For the first time, researchers have identified the successive genetic stages through which an immature ovum passes in order to become functional.
- By analyzing the gene activity of thousands of immature eggs and helper cells, the researchers analyzed two types of follicles, which help to generate and then maintain a woman’s fertility until menopause.
How do the female reproductive sex cells, the egg cells, “program” themselves so that they can be fertilized?
This is the question posed by researchers Allan Spradling and Wanbao Niu, of the Carnegie Institution for Science, in Washington (United States). In a study published in Proceedings of National Academy of Sciences, they reveal for the first time the “user manual” that an immature egg follows to become functional and to be fertilized. A discovery that sheds light on the role played by the ovaries in maintaining a woman’s fertility.
Fertility programmed before birth
To establish the different genetic stages through which eggs pass to reach maturity and thus “generate” fertility, the researchers mapped the gene activity of thousands of immature eggs and helper cells.
It turns out that fertility is being prepared even before we come into the world. Before birth, germ cells (ie cells capable of forming gametes) assemble a finite number of groups of cells called follicles in a woman’s ovaries. Follicles consist of an immature egg and a few “helper” cells, which guide the egg through its maturation process. It is from a follicle that a mature egg is expelled during ovulation.
“The follicles are slowly depleted during a woman’s reproductive life and when they are depleted comes menopause, recalls Allan Spradling. Understanding what it takes for follicles to form and grow successfully helps us learn how damaged genes or adverse environmental factors, including poor diet, can interfere with fertility.”
According to the researcher, drawing up a manual of genetic exploitation of the follicle makes it possible to better understand the possible problems of development of the ovum which could lead to congenital malformations. Understanding these dysfunctions will also help to prevent them.
Two types of follicles with distinct function
To understand the role of follicles and determine gene expression, the two scientists sequenced 52,500 mouse ovary cells at seven stages of follicle development. They then found that the ovaries of mammals produce two distinct types of follicles, which do not have the same genetic activity.
The first, called first wave follicles, are present in the ovary even before puberty. In mice, they generate the first fertile eggs. In contrast, their function in humans is poorly understood, but they can produce useful hormones. The second type, called second wave follicles, are stored at rest, but small groups are activated to mature during the female’s hormonal cycle, which ends with ovulation. The results help clarify the roles of each type.
“We hope that our work will serve as a genetic resource for all researchers studying reproduction and fertility”concludes Allan Spradling.
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