2. The development of the testis,
epididymis and ductus deferens
&
The development of the seminal
vesicle and prostate gland
Topics 34 & 35
3. Anatomy of the testes
Rete testis
(ductus deferens)
Efferent ductules
4. Sexual dimorphism – male and female.
The difference in sexual organs in male and female is caused by the male
having a XY and female having XX chromosomes.
The Y chromosome in males is passed down from father to son, it determines
wether a baby will develop testes instead of ovaries.
In embryologic development the „default“ is female, meaning that if we
inactivate the Y chromosome, the embryo will by default develop into a female
baby.
Mechanism:
On the Y chromosome there is a gene called the Sex-determining region (SRY)
and the transcription factor of that gene creates a protein called the Testes
determining factor.
Under the influence of Testes determining factor the embryo develops into a
male, in it‘s absence, female development is established.
5. Indifferent gonad
Early in development of the gonad, the male and female gonads look identical and are
there called indifferent gonads.
The gonads start forming initially as gonadal ridges in the abdominal region of the embryo,
closely associated with the mesonephros. Collectively called the urogenital ridge.
Gonadal
ridge
6. The gonad is composed of epithelium associated with underlying mesenchyme.
The primordial germ cells don’t appear in the gonad until around 6th week.
They originate from the epiblast in the 2nd week of development and reside in the yolk
sac until they start migrating towards the gonadal ridge in 4th week.
The primordial germ cells reach the gonadal ridge in the 5th week and start invading
it in the 6th week.
7. At the arrival of the primordial germ cells to the gonadal
ridges, the epithelium starts to proliferate into the underlying
mesenchyme.
The gonadal epithelium forms so called primitive sex cords,
which are irregularly shaped and contain the primordial germ
cells. The appearance of the primitive sex chords is identical in
male and female gonads at this stage, hence, called the
indifferent gonad.
8. The testes
Under the influence of the Y chromosome, the indifferent gonad starts to form into
testes.
1. The primitive sex cords continue to proliferate and penetrate deep into the
medulla of the gonad to form medullary cords.
2. A dense fibrous connective tissue layer forms separating the testis cords from the
surface epithelium. Called the tunica albuginea
3. Finally the testis cords now contain primordial germ cells, sertoli cells derived
from gonadal epithelium and leydig cells derived from original mesenchyme.
Leydig cells
Primordial
germ cell
Sertoli cell
9. In the 4th month the testis cords become horseshoe shaped and continuous with
the rete testis. The cords don’t become tubules until puberty, then they are
called seminiferous tubules.
The testis cords and rete testis enter the ductuli efferentes, which is the
remainder of the excretory tubules from the mesonephric system.
The mesonephric duct (Wolfian duct) becomes the ductus deferens (vas
deferens).
10. The genital ducts.
Testosterone from the leydig cells in the gonads stimulates the mesonephric duct (Wolfian)
to persist.
Some of the excretory tubules (epigenital tubules) leftover from the mesonephric system
establish contact with cords of the rete testis.
The rest of the mesonephric tubules (paragenital tubules) do not establish contact and
degenerate.
11. The epigeintal tubules become the efferent ductules.
The mesonephric ducts elongate and become highly
convoluted forming the epididymis.
The mesonephric duct aquires a muscular coat and
forms the ductus deferens(vas deferens).
12. The seminal vesicle and prostate gland.
The seminal vesicle appears as a small bud arising from the ductus deferens
(mesonephric duct).
The bud increasingly lengthens and becomes slightly convoluted.
Both the ductus deferens and seminal vesicle drain into the prostatic urethra and
from there out through the urethra.
The prostate gland appears as an
outgrowth beneath the urinary bladder.
Mesenchyme surrounding the urogenital
sinus is stimulated by testosterone to
form the prostate gland.
The mesenchyme stimulates parts of the
urogenital sinus to form prostatic buds
which penetrate the mesenchyme.
The branching buds will eventually
become prostatic ducts.
13. A hormone called the Müllerian inhibiting substance (MIS) is
secreted from sertoli cells, which causes the paramesonephric
duct to degenerate. The leydig cells produce testosterone
which allows for further development of the mesonephric duct
forming the genital duct.
14. Descent of the testes.
1. The descent of the testes is aided by a structure called the gubernaculum. It
is a ligament which is connected to the lower pole of the testis. As the fetus
grows in length, the testis is tether towards the pubic symphysis of the pelvis.
