Abstract
BackgroundThe complex relationship between oocyte morphology, specific follicular fluid metabolites, gene expression in cumulus granulosa cells, and oocyte competence toward fertilization and embryo development still needs further clarification.MethodsForty-six oocytes retrieved from the largest pre-ovulatory follicle of patients undergoing intra-cytoplasmic sperm injection (ICSI) were considered assessing: (a) oocyte morphological characteristics at polarized light microscopy (PLM), (b) specific follicular fluid (FF) metabolites previously suggested to influence oocyte competence (AMH, markers of redox status and of cytotoxicity), (c) transcription of AMH and AMH type II receptor genes in cumulus cells. Data were analyzed using mono-parametric tests and multivariable logistic analysis in order to correlate morphological and biochemical data with fertilization.ResultsComparing normally fertilized oocytes (n = 29, F group) with unfertilized (n = 17, nF group) we observed that: (a) the meiotic spindle area and major axis were significantly higher in nF group and in fertilized oocytes undergoing an early embryo development arrest; (b) AMH level in FF was comparable in F and nF groups; (c) the FF of nF group contained significantly higher levels of cytotoxicity (lactate dehydrogenase) and oxidative stress (Cu,Zn-superoxide dismutase, catalase, 4-hydroxynonenal-protein conjugates) markers; (d) cumulus cells of nF group showed significantly higher AMH receptor type II gene expression.ConclusionsTaken together, these observations suggest that an excessive cytotoxicity level can alter AMH signal transduction within cumulus cells, in turn leading to partial inhibition of aromatase activity, altered cytoplasmic maturation and increased oxidative stress, factors able to impair oocyte fertilization competence and embryo growth.
Highlights
The complex relationship between oocyte morphology, specific follicular fluid metabolites, gene expression in cumulus granulosa cells, and oocyte competence toward fertilization and embryo development still needs further clarification
It is commonly accepted that the oocyte competence plays a central role in affecting the likelihood of fertilization and subsequent embryo development, even when fertilization is accomplished by direct injection of the sperm in the egg cytoplasm; as a consequence, selecting of the best oocytes would likely lead to an overall improvement of the efficacy of intra-cytoplasmic sperm injection (ICSI) and of the average quality of in vitro produced embryos
Since the acquirement of the competence toward fertilization and embryo development is influenced by a complex interplay between oocyte, Cumulus cells (CCs) and follicular fluid (FF), we applied a combined approach including oocyte morphology, the study of specific metabolites in FF and the expression of specific genes in CCs. We focused on those markers that were previously suggested to be implicated in the modulation of follicular growth and oocyte maturation, and we studied individual follicles in order to better assess the relationship between the follicle and the fate of the oocyte contained inside it
Summary
The complex relationship between oocyte morphology, specific follicular fluid metabolites, gene expression in cumulus granulosa cells, and oocyte competence toward fertilization and embryo development still needs further clarification. The assessment of oocyte morphology, has never been fully satisfactory, allowing to roughly identify negative, rather than positive, predictors of oocyte competence [2]. Some studies showed that PLM is useful to assess cytoplasmic maturation, and is helpful to identify oocytes with the best potential for fertilization [4, 5] embryo growth [4, 6], blastocyst formation [7, 8] and implantation [9, 10]
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