Of mice and men and eggs

Oocytes from male mice

A study this week reported producing eggs from male mice, creating baby mice with two biological fathers.¹ But how did they do it?

The key thing to understand is that it has already been possible to grow eggs from female mouse stem cells. This new paper reported a way to convert male (XY) mouse stem cells to female (XX) cells, which could then be used in the existing method for in vitro oogenesis.

Surely some intricate chromosomal engineering was involved to get rid of the Y chromosome and add another X?

Actually, the researchers did something simpler. Stem cells (ESCs and iPSCs) are somewhat chromosomally unstable, and can randomly lose, duplicate, or rearrange their chromosomes when they divide. Usually this is unwanted behavior. For example, when I’m establishing new gene-edited human iPSC lines, I have to check to make sure this didn’t happen (and in a few cases, I’ve identified problems).

However, for these researchers, chromosomal chaos was a good thing. In particular, male stem cells are prone to lose the Y chromosome. After culturing male mouse ESCs for a few weeks, the researchers isolated single cells and grew new ESC lines from them. Out of 87 lines generated, 5 had lost their Y chromosome.

Now they just needed to duplicate the X. This wasn’t as likely to happen spontaneously, so the researchers treated the cells with reversine, a kinase inhibitor that makes cells randomly yeet² their chromosomes around during mitosis. In order to isolate cells with two X chromosomes, the researchers engineered the X chromosome to express a red fluorescent protein, and sorted cells that were redder than the overall population. This successfully gave female (XX) stem cells, which were otherwise genetically identical to the starting male (XY) cells.

At this point, it was simply a matter of following a long and technically demanding protocol to grow the stem cells into eggs, fertilize the eggs, and implant the embryos into recipient mice. This successfully gave live offspring with two male biological parents. Only a small percentage of the embryos (7 out of 630) developed properly, but the rate was not significantly different from the usual rate with in vitro-derived oocytes.

Overall, this study makes me confident that when a protocol is developed for human in vitro oogenesis (which I believe will be within a few years), it will be possible to grow eggs not only from women, but also from men.

1. They also generated mice with the “father” and “mother” being the same mouse. The mice are inbred anyway, so inbreeding doesn’t matter here.

2. The technical term for this is “mis-segregate”.