Nov 27, 2006 11:07 GMT  ·  By

A team at Harvard Medical School peeked at the processes that occur during the development of the eggs inside mammalian ovaries, finding a protein, called p63, essential for detecting and eliminating DNA damaged ovules, so that only healthy ones will be fertilized.

The study is published online in the journal Nature. Human eggs, or oocytes, start developing before birth. Oocytes (woman's germ cells) develop while the girl is still inside the mother's womb, undergoing meiosis, a type of cell division that leads to the emergence of just one set of chromosomes inside the ovules. Meiosis is common in all living beings with sexual reproduction, otherwise chromosome number would be double than normal, with severe effects. The process is halted before birth and, after puberty, the undeveloped ovules (eggs) start developing.

The researchers found that p63 is active in developing mouse eggs during this arrested period while eggs are waiting for maturation. A very similar protein, called p53, suppresses tumor development.

When exposing ovules to DNA damaging radiation, those lacking p63 survived all, thus there was no selection, while cells lacking p53 but containing p63 were selected. "Thus p63, and not p53, is the critical factor for monitoring the level of DNA damage in oocytes and deciding whether it is better to kill them rather than risking the transmission of mutations to the next generation." said Dr. Frank McKeon.

Genetic analyses suggest that this mutation protecting protein appeared at least 800 million years ago (we do not even speak about vertebrates at that time !). "This (...) sheds light on the process by which the ovary can quality-control the eggs it contains to make sure that damaged ones are not ovulated." said Dr Allan Pacey, senior lecturer at the University of Sheffield and Secretary of the British Fertility Society

"The production of eggs is more complex than sperm production because the immature eggs remain in the ovary for such a long period of time."

This means 12 to more than 40 years on human species. "This makes them vulnerable to DNA damage which could lead to infertility or affect the health of any babies born if there is not a process to repair or remove them before ovulation."

"This work helps us to understand the egg quality control process better and might one day allow us to understand why some women have fertility problems because of the poor quality of the eggs they ovulate."