-pituitary-gonadal axis, by endogenous and/or exogenous factors, causes cyst formation. Secretion of GnRH/LH from the hypothalamus-pituitary is aberrant, which is attributed to insensitivity of the hypothalamus-pituitary to the positive feedback effect of oestrogens. In addition, several factors can influence GnRH/LH release at the hypothalamo-pituitary level. At the ovarian level, cellular and molecular changes in the growing follicle may contribute to anovulation and cyst formation, but studying follicular changes prior to cyst formation remains extremely difficult. Differences in receptor expression between COF and dominant follicles may be an indication of the pathways involved in cyst formation. The genotypic and phenotypic link of COF with milk yield may be attributed to negative energy balance and the associated metabolic and hormonal adaptations. Altered metabolite and hormone concentrations may influence follicle growth and cyst development, both at the level of the hypothalamus-pituitary and the ovarian level.cystic ovarian follicles / pathogenesis / hypothalamus-pituitary / ovary / negative energy balance

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