Insights into the effect of a changed maternal micro-enviroment on follicular growth, the acquisition of oocyte developmental compentece and subsequent embryo quality
Sara Valckx, MSc, PhD
Download thesis (pdf, 5.33 MB)
The work described in this thesis comprised several parts of the interaction between maternal metabolic disorders, the maternal micro-environment, follicular growth, the acquisition of oocyte developmental competence and subsequent early embryo quality and metabolism. Throughout the introduction, several knowledge gaps were formulated in the form of research questions. Now, it is time to answer them.
Research question 1 - How are serum metabolic changes, related to maternal metabolic disorders, like obesity, reflected in the ovarian follicular fluid and how may this affect oocyte developmental competence and subsequent embryo quality?
In Chapter 3, we showed that the serum composition is well reflected within the follicular fluid of women undergoing ART. Even though concentrations in serum and follicular fluid may differ because of the blood-follicle barrier, a strong correlation exists between serum and follicular fluid for most metabolites. Interestingly, even though many serum parameters were associated with BMI, only a few were found in the follicular fluid. Furthermore, some follicular fluid parameters did affect oocyte developmental competence and embryo quality, but these parameters were not related to BMI.
In Chapter 4, we expand on this knowledge by showing that the composition of human follicular fluid, used in an independent bovine in vitro setting, is indeed able to alter oocyte developmental competence and embryo quality. The follicular fluid composition of women with an overall negative IVF outcome or from women with obesity, reduced oocyte developmental competence, while follicular fluid from women with an overall positive IVF outcome did not affect embryonic growth. Furthermore, some changes in bovine blastocyst gene expression patterns were found, indicating that those embryos that did survive, might have an altered quality.
Research question 2 - How are fatty acids distributed in the ovarian follicular fluid and how do they associate with lipolytic conditions, like obesity?
In Chapter 5, we found that most fatty acids in the follicular fluid belong the phospholipid and cholesterol fraction. Only triglycerides and NEFAs were associated with BMI and combined only accounted for approximately 20% of the total fatty acid concentration. Indeed, triglyceride concentrations tended to be higher in overweight women, while NEFA concentrations were higher in obese women, compared to normal weight and overweight women. Our data furthermore showed that only the NEFA fraction presented with abundant BMI-related differences in individual fatty acids (87% of all the fatty acids analysed differed in a BMI-dependent way). Most predominant fatty acids in the follicular fluid were palmitic, oleic, stearic and linoleic acid.
Research question 3 - How do prolonged elevated NEFA concentrations affect the growth and differentiation of the ovarian follicle as a whole and the maturing oocyte enclosed within? Furthermore, how are embryo development, quality and metabolism affected by adverse metabolic conditions during follicular development?
In Chapter 6, we investigated how the biological unit of the growing follicle and the resulting oocyte quality is affected by a long term exposure to elevated NEFA concentrations. We showed that long term elevated NEFA concentrations only moderately affect follicular growth and antrum formation, with the most pronounced effect induced by the HIGH SA treatment. Elevated NEFA concentrations altered gene expression patterns in Day 13 luteinized granulosa cells, revealing that NEFA exposure mainly affected pathways involved in apoptosis, lipid metabolism, oxidative stress and steroidogenesis, which was also evidenced by progesterone, estradiol and inhibin B analyses in spent medium. Most importantly, the oocytes originating from the NEFA exposed follicles displayed a significantly reduced developmental competence (blastocyst formation).
Chapter 7 describes that morula stage embryos, originating from oocytes isolated from follicles exposed to the HIGH SA treatment consume little or no glucose. This opposed to the embryos originating from BASAL or HIGH NEFA exposed follicles. Also, amino acid metabolism tended to be increased in HIGH NEFA embryos. We could not show a difference in intracellular lipid content. Overall, these data point out that the environment in which the follicle grows and the oocytes matures throughout follicular growth, affects the metabolism of the resultant embryo. The following question was raised: ‘Is this effect due to changes throughout follicular growth or is there a time period in development that is particularly sensitive to insult?’ We investigated this window of susceptibility by exposing murine follicles throughout follicular development (Day 1-13) or only during the final maturation phase (Day 12-13) to elevated NEFA concentrations. The results suggest that the prolonged NEFA exposure induces a more severe effect on oocyte developmental competence, compared to an exposure only limited to the final phase of maturation. This thus emphasizes that not only final maturation, but also the crucial follicular development preceding final oocyte maturation are important and sensitive to insult.
Overall, the results described in this thesis enforce our main hypothesis, by confirming that the serum composition is reflected in the follicular fluid and that the follicular fluid composition may affect oocyte developmental competence. One specific characteristic of metabolic disorders, namely elevated NEFA concentrations, has been shown to affect follicular physiology, ultimately resulting in a reduced oocyte developmental competence and an altered embryo metabolism.