Girls will be girls
IN all embryos, sexual development begins at about three weeks, when the
cells that will become the baby鈥檚 own germ cells鈥攖he eggs or
sperm鈥攂egin to form. Tissue around the developing gonad and kidney
differentiates into a variety of ducts and cords鈥攖he precursors for both
male and female internal organs.
Over the next three weeks, the primitive germ cells migrate into the
developing gonad, and in embryos with two X chromosomes the gonad starts to
become an ovary. At least one 鈥渁ntitestis gene鈥�, called DAX1, helps
this process along. Another gene, WNT4, seems to prevent the
development of Leydig cells, which are important testicular cells.
At about 10 weeks an XX fetus normally has none of the masculinising hormone
testosterone, and without it the primitive male structures called the Wolffian
ducts begin to regress. The external genitalia take on the female form, and with
the help of oestrogen another set of ducts called the M眉llerian ducts
develop into the uterus, fallopian tubes and upper part of the vagina. The
internal sex organs are pretty much complete by 20 weeks.
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There are many ways that the XX development pathway can veer off this
standard route, however. An absence of anti-male factors can lead to partially
masculinised internal anatomy. And the presence of testosterone can masculinise
the external anatomy and perhaps, some researchers argue, the brain.
Just add testosterone
A single gene on the Y chromosome, active for just a few hours at about six
weeks, nudges XY embryos onto the male pathway.
The gene, named SRY (for sex determining region on Y), triggers
formation of an important testicular cell type, the Sertoli cells. With the help
of another gene, SOX9, these cause other cells in the developing gonad
become testicular cells.
Leydig cells in the gonadal ridge begin producing the masculinising hormone
testosterone by about eight weeks. This triggers development of the male
reproductive tract from the Wolffian ducts.
At about the same time, the Sertoli cells make a hormone called AMH
(antim眉llerian hormone). It eliminates the M眉llerian ducts, which
would otherwise become the female reproductive tract.
The embryo converts testosterone into dihydrotestosterone, which masculinises
the external genitalia and perhaps the developing brain. At 12 weeks the fetus
is recognisably male, and at 14 weeks differentiation is complete, although the
genitals will continue to grow until birth.
Again, development doesn鈥檛 always follow these rules. An XY embryo can become
female if the SRY signal or testosterone is missing or in short supply,
or if the fetus can鈥檛 respond to testosterone. Some estimates say that there is
one intersexed baby, where chromosomes and anatomy don鈥檛 match, for every 500
births, though many of them will never know it.
