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Unformatted text preview: e testes form, there is another gauntlet
that the developing fetus must run.
Now is the time for hormones. All fetuses produce about the same
amount of estrogen, and if no other hormones appear, baby girls follow.
But if the testes do what they normally do, baby boys follow. Where Our Sexes Come From 53 In the eighty-day-old fetus, neurons in the fetal hypothalamus (a
knot of cells at the base of the brain) begin to secrete pulses of gonadotropin-releasing hormone (GNRH). In response, the fetal pituitary gland
(another clump of brain cells that sits just below the hypothalamus)
begins to release pulses of follicle-stimulating hormone and luteinizing
hormone. These hormones stimulate the developing gonads (either testes or ovaries), which in turn respond with hormones of their own.
For a developing fetus to end up looking like a baby boy two things
have to happen: the testes must produce testosterone, and some of that
testosterone must become 5-alpha dihydrotestosterone, another male
hormone that helps steer fetal development. That conversion requires
an enzyme called 5-alpha reductase. 5-alpha reductase comes in two
forms, poetically named 5-alpha reductase types 1 and 2. Toward the
end of fetal development, the appearance of testosterone, 5-alpha
reductase type 2, and 5-alpha dihydrotestosterone sculpt the fetus into
a baby boy with a penis, scrotum, and all the rest of the external and
internal accoutrements of a male human being.
And since all hormones do their work through specific receptors
on the developing child’s cells, not only do all of the genes that direct
hormone formation have to work in a timely and accurate manner, but
so do all the genes that encode the cell-surface receptors for all of those
hormones. And when each hormone delivers its message to its personalized receptor on a cell’s surface, then a whole series of other molecules
have to tell the nucleus about that message before any critical changes
in that cell’s life can occur. So every one of those message-carrying molecules has to be in place, on time, and ready to go or else everything
produced by the testes or the developing adrenal glands will have no
effect on anything.
That’s a lot—a lot of molecules, a lot of genes, a lot of coordination—that has to occur properly. And all of it happens during the first
ten weeks of pregnancy. At ten weeks, you still can’t tell a male from a
female fetus. Even though they both have genitalia at this point, there
are no differences—the cells that will become either the glans of the
penis or the clitoris sit atop a short stalk that will transform into the
shaft of the penis in males and into the folds of the vaginal opening 54 Between XX and XY in females. The whole fetus is just a little more than an inch long at
this point. The tissues at the base of the stalk hold the still-developing
gonads and will usually differentiate into either a scrotum or the tissues
that surround and buttress the vulva.
While the external genitalia are still struggling to distinguish themselves, at ten weeks the internal genitalia begin to wander down separate paths. In developing females, the uterus, vagina, Fallopian tubes,
and urethra assemble themselves and assume their proper positions.
Similarly, in developing male fetuses the vas deferens, seminal vesicles,
seminiferous tubules, prostate, epididymis, and ejaculatory orifice all
form from the surrounding tissues. That’s a lot of tubes and orifices,
folds and bumps, hollows, vaults, and vesicles to be built and slipped
into position. If the fetal gonads have been pumping out androgens
during the first twelve weeks of fetal life, openings close, the scrotum
forms, and the developing urethra finds its way to the tip of the penis.
If no testosterone shows up, then the glans shrinks into a clitoris, the
vagina fully opens, labia blossom, and the urethra ends up below the
clitoris. A few long solo notes by the gonads, and the deal is sealed.
At about thirty-six weeks in developing boys, the testes descend
into the scrotum. When that happens, the fetus finally has external
genitalia that are fully male. In developing girls, at about the same time,
the finishing touches appear, the excesses get wiped away, and the final
version of the genitalia is formed.
Then the curtain lifts, the conductor raises a hand, a low tune
begins among the bassoons, and a hush sweeps through the audience.
A child is born.
More than any other single factor, the external genitalia, those few
simple tissues, their sizes and their shapes, will imprint in everyone’s
mind an everlasting label of girl or boy, or neither—for some parents
the worst possible outcome.
In spite of the label the doctor slaps on us at birth like a bar code,
this whole sex process isn’t over, not nearly. The pulses of hypothalamic, pituitary, and sex hormones that began in the eighty-day-old
fetus and continued through the last two trimesters of fetal life continue
still throughout early infancy. Then, at about six months...
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This document was uploaded on 02/04/2014.
- Spring '14