Introduction

The female gamete plays a crucial role in determining embryo competence and therefore in vitro fertilization (IVF) results. Oocyte quality is not only influenced by the nuclear and mitochondrial genome, but also by the microenvironment provided by the ovary and the pre-ovulatory follicle that influences transcription and translation, and as a consequence, cytoplasmic maturity. In contrast to in vivo processes, the application of ovarian hormone stimulation protocols for IVF bypasses the complicated selection procedure that usually occurs during oocyte development and maturation of a single oocyte for ovulation, and allows for the maturation of many oocytes, often with compromised quality.

It has been speculated (Van Blerkom and Henry, 1992) that some morphological irregularities, which can easily be assessed at the light microscopy level, may reflect a compromised developmental ability of the oocytes and could therefore represent a useful tool for selecting competent oocytes prior to fertilization. Oocyte morphological assessment in the laboratory is first based on the presentation of the cumulus–corona cells. For mature oocytes, the cumulus–corona mass should appear as an expanded and mucified layer, due to active secretion of hyaluronic acid. This extracellular matrix molecule interposes between the cumulus cells (CCs), separating them and conferring to the cumulus–corona mass a fluffy ‘cloud-like’ appearance. However, stimulated cycles may be characterized by asynchrony between the nuclear maturation status of the oocyte and the expansion of the cumulus–corona cell mass. This has been suggested to be caused by a different sensitivity of the oocyte and the cumulus–corona mass to the stimulants (Laufer et al., 1984).

Following the removal of the cumulus–corona cells in preparation for intracytoplasmic sperm injection (ICSI), oocyte evaluation is more accurate and is based on the nuclear maturation status, the morphology of the cytoplasm and on the appearance of the extracytoplasmic structures. The presence of the first polar body (PBI) is normally considered to be a marker of oocyte nuclear maturity. However, recent studies using polarized light microscopy have shown that oocytes displaying a polar body may still be immature (Rienzi et al., 2005). Only those displaying a meiotic spindle (MS) can in fact be considered as true, mature, Metaphase II (MII) stage oocytes. The presence, position and retardance of the MS have been suggested to be related to developmental competence. In accordance with a recent meta-analysis (Petersen et al., 2009), however, only in vitro development can be related to the morphology of the MS. Analyses of in vivo development are relatively rare in the literature and the meta-analysis failed to show significant differences in implantation rates between embryos derived from oocytes displaying a detectable MS and those without.

Nuclear maturity alone is, in fact, not enough to determine the quality of an oocyte. Nuclear and cytoplasmic maturation should be completed in a coordinated manner to ensure optimal conditions for subsequent fertilization. An ideal mature human oocyte, based on morphological characteristics, should have a ‘normal-looking’ cytoplasm, a single polar body, an appropriate zona pellucida (ZP) thickness and proper perivitelline space (PVS; Swain and Pool, 2008). However, the majority of the oocytes retrieved after ovarian hyperstimulation exhibit one or more variations in the described ‘ideal’ morphological criteria (De Sutter et al., 1996; Xia, 1997; Balaban et al., 1998; Mikkelsen and Lindenberg, 2001; Balaban and Urman, 2006; Ebner et al., 2006; Rienzi et al., 2008). This is also true for oocytes obtained from proven fertile donors (Ten et al., 2007). Morphology, moreover, often fails to predict fertilizing ability and developmental competence (Rienzi et al., 2011). Only a few morphologically detectable features of the Metaphase II oocyte indicate compromised developmental ability. According to the Istanbul consensus workshop on embryo assessment (Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology, 2011) extracytoplasmic anomalies (PBI morphology, PVS size, the appearance of the ZP) are simply phenotypic variations often related to in vitro culture and/or oocyte aging. On the other hand, a special deviation in the cytoplasmic texture, namely the presence of aggregations of smooth endoplasmic reticulum (SER) is potentially lethal and developmental competence of these oocytes should be interpreted with caution. Oocyte morphology may also reflect genetic abnormalities. This is the case for giant oocytes that contain one additional set of chromosomes. These oocytes, when observed with polarized light microscopy, display two distinct MS. Although, the occurrence of giant oocytes is relatively rare after ovarian hyperstimulation, the use of these cells for IVF is dangerous.

Owing to the complex mechanisms related to oocyte maturation and acquisition of competence, it is unlikely that a single characteristic (with the exception of oocyte size and the presence of SER aggregates) can adequately reflect the quality of the cell. Accordingly, to obtain information about the competence of the oocyte, morphological assessment should be combined with other approaches (i.e. cumulus–corona cell gene expression, metabolomics and oxygen consumption). Further predictive value could be obtained by combining the oocyte evaluation with evaluations of preimplantation development (pronuclear stage, cleavage stage and blastocyst stage).