Oocytes grown in the laboratory, starting from a small portion of ovarian tissue: for the first time - after three decades of research - the entire cycle of maturation of female sex cells has been completed in vitro, until reaching a stage suitable for fertilization .

That obtained by Evelyn Telfer, an expert in reproductive biology at the University of Edinburgh (Scotland), is a potentially very important result, for women who risk losing their fertility prematurely.

However, it is still unclear whether the resulting egg cells, matured in just 22 days (as opposed to the 5 canon months used in the female body), are potentially capable of giving rise to a healthy embryo, after encountering a spermatozoon.

The pieces of the puzzle. The maturation of oocytes in the laboratory had already been successfully attempted in mice, and had given rise to normal pregnancies. Partial phases of the process had already been tested for human female sex cells, but none had yet managed to complete it from start to finish, starting from scratch.

How life is born

The key steps. After taking ovarian tissue samples from 10 women aged between 25 and 39, and having isolated 87 ovarian follicles (the tiny "pockets" within which the egg cell precursors mature), the researchers let them develop into different nutrient cocktails, following four stages of growth from the earliest stages to maturity.

Only 9 cells have reached full maturity, ie they have been able to halve their chromosomal outfit in order to be ready for an eventual encounter with a spermatozoon (which did not occur in the study). Before this step, an egg cell cannot be fertilized.

Image The various stages of growth of oocytes in different culture media: in the first they are still immature; in the third they are in an advanced stage of development. | Prof Evelyn Telfer and Dr Marie McLaughlin, the University of Edinburgh

Perspectives. The technique, which has yet to be perfected and will take many years to find clinical applications, is particularly promising for women at risk of early loss of fertility, for example because they are undergoing chemotherapy or radiotherapy treatments.

Currently, for girls who follow oncological therapies before having reached puberty (that phase of development starting from which, every month, follicle rupture frees a mature egg cell), the only way to leave open the possibility of a future pregnancy is excision and subsequent transplantation of ovarian tissue. However, with this procedure there is a risk of reintroducing diseased cells into the body.

How the body changes during adolescence

The new technique would make it possible to grow oocytes in the laboratory and use them for assisted procreation. For women who undergo chemotherapy after puberty, the prospect of cultivating hundreds of oocytes in culture would instead open instead of being forced into fast in vitro fertilization cycles before being able to start treatment (delaying them).

Open questions. However, the research still has many limitations: no genetic analysis of mature oocytes has been carried out to understand if they are healthy, and if the speed of growth has changed their characteristics; "fresh" and non-frozen ovarian tissue has also been used, as would normally be the case in a clinical setting.

Finally the cells arrived at the final stage are few, and some of them exhibit an exceptionally large polar corpuscle (the part of the oocyte expelled before final maturation), a possible sign of an anomaly.

The important work, published in the scientific journal Molecular Human Reproduction, still remains experimental.