We have an article out on how females become females, but this article is how males become males! Literally, the 4 paragraphs of the body are copy and pasted since all fetuses start out the same. So, that information doesn’t change!

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 genitalia 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 possibilities.

The fetus will start developing indifferent gonads, meaning not really male or female. 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! Consider reading through this at the same time as the female reproductive embryo article. The formatting will be pretty close to the same.

Starting with the gonads in males, the testes (ovaries in females). Same start, intermediate mesoderm forming elevation on the dorsal body wall which is the urogenital ridge. Epithelial and underlying mesoderm proliferate to form the gonadal ridge. From here, again the same, we form our primary sex cords which are going to migrate into the gonads from the wall. First difference, though, we’re making a male so we have that TDF floating around. This causes the testes to form instead of the ovaries.

Remember in Jurassic Park how they said that all embryos are inherently female? And how they just need a certain hormone to make them become male and so population control was done by “simply denying them that?” THAT’S WHAT WE’RE TALKING ABOUT!

Primary sex cords doesn’t break apart but it loses its connection to the wall. This makes the tunica albuginea forms (a fibrous envelope that goes around a portion of the penis). The primary sex cords also form the seminiferous cords, tublui recti and the rete testes. The seminiferous cords consists of primordial germs cells and sustentacular (Sertoli) cells which secrete Mullerian inhibiting Factor (MIF). These are super important to prevent the female reproductive system from forming, as we discussed. The mesoderm between the seminiferous cords are also going to give us the interstiial (Leydig) cells, which you probably know secretes testosterone. It’s also good to note that the seminiferous cords we are talking about our solid. They stay that way until puberty, when they hollow out and form the seminiferous tubules.

Just like the ovaries, the testes are formed in the stomach and then descend. Again, it’s the gubernaculum that pulls them down and eventually just anchor them to the bottom of the scrotum. There is peritoneum that is going to grow into the gubernaculum and form the processus vaingalus, which mostly obliterates. The part that doesn’t creates the tunica vaginalis which is a lining of the testes.

Alright, onto the genital ducts. Just like in females, we have the paramesonephric (Mullerian) ducts and the mesonephric (Wolffian) ducts. This time, its the paramesonephric ducts that end up regressing.They still form and fuse and make something called the uterovaginal primordium but since we have MIF that all regresses.

Mesonephric (Wolffian) ducts have something to do now. They’re still going to help make kidneys and the urinary tract. Now, they’re also going to create the epididymis, ductus deferens, seminal vesicle and the ejaculatory duct. A few of the extra tubules that are around the testes also form the efferent ductules of the testes.

Alright, last, let’s make our external genitalia. If you read the female article and know your homologous organs, this shouldn’t be hard. Same proliferation of mesoderm around the cloacal membrane pushes the overly ectoderm up. Here we have the phallus, urogenital folds and the labrioscrotal swellings. Just like before.

Phallus leads to penis (glans penis, corpora cavernosa penis and corpus spongiosum penis).

Urogenital folds will give you the ventral aspect of the penis.

Last, the labioscrotal swellings form the scrotum!