Human cumulus-enclosed germinal vesicle oocytes from early antral follicles reveal heterogeneous cellular and molecular features associated with in vitro maturation capacity.

STUDY QUESTION

Are oocyte size, chromatin remodelling, transcriptional activity and mitochondrial distribution in human immature oocytes from early antral follicles retrieved for in vitro maturation (IVM) associated with the acquisition of meiotic competence?

SUMMARY ANSWER

Oocyte size, chromatin compaction, cessation of RNA synthesis and mitochondria rearrangement around the nucleus are associated with the oocyte's potential to resume meiosis in vitro.

WHAT IS KNOWN ALREADY

Information on oocyte features that confer meiotic competence in human mainly derives from germinal vesicle (GV) oocytes that failed to resume meiosis following an hCG trigger after ovulation induction cycles. Characterization of cumulus-enclosed GV oocytes from small antral follicles prior to IVM provides knowledge on the initial oocyte status and suggests culture requirements in order to promote oocyte competence in vitro.

STUDY DESIGN, SIZE, DURATION: Prospective collection of 107 oocytes immediately after retrieval (before IVM) and of 293 GV oocytes that had failed to resume meiosis (after IVM).

PARTICIPANTS/MATERIALS, SETTING, METHODS: Human oocytes were collected from women with polycystic ovary syndrome (PCOS), receiving in total 450 IU of highly purified-hMG for IVM treatment (patients) or who donated oocytes for IVM research (donors). Oocytes at GV-stage were retrieved from follicles <10 mm (range 2-10 mm) diameter, before IVM (oocytes at retrieval) or those that failed to mature after IVM (meiotically incompetent). Oocytes were allocated for either mitochondrial staining, by incubating in mitotracker red and then fixed; or for nascent RNA staining, which was assessed by fluorescent labelling (Click-iT(®) RNA Assay). In every case, oocyte diameter was recorded and chromatin was stained after oocyte fixation. GV-stage oocytes were analysed by confocal laser-scanning microscopy and their characteristics were compared and related to their meiotic competence.

MAIN RESULTS AND THE ROLE OF CHANCE

Analysis of oocytes at the immature GV-stage revealed that oocytes at retrieval were significantly larger than those that failed to resume meiosis after IVM (112.7 versus 109.6 µm, P < 0.0001). Oocytes assessed at retrieval showed that 50.6% had a condensed chromatin configuration (perinucleolar chromatin rim) and were consistently transcriptionally silent. This rate matched maturation rates in our current in vitro culture system (49%). However, oocytes that had not reinitiated meiosis after 30 h IVM demonstrated, apart of being smaller in diameter, significantly higher rates of dispersed or intermediate chromatin (P = 0.005). Analysis of mitochondrial distribution revealed that many oocytes at retrieval displayed mitochondrial internalization towards the nucleus (12/30) or a perinuclear mitochondrial distribution (6/30). These mitochondrial patterns were observed more rarely in GV incompetent oocytes following 30 h IVM (16/98 and 1/98, respectively).

LIMITATIONS, REASONS FOR CAUTION: Most of the analyses involved the use of invasive techniques. Hence, despite the fact that these data deliver essential information on the intrinsic oocyte maturational and developmental status, a direct match with embryological outcomes could not be established.

WIDER IMPLICATIONS OF THE FINDINGS

The evidence described here can aid in tailoring IVM systems in order to promote completion of nuclear and cytoplasmic maturation of unexpanded cumulus-oocyte complexes.

STUDY FUNDING/COMPETING INTERESTS: This study was supported by research grants by the Institute for the Promotion of Innovation by Science and Technology in Flanders, project numbers IWT 130327 and 110680; the Fund for Research Flanders, project number FWO G.0343.13, the Belgian Foundation Against Cancer (HOPE project) and COOK Medical. None of the authors has any competing interest to declare.