Lab-grown stem cells initiate key steps of human egg and sperm formation

More than sixth of adults around the world know the infertility of their lives. There is a high need not satisfied not only for increased access to affordable and high quality fertility care for those who need it, but, above all, also for new biomedical solutions that can approach the deep causes of infertility.

Some of the first causes of infertility date back to the problems in a biological phenomenon known as “meiosis”, a special type of cell division that cells suffer to form eggs and sperm (gametes). Meiosis starts from a precursor cell with two sets of chromosomes, one inherited from the mother and one of the father, and ends with mature gametes.

Along the way, the maternal and paternal copies of each chromosomes exchange information, generating a new combination of genetic code and, ultimately, one and only one copy of each chromosome must be distributed in the resulting gametes. Errors in these processes can lead to an abnormal number of chromosomes (aneuploids), miscarriage and developmental disorders. However, reproducing meiosis outside the human body, which could possibly allow fertility specialists to create healthy gametes for disadvantaged parents, was extremely difficult.

Now, researchers at the Wyss Institute of Harvard University and the Harvard Medical School (HMS) have developed an in vitro method which allows the differentiation of induced pluripotent stem cells (IPSC) along the way to meiosis. By introducing a cocktail of genes into IPSCs that run on specific gene expression programs, as well as drugs that modify the processing of signals in cells, the team is the first to observe living human cells initiating meiosis outside the body. Their results are published in Scientific advances.

“Healthy eggs and sperm are the product of an extremely complex process and subject to errors. Our study pushes the envelope by reproducing one of its characteristics par excellence in the culture dish,” said George Church, Ph.D. “We are now in an excellent position to find the means to direct the cells throughout the remaining steps of meiosis, which would provide a base to model Number of faults and create healthy gametes for people who cannot get there effectively. ” Church is also a professor of genetics at HMS and professor of health and technology sciences at Harvard and MIT. He also directs the synthetic biology platform of the Wyss Institute.

Shortcut towards meiosis

“To create eggs and sperm themselves, we must be able to drive cells through the divisions of meiotic cells,” said Merrick Pierson Smela, Ph.D., the first author of the study. In the body, the precursor cells of eggs and sperm transit through what is called the “primordial germ cell” (PGC) before entering meiosis. The previous cell culture methods were able to reach the state of PGC, but the resulting PGC type cells could not successfully perform meiosis. “Our protocol completely bypasses the state of PGC to considerably simplify the process of launching meiosis,” said Smela, who has done her work at Wyss as a graduate student from the church group.

To allow this shortcut and reach meiosis, Smela and her colleagues have found combinations of genes which, when activated in cells, bring cells to initiate meiosis. First, the researchers designed stem cells to become fluorescent if they were starting to do meiosis. They then activated combinations of genes planned to play a role in meiosis. In some of these combinations, cells have become fluorescent, indicating that they were doing a meiosis.

The researchers also found that the addition of two different chemicals to cultivation environments – a synthetic imitation of vitamin A, and a DNA methylation inhibitor – has strengthened the effectiveness of entry into meiosis. So-called methyl groups remove the expression of genes nearby and are also eliminated during the normal development of gametes to create a “clean slate” for their differentiation.

By testing combinations of factors in 646,493 individual cells, Smela and her colleagues found three regulatory genes, Boll, Meioc and Hoxb5, which can activate meiosis in their system. The first two of them were previously known to regulate meiosis, but the role of Hoxb5 was unexpected. In addition, BCL2, which stabilizes mitochondria, was necessary to prevent programmed cell death during the induction of meiosis.

Take the meiosis temperature

Thanks to a careful analysis of protein expression and RNA during the induction of meiosis, as well as varying their culture conditions, the team made other interesting observations. The cells have effectively progressed during the first two stages of meiosis (leptonema and zygonema) over approximately 12 days, which, respectively, is when they condensed considerably and when they corresponded to the chromosomes of the mother and the father. On day 15, some cells reached the third step (pachynema), when the paired chromosomes exchange information, but have not progressed. Researchers are currently optimizing their system to allow cells to continue throughout meiosis.

The male and female cells derived from the male and female IPSCs have more effectively entered meiosis when they are cultivated at 34 ° C, the temperature of the testicles, instead of 37 ° C, or the body temperature. The development of sperm was previously known to require lower temperatures, but the fact that lower temperatures also helped female cells. In addition, in addition to differentiated cells not passing through a PGC type state, most had ovarian cell signatures and fewer testicular cells, which suggested to the team that their conditions have preferentially activated an egg cell differentiation program, even when they start from male IPSC.

According to Smela, short -term applications of the new protocol include the development of male contraceptives and the test of new drug candidates for potential reproductive toxicities. In the longer term, he said: “I am enthusiastic about the potential of this technology to resolve infertility by cultivating healthy eggs and sperm for people who need it.” He now advances technology as a scientific director of Ovelle Bio, a startup in reproductive medicine that the Church supports as a scientific advisor.

In 2023, Smela and other members of the Church team published a method that allows generation of so -called ovarian cells of the Granulosa type IPSC, but using different stimuli. Organized in ovarian organoids (ovaroids), these cells can help the maturation of egg cells obtained from mothers who require significantly lower hormonal exposure in an in vitro fertilization procedure. The technology has led to the success of Gameto and the birth of the first baby using this new procedure for maturing eggs. Ovaroids or similar engineering tissue environments could possibly help mature gametes that have completely finished meiosis to be matured.

“The fertility rate in the United States being at a historic level and an increasing number of couples struggling with fertility problems in their lives, this advance by the group of George Church offers researchers a new platform to work on a solution for many underlying causes,” said Wyss Founder Dongber, MD, PH.D., who is also the folkman of Judah, Volkman, the Volkman Profesor Vascul Hansjörg Wyss Biological Inspiration Professor at the Harvard John A. Paulson School of Engineering and Applied Sciences.

The other authors of the publication are Jessica Adams, Carl Ma, Laura Breimann, Ursula Widocki, Bogdan Dobre and Toshi Shioda.