In summary, we extend our initial observations [7
] with additional evidence in support of a role for the HDL particle in the developmental potential of the human oocyte during early embryogenesis. We have now demonstrated that multiple components of FF HDL particles including cholesterol, ApoAI, and the micronutrients, ß-cryptoxanthine and γ-tocopherol, represent unique clinical predictors of embryo fragmentation during IVF. From these results it is unclear whether the HDL particles themselves or co-linear HDL particle components such as ß-crytoxanthin and α-tocopherol are responsible for the protective effect against embryo fragmentation. It is clear that the strong correlations between HDL and lipophilic micronutrient levels in FF prompt all future studies to consider adjusting for associated covariates during statistical analyses. While previous studies have not demonstrated significant clinical effects associated with carotenoids, retinoids, or tocopherols, it is not clear that either study analyzed the relationship with embryo morphology parameters [12
]. Consistent with Schweigert et al. we observe similarly reduced levels of micronutrients in FF compared to plasma [13
]. Our observations suggest that numerous lipophilic components of HDL particles including micronutrients may be influencing the membrane and cytoplasmic properties of the oocyte with downstream effects on embryo fragmentation occurring during in vitro cytokinesis.
There are several notable limitations to this study: (1) we cannot make any conclusions regarding a role for sperm factors even though we considered total motile sperm count in our multivariate analyses; and (2) these findings are simply associations and do not necessarily imply causal effect. We acknowledge that more detailed human and mammalian studies need to be performed in order to assess causality.
To what extent the constitution of the FF HDL particle is a reflection of intra-follicular metabolic processing versus extra-follicular modeling is not known. Our understand of FF HDL metabolism during folliculogenesis and oocyte development is limited and further complicated by the multiple proteins and lipids that interact with and contribute to the biological roles of HDL particles [8
]. Given the relative predominant existence of HDL particles in FF during folliculogenesis, it remains to be determined if these lipophilic micronutrients are influencing oocyte development independent of intra-follicular cholesterol and phospholipid transport mechanisms [8
]. Our findings raise interesting questions regarding the biological mechanisms that are directing apparent effects of HDL-associated micronutrients on oocyte development and competence. Furthermore, the negative correlations between multiple FF HDL components and embryo fragmentation provide reasons to consider HDL particle composition as a useful biomarker of embryo fragmentation during IVF.