Also of great importance is that there is an outpouching of peritoneum
invaginating towards the scrotum, called the vaginal process.
Note: the testis is covered by peritoneum (mesothelium) in the abdominal
cavity and is therefore passing behind the peritoneum (displayed in blue in
pictures A and B)
15. 2. Once the testis is close to the pubic symphysis, the gubernaculum becomes
contractile and pulls the testis into the testicular sack(scrotum) behinds the
vaginal process.
The vaginal process closes, leaving a layer of peritoneum to cover the testis
called the tunica vaginalis.
16. 3. In addition to being covered by layers of
peritoneum called the tunica vaginalis, the
testis becomes ensheathed by layers
deriving from the abdominal wall (muscles
and fascia).
As it is pulled to the scrotum by the
gubernaculum, the testis must pass
through the abdominal wall muscles.
Transversalis fascia
Internal spermatic fascia
Internal abdominal oblique muscle
Cremasteric fascia and muscle.
External abdominal oblique muscle
External spermatic fascia.
17.
18. The development of the ovary
&
The development of the uterine
tube and uterus
Topic 31 & 32
19. The ovary
In the indifferent stage of gonad development, we have primary sex cords fromed from
epithelium, with primordial germ cells encased in the cords, surrounded by
mesenchyme.
1. In the female ovary, the surface epithelium of the female gonad continues to
proliferate. Giving rise to secondary sex cords, called cortical cords, which penetrate the
underlying mesenchyme but remain close to the surface.
2. The medullary cords
degenerate and soon the
cortical cords split into
isolated cell clusters.
The cells in those
clusters surround each
oogonium (PGC) and
form a layer of epithelial
cells called follicular
cells.
Together called a
primordial follicle.
20. In both female and male embryos, the
mesonephric duct and the paramesonephric
duct arise.
In females the paramesonephric duct
persists and the mesonephric duct and
nephrons degenerate (leaving some
remnants).
Due to the absence of Müllerian inhibiting
substance (MIS) and testosterone, and the
higher levels of estrogen, the
paramesonephric ducts (Müllerian ducts)
will develop further into the uterus and
uterine tubes.
In the absence of testosterone, the
mesonephric ducts degenerate.
Note: the default development is towards
female sexual organs.
Genital ducts in the female.
21. The paramesonephric duct and the formation of the uterine tube and uterus.
The paramesonephric duct can roughly be described in 3 parts:
1. Cranial part, which opens into the abdominal cavity.
2. Middle part, which horizontally crosses the mesogastric duct.
3. Caudal part, which fuses with its partner duct from the opposite side.
During descent of the ovary towards the pelvis, the two cranial part develop into the
uterine tube (fallopian tube).
The caudal part fuses to form the uterine canal. Where it connects to the urogenital
sinus is called the sinus tubercle.
The urogenital ridges move mediocaudally causing them to lie in a transverse plane.
Note:
The gubernaculum exists in
females as well and pulls the
ovary towards the pelvic cavity. In
the adult it becomes two parts:
the ligament of ovary and round
ligament of uterus.
The suspensory ligament is simply
a remnant of the mesonephric
duct.
22. After the ducts fuse in the midline a broad transverse pelvic fold is established,
This is the broad ligament of the uterus.
The uterine tubes lie on it’s superior border and the ovary lies on its posterior surface.
The broad ligament divides the pelvic cavity into 1. uterorectal pouch behind the uterus
and 2. uterovesical pouch in front of the uterus.
23. The fused paramesonephric ducts give rise to the corpus and cervix of the uterus and upper
1/3rd of the vagina.
The uterus is surrounded by a layer of mesenchyme that form both its muscular coat
(myometrium) and peritoneal covering (perimetrium).
25. Vagina.
1. Shortly after the joined
paramesonephric ducts contact the
urgenital sinus, a dense structure
called the sinus tubercle starts
condensing and proliferating forming
a sinuvaginal bulb (vaginal plate)
2. Proliferation of the cranial end of the
plate increases distance between the
uterus and urogenital sinus.
3. At 5th month of development the
vagina is fully canalized.
4. Upper portion deriving from uterine
canal and lower portion from the
urogenital sinus.
5. The vaginal lumen remains separated
from the urogenital sinus by the
hymen (a thin tissue plate).
26. 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)
27. • 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
