Embryonic Sexual Development


Men are from Mars. Women are from Venus.

Nope. All humans have been born on Earth so far.  Culturally, men and women may interact as aliens, but anatomically, they are strikingly similar.


Well, men are different than women, right?

Between the sexes, there are more similarities than differences, so I’d go with mostly the same. One chromosome out of 46 is different between males and females (that’s just over 2% difference!). Most of the basic body parts are from the same tissue origins, with the same blood supply, the same nerve control, and the same function. 

It’s the shape of people’s genitals that is confusing the issue. Humans start with the same basic embryonic tissues. As development and differentiation progresses the shape of the genitals is what changes. It is the differences in shape that we recognize as “male” and/or “female”.


Why does it matter?

First, if you consider that the shape of your genitals determines what you’ll get paid at work, it matters.

  • Sexually, when we believe that males and females are so different in their function and beliefs that they must have been born on different planets, we have a major education problem.
  • If you start with the assumption that men are different than women in sexual interest, drive, performance, and satisfaction, then you will medically diagnose and treat women differently than men.
  • If, instead, you start with the correct view that men and women are slightly different, you can more successfully grapple with questions of sexual function.

Adam was Eve, for 10 weeks anyway.

Perhaps Ev-adam is a better name. From conception until the 10th week of gestation, all embryos have the parts to become female and/or male. Development drives the embryo towards a female body shape unless testosterone pushes the embryo off that path. This is called unimorphic development, because all fetuses will develop into a female shape unless the Y chromosome is present AND produces sufficient levels of testosterone.


Mothers always give an X chromosome.

Inside each cell, all humans have one “X” chromosome which they got from their biological mother. X chromosomes are critical for survival so everyone has at least one in every cell.


Fathers determine chromosomal sex of the fetus.

Men are responsible for the sex of a fetus, because men produce X and Y chromosomes. Which ever one gets to into an egg (where one X is already waiting) determines which sex that particular fetus will be. If an embryo gets an X-laden sperm from the biologic father, congratulations--it’s a girl. If an embryo gets a Y-laden sperm from the biological father, congratulations--it’s a boy.  

In this sense, the wives of English King Henry the VIII really got a bad rap: he killed several because they only produced female children, even though he was responsible for the sex of his offspring. 


What does the Y chromosome do?

  • From conception up until 10 weeks. two systems develop side-by-side. One system is the Paramesonephric System (female, aka Mullerian), and the other is the Mesonephric System (male, aka Wolffian). Each system has the potential to develop, and both are “standing by” in every fetus until the presence (XY=Male) or absence (XX=Female) of the testosterone surge. 
  • At 10 weeks gestation, the Y chromosome causes the hormonal system of the male fetus to produce a surge of testosterone that is 3-5 times larger than any adult male will ever experience.
  • This testosterone surge essentially re-forms most of the existing body parts, lengthening the clitoris, paraurethral and urethra tissue, while zipping up the labia minora.
  • The labia majora are relatively flat, but will fill out into the scrotum when the testes drop down through the abdominal wall sometime at or just after birth.

Geek ALERT: Technically, the change from the undifferentiated gonad to a testis requires a gene called SRY, which is located on the short arm of the Y chromosome. Without this gene, the undifferentiated gonad will develop into an ovary, complete with ova. If present, the gene SRY causes fetal testosterone production to surge. This surge stimulates development of the wolffian structures (epididymis, vas deferens, and seminal vesicles), whereas a substance called antimüllerian hormone suppresses development of the fallopian tubes, uterus, and vagina.

The main point is that although men and women may appear somewhat different as adults, they all embryonically developed from the exact same structures. Men and women have the same nerves, same genital arousal structures (clitoris = clitoral caverns), same blood vessels, same biochemistry, etc.
Dr. Myrtle