We will continue our discussion of the reproductive system’s development
by looking at the development of the external genitalia
and how the same features will diverge
to produce either masculine or feminine appearance.
The external genitalia initially develop from the somatic layer of lateral plate mesoderm,
the somatopleure that’s growing anteriorly to form the thoracic, abdominal,
and then, pelvic body wall.
As these wrap around,
they’re going to come into existence on either side of the cloacal membrane.
You may recall that the cloaca is a common chamber for emptying of the urinary,
the genital, and the gastrointestinal system, and the cloacal folds
on either side of it are going to become some of the external genitalia.
As development proceeds,
an urorectal septum separates the cloaca into either the urogenital sinus or the rectum,
one anteriorly, one posteriorly.
On the outside, we have a continued separation of the membrane into an anal membrane
and a urogenital membrane
that’s covering the opening of the reproductive and urinary tracts.
As that’s occurring, labioscrotal swellings
are going to develop on either side of these folds.
So the urogenital membrane will be surrounded by labioscrotal swellings,
with a genital tubercle, a little bump located just anterior to it.
Now, both male and female external genitalia develop from the same starting point.
But exposure to estrogen or Dihydrotestosterone,
DHT, is going to move it into one or two different directions
so that it can form either male or female external genitalia.
In female, the urethral folds on either side of the urethral and vaginal opening
are going to remain separate.
Posterior to that will be a closure of that fold
and then, a little further back, the opening of the anus.
Further development is gonna cause labioscrotal swellings further lateral
to enlarge and create the labia majora and more anteriorly, the mons pubis.
The genital tubercle will become the head of the clitoris,
also known as the glans of the clitoris.
And once again, the opening of the urinary system and the vaginal opening
will be found inside the vestibule.
In male development, starting from the indifferent stage,
we have the urethral folds on either side,
and the labioscrotal swellings just lateral to those.
The glans of the penis will develop from the genital tubercle
located just ahead of the urethral folds.
Now, in the case of male development,
the labioscrotal swellings will become the scrotum, the urethral folds will seal together,
and the opening of the urethra will be eventually found at the head of the penis,
also known as the glans of the penis.
So as the urethral folds seal themselves shut,
they kind of zip forward from the base of the penis,
out towards the glans, and the labioscrotum swellings enlarge
and have the testes descend into them and then, will fuse on the midline,
leaving us with a scrotal raphe or a continuous seam
between the two sides of the scrotum
and similarly, will have the penile raphe extending onto the shaft of the penis
on its ventral side as the urethra is completely enclosed in the spongey cavernous body,
or pardon me, the spongey body of the penis.
The endoderm lined urethra almost reaches the end of the penis
but does not actually reach the outside.
Instead, before the urethra can actually empty urine to the outside,
ectoderm from the glans of the penis invaginates
and meets the endoderm lined portion of the spongey urethra.
And when that membrane between the two ruptures,
urine can then be released from the penis and the kidneys can do their work
and contribute the urine back into the amniotic fluid.
As this is happening, the glans of the penis develops some furrows within it
that will move inward and create the prepuce or the foreskin of the penis.
If the labioscrotal swellings or urethral folds fail to fuse together
to create the scrotal or penile raphe along its ventral surface,
the urethra in the male can open a little too soon
and not actually make it to the glans of the penis.
This is called hypospadias.
It’s an irrelatively common anomaly to find in male infants.
If the scrotum has an opening allowing urine out, that is called peroneal hypospadias.
Between the scrotum and the shaft of the penis would be penoscrotal.
And further down onto the shaft, we would have penile hypospadias.
The further the hypospadias extends outward,
the more it’s gonna replicate the normal appearance
emptying from the glands of the penis
and if it’s emptying just underneath the glands of the penis in a balanic/coronal manner,
or on the tip of the penis but facing downward, also known as glandular hypospadias,
it will probably not be surgically repaired.
But the ones that are emptying a little further back
may have elective surgery done to make sure that urine can drain in the typical manner.
In a seemingly related condition,
you can have epispadias where the urethra opens on the dorsal side of the penis.
This is due to a completely different set of events during development.
If the lateral plate mesoderm that forms the pelvic wall
fails to fuse completely on the front,
it can wind up causing an opening of the penis on the dorsal surface
or even an opening of the penis, genitalia, and bladder
from the anterior wall of the pelvis.
This is known as bladder exstrophy
and is similar to gastroschisis and ectopia cordis
and that it is a body wall defect only this time, manifesting in the pelvis.