I'm not sure if that's a realistic hope. IANAB, but FtM individuals don't have a Y chromosome to work with, so I don't see how they could get the genetic coding to make testes work. You could use a biological male's DNA, but then you're trading hormones for anti-rejection pills.
I believe there's only one gene on the Y chromosome that really kicks off "maleness", the SRY gene, and it doesn't code for "testicles". The SRY gene just kicks off another process, the genes for which are present in everybody. I think there would still be a few steps left to create the seed cells, but that would be the case in trans women as well, as it seems the mouse pseudo-ovaries were seeded with the mice's harvested eggs.
I think there are genes on the Y chromosome which code for sperm cells, so trans men wouldn't be fertile with one of these testicle-chips, but even getting an endogenous source of hormones would be awesome.
Also, I'm not sure what "IANAB" means. I am not a "?"
I know there's SRY, and that does kick things off, but I also know there's a disorder where SRY translocates to the X chromosome. It causes otherwise XX individuals to develop as a male, but they don't produce testosterone and they require augmentation. That makes me believe that the testosterone producing genes are probably on the Y chromosome as well.
Testosterone levels vary wildly from person to person based on different conditions, but if you look around you can find reference ranges for men and women. The reference ranges for "Average" male and female adults from one source are:
So the top end for women is much smaller than for men, but it's not actually that far from the lower end of the male range. These are "normal" ranges, and some (otherwise) perfectly healthy men are below the bottom end, and some women are above the top end.
One might expect that ovaries don't produce testosterone, but in a typical healthy XX woman the testosterone in her bloodstream is produced by multiple parts of the body. So while an XX individual has low levels, there are multiple sources of it in their physical makeup, and it's at least theoretically possible you could increase production.
As far as estrogen goes there are some easy ways to produce it for an XY individual but the side effects are pretty severe :) Some otherwise healthy XY men have various genetic or other maladies that cause their body to produce unusually high levels of estrogen by itself, so assuming you could control for the undesirable side effects, it's clearly possible to do that too.
Personally, I don't think we're going to get a good handle on producing the hormones necessary for MtF and FtM trans people in the body naturally anytime soon, but it seems feasible to me. And it would certainly be helpful to cis people with unpleasant hormonal disorders.
Interesting, I truly had no idea women produced any amount of testosterone. Maybe it would be possible to ramp that up, and that would be a big development!
Re: Males producing estrogen, that's why I focused on the FtM in my previous reply. I knew it was possible for men to produce estrogen pretty easily, and I figured that's a much quicker path than the other way around.
Thanks for the information! It was truly informative.
I wonder how immunologically different the proteins on an artificial testicle that had the host's DNA aside from a single close-relative donor Y chromosome would be.
