Basically, a study of a rare but naturally occurring developmental oddity in chickens that results in animals bilaterally divided into male and female (seriously, look at the picture) has found that male and female cells are scattered throughout the body, male dominating one side and female dominating the other, regardless of the hormonal signals that previously were thought to override this cellular sex.
Subsequent experiments supported the idea of strong, cell-by-cell sex identity. When the researchers transplanted tissues from genetically female embryos into what would become the gonads of genetically male ones and vice versa, the transplanted cells didn’t start expressing opposite-sex characteristics.In some ways this seems like it could be a bit disheartening for humans using hormone therapy as part of a transition process*, but to me, taken together with a lot of other research (like the experiment I pointed out a while back in which female mice were turned into male mice by switching one gene), I find this fantastic. It's yet another scientific finding chipping away at the notion we have that sex is a simple binary, and yet another encouraging sign that we may eventually have far greater control over how our bodies reflect gender than we do now.
In combination with other recent papers, says UCLA geneticist Art Arnold, the new study calls for fresh thinking about sex determination, and not just in birds. “The old hormone-only theory is no longer viable, for birds or mammals,” he says.
*Though of course hormones do have a huge, demonstrable effect that no amount of scientific findings about cellular sex can take away, and knowing more about this stuff can only help the hormones to be more effective.