Where Wet Diapers Come From (Urinary Embryo)

Let’s discuss the development of the urinary system a bit more. Specifically, the parts that help us get urine out.

The urinary bladder comes from the upper portion of the urogenital sinus. Do you remember the allantois? The urogenital sinus is continuous with that.

The allantois is the fetal membrane that lies below the chorion. It becomes the urachus, which is a type of fibrous cord. As an adult, we can still see it, but we call it the median umbilical ligament.

Moving back to the lower side of the mesonephric duct, we can see the trigone of the bladder development. That is essentially the bottom portion of the developed bladder. All along here, the lining comes from the endoderm because we need that super special transitional epithelium.

Now we’re holding urine, let’s get it out of the body. The formation of the female and male urethra is slightly different.

In the female, it comes from the lower portion of the urogenital sinus. The endodermal develops into the mesoderm around it to make the urethral glands and the paraurethral glands of Skene. These are homologous to the prostrate in males. It’s incredibly helpful in these studies to learn the homologous structures. It takes what feels like twice the work and provides the framework to understand them as half the structures.

Anyway, the female urethra goes as far as the navicular fossa which in turn empties into the vestibule of the vagina. Here, more endodermal outgrowths in the the surrounding mesoderm and we’ve got the lesser vestibular glands and the greater vestibular glands of Bartholin. These are homologous to the bulbourethral glands of Cowper in the male.

Bartholin opens on each side to the vaginal force, throw on your transitional and stratified squamous epithelium from the endoderm and there is the female urethra!

Now, on the guys. The urethra is quite a bit longer since it doesn’t just go to the end of the pelvis, it has to reach the end of the penis. Still coming from the urogenital sinus, we have the prostatic urethra, membranous urethra, bulbous urethra and the proximal part of the penile urethra. If you’re reading this for class, check your textbook! These are often labeled and even partitioned out differently depending on the source you are using.

Prostatic urehtra does the endodermal outgrowth thing to make the prostrate gland. Prostratic urethra will also have a sort of bump on the back wall that is the urethral crest. There is a groove on either side that is the prostatic sinus which will allow the prostate gland to secrete into the sinus. There is also an ovoid enlargement called the seminal colliculus, usually between the prostatic sinuses. Here are the openings of the ejaculatory ducts and the prostatic utricle. Don’t worry so much about the prostatic utricle, it’s just a leftover of the paramesonephric ducts which is more important in females.

Membranous urethra, same outgrowthing, making the bulbourethral glands of Cowper.

Bulbous urethra has the openings of the Cowper.

Proximal part of the penile urethra, outgrowthing again, boom! Urethral glands of Littre

Last, the distal part of the penile urethra. See why we started with the female urethra? Instead of outgrowth, we actually have in-growth of ectoderm (not endoderm!). This forms the glandular plate which is canalized. Now we’ve got our navicular fossa. There is separation that works its way in called the ectodermal septa. This makes the foreskin. Throw in some stratified squamous epithelium lining from the ectoderm and we’re done!

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