What is the difference between “Mature Oocytes” and “Early Oocytes”?

Embryologist will select only mature oocytes (MII stage) in order to bring them to ICSI procedure. During ICSI step, the embryologist will evaluate the oocytes from their physical characteristics.


This is the primary assessment that helps to predict the fertilization rate and embryos quality.

Characteristics of good mature oocytes (MII stage):
Round shape inside the zona pellucida, about 120 micrometers in diameter
The cytoplasm is even and smooth.
The polar body is suitably proportional to the perivitelline space (the space between the cytoplasm and the zona pellucida of an oocyte).

Immature oocytes (GV, MI stage), the stage that is not yet suitable for doing ICSI. Characteristics of these oocytes are as follows:
An oocyte in GV stage has the presence of germinal vesicle but the polar body is not yet found.
An oocyte in MI stage doesn’t have the polar body as well as the perivitelline space yet.

In case of immature oocytes, the embryologist of Prime Fertility Clinic will culture them. Waiting for the chance that the early oocytes may develop to the mature oocytes. Then the embryologist will be able to fertilize them with the sperm in ICSI step respectively. This is to increase the chance of getting more embryos consequently.

Reference: Prime Fertility Center Co., Ltd.

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