An oocyte is an immature egg, the female gametocyte or germ cell involved in reproduction. Oocytes are produced in the ovaries during female gametogenesis.The oocyte matures and divides during the follicular phase and then again during ovulation, and becoming a mature ovum or egg.

What is an oocyte?

Oocytes are immature, developing eggs. Eggs are the gametes (cells involved in reproduction) produced by biological females of mammal species. Eggs provide half the genetic material for a future child.

How oocytes are produced

Oocytes start in a form called “germ cells.” During fetal development, these cells move to the developing ovaries and begin to replicate through a process of cell division, or mitosis. They then undergo another, more advanced type of cell division called meiosis.

Oocytes remain at this stage from birth through puberty. Then, each menstrual cycle, a group of several eggs begin to mature into secondary oocytes. Ultimately, one is released through ovulation into the fallopian tube, where they can be fertilized by sperm. Upon fertilization, an oocyte undergoes its final cell division, becoming a mature egg cell known as an “ovum.”

Oocytes and fertility

Females are born with millions of oocytes, but because reproductive years are limited, only around 400 will mature into eggs over the course of their lifetime. The oocyte cell division process sometimes involves errors that give an oocyte an abnormal number of chromosomes, which may cause infertility, miscarriage, or conditions like Down syndrome. These egg quality issues increase with age.

Oocytes can be retrieved and frozen to preserve fertility. This process, called egg freezing or egg cryopreservation, may be helpful for those who:

  • will be undergoing cancer treatments like chemotherapy, which may lead to infertility
  • have genetic conditions that may lead to early menopause
  • need surgery that may damage the ovaries
  • anticipate trying to get pregnant at an older age, when their natural fertility will have declined

History of the oocyte

Karl Ernst von Baer discovered the ovum (mature egg) in 1826. Once the oocyte development process and fertilization were better understood, egg and embryo transfers were achieved in animals.Starting in the early 1980s, oocytes began being surgically retrieved for in vitro fertilization in humans. In 1986, the first human pregnancy using a frozen oocyte occurred. Oocyte cryopreservation has been used to help preserve fertility since the 1990s and success rates for achieving pregnancy have improved.

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