And all of that happens before we are even four weeks

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Unformatted text preview: f tightly clustered grapes. Then, following some direction laid down a hundred million years ago, a few cells condense into a mass on one side of the cluster, and a fluid-filled cavity opens at the opposite side. Surrounding all of this is a thin crystal-clear membrane, a shell. At the end of the fifth day, the embryo begins to rhythmically expand and contract. As those contractions intensify, the shell bursts, and the embryo hatches. All of this happens as the embryo slowly wanders through the labyrinth of the Fallopian tube. But by about day six this odyssey comes to an end, and the embryo implants itself in the frothy wall of the uterus. The embryo now has two inner cavities—one will become the amniotic sac that will surround the developing fetus, and the other the secondary yolk sac. In between those two cavities is a thin strip of cells. Where Our Sexes Come From 51 Within that narrow thread live the cells that will become the child—the living bridge between placenta and yolk, a bridge between amoeba and ape, between an exploding universe and the darknesses still to come. Out of that cyclone of dividing cells, one of the first groups of cells to commit itself to a final future is the group that will form the gonads. These cells may become ovaries inside a new baby girl or testes in a new baby boy—bits of life destined by the finger of a force beyond our imagining to generate eggs or sperm. And all of that happens before we are even four weeks old. By about week six, the embryo has assumed a vaguely recognizable form, and at a point just below the umbilical attachment the germ cells have settled into a structure called the gonadal or urogenital ridge. If some mutation prevents the formation of the urogenital ridge, no gonads and no sexual characteristics will ever develop, and usually no kidneys or adrenal glands will develop either. In this case, the embryo turns out its small lamps and slips back beneath its mother’s blankets; it stops developing, and the uterus reabsorbs it. But mutations that prevent the formation of the urogenital ridge are rare, which seems nearly a miracle, because, along the way to the gonadal ridge, dozens, perhaps hundreds of things need to occur with Swiss precision—enzymes must appear just as they are needed, then just as quickly fade. Pieces of DNA must be awakened at a precise hour, then lulled again to sleep. RNA must ratchet through the embryo’s world, grinding out proteins like cars off an assembly line. Groups of cells must migrate from south to north, east to west, and back again. Like an orchestra, each section plays its part—the reeds, the brass, the percussion, and the strings all make precisely timed and critical contributions. Occasionally in the brass section, French horns are swapped for flutes, and among the violins, snare drums appear; new musicians arrive and depart, the floor raises and lowers periodically, as does the curtain. And without any one piece of this, things change, the music shifts, often in unpredictable ways. But if everything goes more or less as usual, at about seven weeks a pair of gonads appears near the urogenital ridge—gonads already filled with cells that will one day become eggs or sperm. At this point 52 Between XX and XY the gonads are somewhere between male and female; the final sex of the fetus depends on what happens next. If nothing (or nearly nothing) happens, the primitive gonads become ovaries, and the embryo develops the external and internal machinery of a girl—ovaries, uterus, Fallopian tubes, vagina, clitoris, labia. Female, in other words, is development’s default mode; without orders to the contrary, women’s bodies make more women. Perhaps that isn’t as surprising as it might seem, given that sponges, starfish, whiptail lizards, and others do the same thing—all without males. Since we evolved from one of those all-female species, maybe it isn’t startling that reproduction’s first choice is female. Only the gradual evolution of the Y chromosome (from an X chromosome) changed that. Compared to making girls, making boys is harder work. And though many things can go wrong on the way to making girl babies, there are even more potential detours, pitfalls, and wrong turns on the road to making baby boys. The human X chromosome is long, wasp-waisted, and full of information. The Y chromosome, on the other hand, is an odd-looking little blob of DNA. But there are genes on the Y chromosome that can make all the difference when it comes to determining sex. One part in particular of the Y chromosome plays a critical role. We call it the sexdetermining region of the Y chromosome, or SRY, and it is essential for making baby boys. Genes in this part of the Y chromosome operate like switches to turn on other genes that are critical to male development. Without SRY, the primitive gonads do not develop into testes, and sometimes don’t develop at all.1 With SRY, the gonads, until now amorphous little tissues, transform into testes and by about four months begin to do some of the things that testes do. The SRY part of the Y chromosome is essential for making baby boys, but it is not sufficient. After th...
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This document was uploaded on 02/04/2014.

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