Oocyte assessment: Polscope
The oocyte is assessed according to two criteria: nuclear maturity and morphological appearance.
The oocyte is considered to be mature (from the nuclear point of view), and hence usable for in-vitro insemination, if it has reached the metaphase II stage. At this stage the oocyte features the meiotic spindle (a birefringent structure consisting of microtubules that keep the oocyte chromosomes alonged on the metaphase plate). A study conducted at our center (Rienzi et al., 2003) showed a correlation between the presence and position of the meiotic spindle analysed using a special instrument (Spindle View) and the potential for development of the oocyte subjected to ICSI.
From a morphological point of view, the oocyte cytoplasm may have different types of polymorphisms: graininess, presence of organelles or vesicles, accumulation of smooth cytoplasmic reticulum, presence of areas of the cytoplasm with no organelles and/or vacuoles. Our team has recently demonstrated a correlation between some of these characteristics and the growth capacity of oocytes (Rienzi et al., 2008). At our center we select oocytes not only based on their maturity, but also on their quality.
In some cases in the presence of oocytes with particular morphological characteristics Laser-ICSI can be performed (Rienzi et al.). This technique allows thinning a small part of the outer membrane of the oocyte (pellucid zone) using a laser to reduce the pressure exerted during the injection. This reduces the mechanical stress due to the injection which can damage the more fragile oocytes, compromising their survival.