Embrylogy notes made by Jón Kolbeinn Guðmundsson for University of Debrecen Medicine and Dentistry students.

1. Embryology part 7
Jón Kolbeinn Guðmundsson

2. The urogenital system
What is collectively called the urogenital system will give rise to both the urinary system
and genital system.
Urinary system: kidney, ureters, urinary bladder, urethra
Genital system: - female- ovaries, oviduct, uterus, vagina.
- male - testes, vas deferens, penis.
The development of both of these systems is closely linked.

3. The urogenital ridge
Along the posterior wall of the abdominal cavity (retroperitoneal) there forms a common
mesodermal ridge, deriving from intermediate mesoderm.
The ridge is called the urogenital ridge since both the urinary system and genital system
derive from it.

4. The development of the kidney
Topic 29

5. Anatomy of the kidney

6. The nephron.
The functional kidney is essentially made up of a:
1. Collecting system (collecting ducts)
2. Excretory system (nephrons)
These two systems make up the functional kidney but each has separate origins. The
collecting system originates from the ureteric bud and the excretory system from
metanephric mesoderm.
Collecting system
Excretory system

7. 3 stages of kidney development
During development there are 3 kidney systems that form, and one of them persists as
the definitive kidney. The other systems degenerate eventually, or form other structures.
Pronephros - forms rapidly in the beginning of the 4th week and disappears by
the end of the 4th week.
Mesonephros - consists of a mesonephric duct which runs down the length of
the embryo and inserts into the cloaca. It forms around the 4th
week and most of it disappears around the end of the 2th month,
however, parts of it participate in the male genital system.
It disappears in females.
Metanephros - appears last as a ureteric bud growing from the mesonephric duct
into surrounding mesoderm. This is the definitive kidney.

8. Paramesonephric duct
(Müllerian duct)
Gonad
Mesonephros
Mesonephric duct
(Wolffian duct)
In the middle of the second
month, the mesonephros has
developed into a long ovoid
structure with a complicated
tubule system. It is associated
with the gonad on its medial side
and this structure is therefore
called the urogenital ridge.

9. Pronephros disappears
The mesonephric duct originates from the intermedate mesoderm and
eventually connects to the cloaca.
The pronephros disappears and from the mesonephric duct the ureteric bud
grows.

10. Mesonephros
It’s important to understand that the mesonephric duct forms mesonephric tubules and
becomes associated with glomeruli formed from the aorta.
The mesonephros will however not produce urine, it will handle fluid, but all waste products
are handled by the placenta.

11. Pronephros
Mesonephros
Metanephros
(*ureteric bud)
Metanephros & the ureteric bud
The ureteric bud appears
as an outgrowth from
the mesonephric duct at
around 5th week. It
appears close to the
cloaca.
The bud soon becomes
associated with
metanephric mesoderm,
which starts to cover its
distal end like a cap.
Metanephric
mesoderm
Ureteric
bud

12. 1. The ureteric bud divides and expands into the metanephric blastema
(mesoderm) and gradually forms a primitive renal pelvis, which now
has a cephalic and caudal portions.
2. The renal pelvis invaginates further into the metanephric blastema
forming major calyces.
3. The major calyces form minor calyces, etc.
4. From the minor calyx we have collecting tubules burrowing further into
the metanephric blastema.
The collecting system

13. The excretory system
The collecting tubules formed from the minor calyx are capped at their ends by
metanephric blastema.
The tubule induces those cells to differentiate into renal vesicles, which will
eventually give rise to primitive nephrons.

14. The renal vesicles start to form S-
shaped tubules with a bowman’s
capsule forming at it’s proximal
end.
The distal end now starts to fuse
with the collecting duct and the
two will eventually becoming one.

15. The bowman’s capsule forms along with
a glomerulus.
As the tubule grows and elongates, it
starts to become convoluted in certain
areas, and the loop of Henle starts
appearing.
Loop of Henle
Bowman’s
capsule
Glomerulus
Distal convoluted
tubule
Proximal convoluted
tubule

16. Nephrons keep forming until birth at which there are approximately 1 million in
each kidney.
Urine production begins early in gestation, soon after the glomerular capillaries
have differentiated, which start to form around 10th week.

