This latest breakthrough recreates the distinct stages of sperm development from a primordial germ cell-like cell, through the stages of sperm development known as spermatogenesis into a sperm that can be used to create viable and fertile offspring. Researchers from the ASHBi Institute for the Advanced Study of Human Biology in Kyoto, Japan demonstrated their model worked in vitro and in vivo, in a breakthrough for in vitro gametogenesis, as previous work had not recreated the development of sperm entirely in vitro. Earlier this year mouse egg cells were developed from mouse stem cells entirely in vitro (See BioNews 1104).
'This is the first study to reconstitute functional sperm from mouse pluripotent stem cells in a test tube. This opens new possibilities for male germ cell differentiation,' said ASHBi director Professor Mitinori Saitou.
Germ cells remain difficult to generate from pluripotent stem cells, due to the complex differentiation processes they undergo to form gametes which have just half the chromosomes of other cells, via meiosis.
To achieve a reliable process researchers attempted to mimic the three essential stages that occur in the testes. Firstly, the stem cells are differentiated into primordial germ cells, then into spermatogonia stem cells, and finally sperm. It is the second stage of this process which has been a key challenge to replicate in the laboratory.
The study used mouse embryonic stem cells which were induced to form primordial germ cell-like cells. Ten thousand of these were examined under eight different conditions. To determine the conditions that were best for manufacturing spermatogonia stem cells, they looked at the expression of key genes and markers of cell activity.
Researchers then used these optimal conditions to recreate spermatogenesis both on transplantation into testes in vivo and in a culture of testes transplants in vitro. The resulting sperm did not have tails but using spermatid injection they were able to fertilise an egg and create embryos that resulted in viable fertile offspring.
Epigenetic differences due to high levels of methylation were observed in the sperm that were created using this method, and further research needs to be done into the impact of this on resulting offspring, the researchers wrote in the paper published in the journal Cell Stem Cell.