And since all hormones do their work through specific

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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.

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