Maturation of human oocytes in vitro and their developmental competence

Abstract

Oocyte maturation remains an enigmatic process that is generally understood to span the time from when messages initiate germinal vesicle breakdown (GVBD) to completion of the nuclear changes resulting in expulsion of the first polar body. The process of maturation encompasses a complex series of molecular and structural events, culminating in the arrest of the oocyte chromosomes on the metaphase II plate in anticipation of sperm penetration and activation for fertilization. There is no agreed assay for the completion of oocyte maturation, other than the full developmental competence of the fertilized oocyte to fully formed live born offspring. Completion of the nuclear changes to produce a metaphase II oocyte does not identify developmental competence and does not reflect the molecular and structural maturity of an oocyte, which is sometimes termed cytoplasmic maturation. It is well known that oocytes will progress spontaneously through the nuclear changes characteristic of oocyte maturation when they are liberated from the antral follicle and cultured in vitro (Pincus and Enzmann, 1935; Edwards, 1965). GVBD may be observed in oocytes in advanced stages of follicular atresia when follicular support cells have died. Hence progression of meiosis to metaphase I or II could be representative of either oocyte degeneration or oocyte maturation. Studies in the mid-1970s by Thibault et al. (1975) and Moor and Trounson (1977) showed that, in species such as rabbits and sheep that have an obligatory phase of protein synthesis during maturation, a complete follicle was necessary to mature oocytes fully. By maintaining the primary elements of follicular culture and retaining granulosa cell viability and function, Staigmiller and Moor (1984) were able to retain maturation and developmental competence of sheep oocyte-cumulus cell complexes in culture when they were removed from the follicular environment. Complete maturation of oocytes is essential for the developmental competence of embryos. Any interventions in the growth phase of the oocyte and the follicle in the ovary will affect oocyte maturation, fertilization and subsequent embryo development. Oocyte size is associated with maturation and embryo development in most species examined and this may indicate that a certain size is necessary to initiate the molecular cascade of normal nuclear and cytoplasmic maturation. The minimum size of follicle required for developmental competence in humans is 5−7 mm in diameter. Maturation in vitro can be accomplished in humans, but is associated with a loss of developmental competence unless the oocyte is near completion of its preovulatory growth phase. This loss of developmental competence is associated with the absence of specific proteins in oocytes cultured to metaphase II in vitro. The composition of culture medium used successfully for maturation of human oocytes is surprisingly similar to that originally developed for maturation of oocytes in follicle culture in vitro. The presence of follicle support cells in culture i...

The pivotal role of glucose metabolism in determining oocyte developmental competence

“…Alternatively, spontaneous completion of maturation will occur following mechanical release of the oocyte from ovarian antral follicles (Edwards 1965). This latter phenomenon is referred to as in vitro oocyte maturation (IVM) and is commonly used to study oocyte development in several mammalian species, and has application to assisted reproduction in humans and livestock animal production (Trounson et al 2001, Gilchrist & Thompson 2007, Hashimoto 2009). …”

In vitro follicle growth under non-attachment conditions and decreased FSH levels reduces Lhcgr expression in cumulus cells and promotes oocyte developmental competence

“…Given the fact that competence of oocytes obtained with in vitro techniques is as yet suboptimal when compared to oocytes grown and matured in vivo [3,[5][6][7][8][9], the development of more appropriate in vitro conditions is a prerequisite for obtaining large numbers of competent oocytes in a reproducible manner.…”

Comparing Pig and Amphibian Oocytes: Methodologies for Aneuploidy Detection and Complementary Lessons for MAPK Involvement in Meiotic Spindle Morphogenesis

“…MAPK is inactive in immature oocyte while its activity increases during maturation and drops after fertilization. Thus, the functional role of MAPK in human oocytes remains an opened question since dynamical observations have been solely gathered (Combelles et al, 2005;Sun et al, 1999;Trounson et al, 2001). MAPK and Mos have been then assumed to exert similar functions to those of other species.…”