|MadSci Network: Anatomy|
While I understand your reference to the embryonic stage, others may not. And we need to be clear on what we mean when we say that embryos start out as "females".
Developing human embryos initially contain two tubes or ducts that eventually give rise to the majority of the internal sex organs in the body. These are called the mesonephric and the paramesonephric ducts. Roughly speaking, the mesonephric ducts give rise to the male epididymis, vas deferens and seminal vesicle, while the paramesonephric ducts give rise to the female Fallopian tubes, uterus and part of the vagina.
All embryos, whether male of female, start out with both "primordial" ducts. Which duct develops further depends on whether the embryo is male or female. Specifically, a male embryo produces testosterone that promotes development of the mesonephric ducts, and Mullerian inhibiting factor that prevents development of the paramesonephric ducts. The result is that only male organs develop. In contrast, females lack testosterone, so the mesonephric ducts do not develop, and lack Mullerian inhibiting factor, so the paramesonephric ducts do develop. The result is that only female organs develop.
The effects of testosterone go beyond the internal sex organs, however. Testosterone is also important in shaping the development of the external sex organs. Again, female genitalia are the "default", with testosterone being required to induce proper formation of the penis, etc.
With those details explained, we can now see why people say that embryos start out "female". If no hormones are present, the female organs will still develop normally! Male embryos must specifically make the hormones if they want the male organs to develop. Thus, it is not actually correct to say that embryos "appear as females". Rather, all embryos initially appear as an indeterminate gender, with female organs being the "default pathway" that occurs in the absence of the male hormones.
What may not be obvious to some readers, then, is that most components of the internal sex organs do not have direct counterparts between males and females, since they develop from entirely different ducts. In contrast, the external sex organs do develop from the same original tissue masses, and just fold and shape differently depending on the hormones that are present during development. Therefore, the clitoris is roughly the counterpart of the penis, the labia are roughly the counterpart of the scrotum, etc.
So, finally, what about the prostate gland? I hope you now see that it does not necessarily have to have a female counterpart. But in fact, it does. The prostate develops from a portion of the urethra (the tube connecting the bladder with the outside world, developed from the same tube in both males and females). Specifically, testosterone causes numerous outpouchings or invaginations to project from the urethra into the surrounding tissue, ultimately making the prostate. In females, in the absence of testosterone, a roughly similar but much more limited outpouching occurs to form the very mysterious "paraurethral glands of Skene". These tiny glands are located on either side of the urethral opening in females. They have no important function, but probably do secrete a small amount of fluid during female orgasm. They are virtually impossible to see or find unless they get infected, in which case they become enlarged, painful and leak fluid.
Thanks for the question.
Tom Wilson, MD PhD
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