17. Position of the kidney in the abdomen.
In the adult the kidneys are positioned high
up in the abdomen, at around T12-L3.
In the embryo, the kidneys are initially in
the pelvic region but start to ascend
upwards in the abdomen.
This is caused by the relative growth of the
body in the lumbar and sacral regions.
In other words, as the embryo lengthens,
the kidneys change their relative position
from pelvis to upper abdomen.
Important note: In the pelvis, the metanephros receives its arterial supply from a
pelvic branch of the aorta. During its ascent to the abdominal level, it is
vascularized by arteries that origínate from the aorta at continuously higher levels.
The lower vessels usually degenerate, but some may remain.

18. The development of the ureter,
urinary vesicle and urethra
Topic 36

19. During the 4th to 7th week of development:
The cloaca divides into the 1. urogenital sinus anteriorly and the 2. anal canal
posteriorly.
The urorectal septum is a layer of mesoderm between the primitive anal canal and the
urogenital sinus.
The urinary vesicle (bladder)
Urogenital
sinus
Anal canal
Urogenital
septum
*The bladder and
urethra originate from
the urogenital sinus.*

20. Allantois
Urinary
bladder
The cranial part of the urogenital sinus
gives rise to the urinary bladder.
The allantois is an evolutionary remnant
which is connected with the urinary
bladder. It will become a ligament called
the urachus and will connect to the apex
of the bladder.
In the adult human the urachus becomes
the median umbilical ligament.

21. Proctodeum
Anal canal
The proctodeum is a membrane
covering the cloaca.
Once the urorectal septum devides
the cloaca into urogential sinus and
anal canal, we essentially have two
membranes.
One covers the anal canal called the
anal membrane.
The other covers the urogential sinus
and is called the urogenital
membrane.

22. Urogenital
membrane
Anal
membrane
Phallus
Perineal body
Now we have two separate canals,
the anal canal (rectum) and the
primitive urethra.
A primordial phallus is seen forming
at this stage, which will become the
penis in males and clitoris in
females. The development differs
greatly between the sexes.
The site where urorectal septum
divided the two, is called the
perineal body and becomes an
important attachment site for
muscles of the pelvic floor. In the
adult it becomes the perineum.

23. The ureter
It is important to show the development of the ureters and urethra in male and
females separately. There are differences in the way they develop.
We will start with the male development.
The ureteric bud originates
from the mesonephric duct
which connects to the
posterior side of the
primitive urinary bladder.
(A)
Eventually the caudal
portion of the mesonephric
duct is incorporated into the
bladder, making the ureter
and mesonephric duct enter
the bladder separately
(B)

24. The part of the mesonephric duct that
was incorporated into the bladder is
going to give rise to the trigone of the
urinary bladder.
Meanwhile, the mesonephric duct
inserts into the bladder inferiorly to
the ureter.
(C)
Later in development the mesonephric
duct is involved in formation of the
ductus deferens which connects to the
testes. The testes will eventually
descend into the scrotum and
therefore the mesonephric duct lies
over the ureters.
(D)

25. The urethra
In the urogenital sinus, just below the bladder there will be
outgrowths developing.
In males these outgrowths will form the prostate gland and the
prostatic urethra, the rest makes up the membranous and penile
urethra
*In females they form the urethral and paraurethral glands.*

26. The development of the penis
Topic 37

27. The indifferent stage
At early stages, male and female external sexual organs look identical, and
feature a large genital tubercle (primordial phallus).
In the center there is the cloacal
membrane.
Then a fold of ectoderm called
the cloacal fold.
Surrounding the cloacal fold is
the genital swelling
(labioscrotal)

28. Later as the urogenital sinus and
anal canal become separate. Then
we see a division between the two
separate structures.
The cloacal membrane essentially
divides into urogenital membrane
and anal membrane.
The urogenital folds separate from
the anal folds.

29. • In females there are relatively small morphological
changes.
The genital tubercle becomes the clitoris,
The urogenital fold becomes labia minora
The labioscrotal fold becomes labia majora
* More detail in other topics*

30. In the development of male external genitalia, there are more marked
changes and involves tissue rearrangements.
The genital tubercle becomes the glans penis.
The labioscrotal swellings become the scrotum.
The urethral groove needs to “zip up” towards the phallus in order to
enclose the penile urethra. ** This can often go wrong**
The phallus elongates and pulls the urethral folds forwards with it.
Urethral
groove

31. The epithelial lining of the urethral groove is endoderm derived from the urogenital
sinus.
This is called the urethral plate and once it “zips up” it makes up the penile urethra.
Urethral plate
This canal does not extend to the phallus,
at the end of 4th month some ectodermal
cells from the glans penis forms a canal
towards it that eventually forms a lumen.
This will be the external urethral meatus
(urethral opening).