How do boys become boys and girls become girls? And when? Let’s take a look at that in this article. Well, how girls become girls, at least. We’ll do a separate discussion on boys being boys!

All the way until week 7, it’s all the same. Even now, it’s only a phenotypical sexual differentiation. By week 12, the external genitialia can be recognized. Week 20 is when things really start rocking.

Phenotypic sexual differentiation is done at this point. Phenotypic sexual differentiation is ran by the SRY gene (which codes for a Sex-determining Region Y, also known as TDF – Testes Determining Factor). It can result in females, males and the intersex possibilites.

The fetus will start developing indifferent gonads, meaning not really male or femal. Those are the testes, Leydig cells and Sertolic cells. These produce testosterone and Mullerian-inhibiting factor (MIF). Know these hormones!

If we’re making a boy, we have all three. If we’re making a girl, those are not produced and all of those components are remodeled to form the female reproductive system. Just like in so many other things, men and women don’t really have different body parts. It’s more like they have the same ones, just different versions. They are pretty much all homologous!

In males, the gonad is the testes. In females, it’s the ovary. The ovary comes from intermediate mesoderm along the dorsal body wall forming the urogenital ridge. This should be a familiar aspect if you read the development of the urinary system. The epithelium and mesoderm of the urogenital ridge start reproducing to form the gonadal ridge. That is going to create the primary sex cords. From there, the primary sex cords extend into the medulla and develop into the rete ovarii (which are just going away, anyhow).

Keep going a little later and then the secondary sex cords develop and roll into the primordial germ cells making the tunica albuginea. Again, the secondary sex cord breaks apart but then gets back together! Like a high school relationship. This time, though, when they get back together they’re isolated from each other in clusters. These are the primordial follicles – finally, something we can recognize! These primordial follicals contain the primary oocytes surrounded by the simple squamous cells. All this is coming from the mesoderm by the way.

The ovaries are developed in the abdomen but then descend, much like the testes. We’re not actually entirely sure how all this works, other than the disproportionate growth that happens (much like the kidneys). The gubernaculum is the muscle that causes the testes to descend into the scrotum, and since testes and ovaries are homologous, it’s probably involved. We do know that in a female, that is going to stick around to form the ovarian ligament (connecting the ovary to the uterus) and the round ligament of the uterus (so it continues all the way into the labia majora which is homologous to the scrotum).

There are two genital ducts we should discuss, too. These are the paramesonephric ducts (Mullerian ducts) and the Mesonephric (Wolffian) ducts. The mesonephric ducts develop to make the metanephric kidney, but in females they degenerate after that. So, we’ll focus on the paramesonephric ducts.

Let’s go back to the urogenital ridge and look at the lateral surface. This is where the paramesonephric ducts form. On the top side, or the cranial side, we have the uterine tubes. On the bottom, or caudal, side the two ducts actually form. This is our uterovaginal primordium and the tissue they were connected two also get pulled towards each other. Those form the broad ligaments. From here, in our uterovaginal primordium, we’re making the uterus, cervix and the top 1/3 of the vagina.

The other thing the paramesonephric ducts are going to do is shoot back into the posterior wall and create the sinovaginal bulbs. These form to make a vaginal plate, which then canalizes and forms the rest (inferior 2/3) of the vagina. What’s interesting is that even though the vagina comes from two different places, the epithelial lining of the whole thing comes from the endoderm.

Alright, almost done. Last, we make the external sex parts. Basically, there is all sorts of mesoderm growing around the cloacal membrane. The ectoderm that rises up around it are the phallus, urogential folds and the labioscrotal swellings. The phallus forms the clitoris (homologus to the penis), the urogenital folds form the labia minora (penile skin) and the labioscrotal swellings form the labia majora and the mons pubis (scrotum).

Next week, we’ll discuss how it is different in males!