HOW ARE EGGS MADE?
- In humans a female is born with all of her eggs (Oocytes) stored in her ovaries. These are all the oocytes she will have for her whole life.
- Oocytes are made from stem cells in the ovary before birth, and protected within a follicle.
- After puberty the oocytes mature on stimulation by hormones and one fully mature oocyte is released from an ovary every month.
STEP ONE –THE FEMALE FOETUS
- Oocytes are made from ovarian stem cells in the foetus during the first and second trimester of pregnancy.
- In the second trimester the stem cells develop into primary oocyes by undergoing a special type of cell division which halves the genetic information they contain.
- That is so they contain half the genetic information needed to make a baby. Male sperm cells contain the other half.
- The primary oocytes are stored in the ovary and protected within a follicle (small layer of cells).
STEP TWO – OOCYTE DEVELOPMENT AND OVULATION
- The primary oocytes remain unchanged until puberty when hormones stimulate the development of one oocyte each month during the menstrual cycle.
- The oocyte matures into a secondary oocyte that is surrounded by a layer of proteins and small granulosa cells and is released by the ovary at ovulation.
- Only one ovary releases an egg each month, we are not sure how this happens yet.
STEP THREE – FERTILIZATION
- After ovulation the oocyte is ready for fertilization by a sperm cell. This usually takes place in the fallopian tube.
- If fertilization does not occur within 24 hours, the oocyte is no longer viable and the cycle begins again next month.
- However if the oocyte is fertilized it attaches to the uterus wall and signals for the body to prepare its self for pregnancy.
A HELPING HAND
After ovulation the oocyte releases special chemicals that sperm are attracted to. These chemicals excite the sperm so that they swim faster while also guiding the sperm towards the egg.
The sperms job is to deliver half the genetic information needed to make a baby. The oocyte contain the other half but also provides all the nourishment necessary to sustain the new almost a week. The volume of the oocyte is therefore much greater than that of the sperm.
A human oocyte is 20x the size of a human sperm
The MRC center for Reproductive Health is currently researching new treatments for female infertility caused by endometriosis, polycystic ovary syndrome and fibrosis.