What is an Oocyte?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 22 September 2019
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Difficulty with defining the term oocyte exists, since it may occasionally mean different things. Technically it refers to one early stage that a female egg goes through prior to being fully formed, ovulated and ready for fertilization. Many people simply define the oocyte as the female egg, especially in humans, and this usage is extremely common, if inaccurate. Egg donation might be called oocyte donation, for instance, though technically a true oocyte donation would not be helpful in the fertility treatment of others because an immature egg cannot be fertilized.

In the strictest definition then, the oocyte is produced each month and begins to mature. Ultimately in the healthy female at least one of these “germ cells” matures fully into becoming an ovum. Occasionally, two oocytes complete this maturation process and are both ovulated, resulting in some forms of twins if both ova are fertilized. This is considerably more likely if women are taking fertility medications, which can hasten maturation of germ cells into ova, in amounts that exceed normal standards.


While many people view the oocyte as the potential ovum, as mentioned, some use the term much more loosely, especially when it comes to human egg donation. A number of egg donation programs are referred to as oocyte donation, and this may lead to general confusion on just what this cell is. In various types of egg donation, women usually take fertility medications and may take drugs that can stimulate ovulation at a certain point in a cycle. More oocytes become ova as a result of this medication, and these can then be harvested at the point of ovulation. Since implantation of a fertilized egg is often done in multiple numbers to increase the chances of pregnancy, focus on getting oocytes to reach ovum stage is important so egg harvest is bigger.

It is important for women considering this form of donation to determine how this reduces fertility in the future. If women are not planning to have children until much later in life, this can especially become a factor. The reason for this is that women, unlike men, essentially have all the germ cells in their body that they will ever have. Men tend to produce a fairly constant and new supply of sperm cells, but women do not do the same with oocytes and ova. Prior to donation, women may want check to determine if egg donation poses any risk to future fertility, though technically this risk is extremely minimal and might only affect a small population of oocyte donors.

Another issue that affects in vitro fertilization programs and women who might use them as a source of donor eggs is that occasionally oocytes do not mature, despite drug intervention. This very reason may be present in women seeking out donor ova; due to a number of different medical scenarios oocyte maturation isn’t occurring and women might not be able to get pregnant. The medical community studies this subject extensively and has found several potential explanations for what may thwart the maturation process. This matter could be investigated by individual doctors also, to understand more clearly why a patient might not be able to achieve pregnancy.


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