Hello Everyone
This is the third time this extremely interesting subject has come up in the last three months.
Thank you Josie for your scientific analysis and I just wish to copy a similar analysis from HughE dated November 24th 2018 if I may please:
""It goes back to before you were even born.
There's a popular misconception that the sex you develop as is determined by X and Y chromosomes. In fact, all being XX or XY does is determine whether you develop ovaries or testicles, everything from that point forward is driven by hormones. More specifically, in the presence of testicular hormones, a foetus develops as male. In the absence of those hormones, it develops as female instead (ovarian hormones aren't actually necessary for female development to occur, female development is what always happens if there are no testicular hormones present). This is easily demonstrated by a condition called Swyer's syndrome, in which the testicles of a genetically male (XY) foetus fail to develop. People with the condition look female at birth, and they grow up to look and behave just like ordinary women. Often, the condition isn't even spotted until, as teenagers, they fail to start menstruating.
The genitals undergo their development from week 7 to week 12 after conception, so by the end of week 12, you already have male or female genitals, something which can no longer change (unless you have GRS later in life of course!). The brain is different though. The early stages of brain development involve very rapid cell division (to produce the enormous numbers of cells that will ultimately make up the brain), and the migration of those cells to where their final place in the brain will be (which is often far distant from where they formed). Those early steps don't appear to have any major sex differences, so hormones during that part of brain development don't make any difference to the eventual sex of the brain.
By about week 16, the very first cells have reached their final position in the brain. Once in position, they start to grow the nerve axons that will permanently connect them to the other brain cells they're supposed to be interacting with. More and more cells reach their final position and begin to grow their permanent connections to other cells, and by week 21, the cell migration stage of brain development is over, and the main task (ongoing for the remainder of prenatal development) is the growth of nerve axons and dendrites (the "wires" that connect up brain cells), and synaptogenesis, or creating the junctions between those wires. During that time, a process of programmed cell death takes place as well, in which brain cells surplus to requirements are removed. This also appears to be the time when hormones have the biggest impact in determining the sex of your brain, so I'm guessing that there's a male way and a female way of connecting up brain tissue, which are subtly different at the microscopic level. It may be that different cells are removed during the programmed cell death stage if high levels of testosterone are present (testosterone is the main testicular hormone that drives male development), than if there's little or no testosterone there.
So, what appears to make people MTF transgender is that their testicles developed as normal and, to begin with, were producing enough testosterone for male genital development to occur. However, things then went south, and their testosterone production slowed or stopped altogether, so that during the crucial week 16 to birth period for determining whether the brain gets wired up along male or female lines, there wasn't enough testosterone present for the brain to be wired up as male. Instead, it got the patterning that happens by default, the female kind.
As to what can cause testosterone production to go wrong, any of the conventional genetic causes of intersex can. However, so can environmental factors, for instance exposure to external estrogens. Unfortunately, doctors didn't realise this, and for several decades during the mid 20th century, they were giving pregnant women high doses of an artificial estrogen called DES, a drug which acts as a chemical castration agent in men. Many of us in the older age bracket either know or suspect we were exposed to that drug. My own view is that the effect probably isn't limited to just estrogens though, and any hormones or other drugs that interfere with testosterone production in adult men will, if they're administered during pregnancy, run the risk of producing MTF transgender babies. This is a hugely controversial thing to say of course, because an awful lot of hormones and other drugs used in medicine do interfere with testosterone production!
Anyway, hopefully that answers your question. You actually are the person you perceive yourself to be, someone whose body developed along male lines, but whose brain developed along female lines instead."
Hugs
Pamela
