Category: Oocyte

Oocyte

Female gamete plays a crucial role in determining embryo competence and
therefore in-vitro fertilization (IVF) results. Some morphological
irregularities in oocytes can be easily assessed at the light microscopy level.

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.

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 polar body (PBI) is normally considered to be a marker of oocyte nuclear maturity.

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). However, the majority of the oocytes retrieved after ovarian hyperstimulation exhibit one or more variations in the described ‘ideal’ morphological criteria.

Only a few morphologically detectable features of the Metaphase II oocyte indicate compromised developmental ability. Extracytoplasmic anomalies in 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 occurrence of giant oocytes is relatively rare after ovarian hyperstimulation, the use of these cells for IVF is dangerous.

References:
http://atlas.eshre.eu/es/1454658456603810













Maturation Stage Of Oocyte

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.