male fetal hormonal development

[The Lone Ranger]

I'm not sure it's very helpful or correct to say that there's a gender change during fetal development. In early development, the gonads and genitalia exist in an undifferentiated state and can go either way, depending upon hormonal influences. It's not like ovaries are converted to testes.

Each developing fetus has two sets of ducts -- the Wolffian ducts and the Müllerian ducts.


If the fetus is genetically male, the Sex-Determining Region of the Y chromosome (SRY) triggers production of the H-Y antigen, which causes the developing gonads to differentiate into testes. As the testes grow, they produce androgens, chiefly testosterone. Infusion of androgens causes the testes to ultimately migrate outside the abdomen (as they do, they "punch through" and weaken the abdominal wall, which is why men are so much more vulnerable to hernias). It also causes the Wolffian ducts to differentiate into the male reproductive tract -- the epididymus, ductus deferens, ejaculatory duct, and seminal vesicles. It causes the clitoris to enlarge and ultimately incorporate the urethra inside itself, becoming a penis. And it causes the external genitalia to fuse together, becoming the scrotum.

The testes also secrete the hormone Müllerian-Inhibiting Factor, which causes the Müllerian ducts to regress.




In the absence of testosterone and Müllerian-Inhibiting Factor, the gonads develop into ovaries; the Wolffian ducts regress; the Müllerian ducts differentiate into the female reproductive tract (the oviducts, uterus, and vagina); the clitoris remains small; and the external genitalia, instead of fusing into a scrotum, remain separate and form the labia majora and labia minora.


In rare instances, a fetus is genetically male, but a mutation at the SRY means that the H-Y antigen isn't produced. Or, alternately, the SRY may be normal, but the hormone Human Chorionic Gonadotropin -- which is normally produced by the mother's placenta and which "activates" the testes, causing them to start producing testosterone and Müllerian-Inhibiting Factor -- isn't produced. In either event, since the fetus isn't producing normal levels of testosterone and Müllerian-Inhibiting Factor, it develops into a "female." Since it's genetically male, however, oogenesis cannot occur, and the individual is sterile.


In any event, while it's true, at least in a sense, that the "default" sex for mammals is "female"*, it's not correct to say that the fetus ever switches from female to male.


*This is not true for all animals -- or even all vertebrates. The XX - XY sex-determining pattern is true of most mammals, but it's by no means universal in vertebrates. For instance, the "default" sex in birds seems to be male.

In many reptiles, genetics seem to have little or nothing to do with sex determination -- incubation temperature is what determines whether an embryo develops into a male or female.


Cheers,

Michael



[ETA: By the way, biologists don't normally use the word "gender." Strictly speaking, the word "gender" refers to words, not living beings. Biologists distinguish between "genetic sexes," "gonadal sexes," and "phenotypic sexes," but not between "genders."]

[angrybellsprout]

There you go with one of the most common misconceptions, that gender and sex are the same thing. I made note of that in the origonal post and in the one before this one, as the sex change is an observable event within the fetal development.

Though since you wish to talk about the sex change that takes place in male humans in the first few weeks of their existance, that is fine as well.

One of the issues that comes up with the male fetal hormonal development is that if something disturbs one of the three main androgen bursts that something such as what you described can take place. If something disturbs the process, then the default female settings won't change and it will continue to bear the traits of such.

I do guess that I should have been more specific earlier on in the thread that I was specifically talking about male humans, though the concept does apply to most mammals.

The only thing that I disagree with what you stated was that you make it seem as if a distortion in the Y chromosome was the only way to prevent the sex change from taking place. The theory that I've always seen puts more focus on the mother being the source of the androgen, or at least that if something happens to her that her body can disrupt the process of the androgen production within the male fetus.

Now obviously this is just a theory, and I'm not 100% sure which way it is in reality, but I just wanted to point that out.

[The Lone Ranger]

Apparently, you haven't read my entire post. (I amended it to clarify that "gender" isn't a word used in biology before your post, by the way.) Also, I specifically pointed out that there are at least two reasons for a genetically male fetus to develop into a phenotypic female: damage to the Y chromosome or failure of the mother's placenta to produce sufficient quantitites of the hormones that "activate" the developing testes.


But there is no sex change, since the undifferentiated fetus doesn't have ovaries and does have Mullerian ducts -- and so is neither male nor female in a gonadal or phenotypic sense. What happens is that it becomes one sex or the other. Even though the "default" sex is female (in humans), that in no way means that the fetus is a female until and unless acted upon by androgens.

Cheers,

Michael

[angrybellsprout]

Gender isn't a biological term, it is a psychological term, as I have heard of this more in the relm of psychology than biology.

[Angakuk]

Originally Posted by The Lone Ranger

Apparently, you haven't read my entire post. (I amended it to clarify that "gender" isn't a word used in biology before your post, by the way.)

Originally Posted by angrybellsprout

Gender isn't a biological term, it is a psychological term, as I have heard of this more in the relm of psychology than biology.

Is it a problem you have with reading English, or just comprehending it, angry?

[angrybellsprout]

What's that? Still running away from conversation? Seems to be the only thing that you are able to do.