Oocyte nuclear maturity, as assessed by light microscopy, is
assumed to be at the MII stage when the PBI is visible in the PVS .
The MII stage is characterized by the alignment of the homologous chromosomes on the spindle equatorial plate during metaphase of the second meiotic division. It is generally recognized that 85% of the retrieved oocytes following ovarian hyper stimulation display the PBI and are classified as MII, whereas 10% present an intracytoplasmic nucleus called the ‘germinal vesicle’ characteristic of prophase I of the first meiotic division. Approximately 5% of the oocytes have neither a visible GV nor PBI and these oocytes are generally classified as MI oocytes. These oocytes may, however, be at the GV breakdown stage where the nuclear envelope has broken down but has not as yet progressed to true MI where the chromosomes are aligned on the metaphase plate in preparation for the completion of the first meiotic division.