In species that depend on sexual reproduction for perpetuation, children only arrive if the male’s sperm fertilizes the female’s eggs. For this reason, current assisted reproductive techniques always depend on donors. However, new work just published in the journal Cell Stem Cell has shown that mouse pluripotent stem cells can differentiate to give functional sperm. These sperm were used successfully to produce healthy and fertile offspring and provide the most comprehensive model yet generated for generating male germ cells in a test tube.
Does this mean that sperm donations will no longer be needed for assisted reproduction processes? Not so fast: the model has been validated only in mice, and at the moment we are moving in the realm of basic science. If it is effective and safe for human application, it will still take time to be reflected in a clinical application. However, the work represents a breakthrough in this field and opens up new possibilities for the differentiation of male germ cells.
Pluripotent stem cells have allowed scientists to study how each and every cell in the body forms. Brain cells, heart cells, and liver cells are just a few examples of the types of cells that are derived from these stem cells and are now being used in patients as experimental cell therapies. However, some cell types remain difficult to produce from pluripotent stem cells, particularly sperm.
The difficulty of producing sperm
Among all cell types, germ cells are unique for many reasons. First, unlike all other cells, which carry 46 chromosomes, germ cells only have 23 chromosomes, the egg has all of its mother’s chromosomes, and sperm cells have all of its father’s chromosomes.
In addition, they are the only cells that parents actually transmit to their offspring, which makes them, according to Mitinori Saitou, one of the authors of the study and director of the Institute for the Advanced Study of Human Biology at the University of Kyoto, “the driving force that sustains and makes a species evolve. “
Although more research is needed, scientists have made significant advances in the production of sperm from pluripotent stem cells, at least for mice. The process is generally divided into three stages that mimic natural development. Stem cells first differentiate into primordial germ cells, then into sperm stem cells, which is when male sex is determined, and finally into sperm.
The stem cells of spermatogonia are what allow the male to produce sperm throughout life, but this second stage has proven to be the most difficult to recreate in the laboratory.
It is hard but not impossible. Mouse spermatogonia stem cells can be produced, but inefficiently, which is why Yukiko Ishikura, another collaborator on the study, concluded that it was necessary to optimize the differentiation process.
“The rate of differentiation is about a week slower than in the mouse body and the contribution of spermatogonia stem cells to spermatogenesis is low,” he explained.
Starting with mouse pluripotent stem cells, the researchers prepared primordial germ cells and examined more than 10,000 under eight different conditions using what they call a “new reconstituted testis method.”
To validate the best conditions for the manufacture of spermatogonia stem cells, they confirmed that the cells shared several properties with those of mouse testes, including key gene expression, epigenetics, and transient upregulation of retrotransposons. The identical epigenetics was also crucial. Although genes are made of DNA, their expressions depend on epigenetic factors such as DNA methylation. Germ cells show different patterns of DNA methylation during their development, patterns that are considered crucial to their ability to produce offspring. To confirm that spermatogonia stem cells behaved like those produced in the body, the researchers injected spermatogonia stem cells made in their laboratory into testicles of mice, where the cells were allowed to develop into spermatids . These spermatids were collected and injected into eggs to grow the embryos. The embryos were then used to fertilize mice, which then gave birth to healthy pups that were also fertile.
Text: Kyoto University
Photo: Frozen sections of 52-day-old samples cultured by in vitro sperm induction method were immunostained to confirm sperm formation. Red indicates GSCLC markers, green indicates sperm markers, and white indicates nuclei.
