Download Developmental Anatomy 13

20.02.2006
Developmental Anatomy 13
Last day: more or less finished the urinary system.
From last day, a complete hypospadias may lead to misdiagnosis of sex which may have
severe psychological repercussions for the future.
A.
Congenital anomalies of the kidney:
The kidney develops at sacral levels and later the body wall grows faster and the kidney
moves up into the abdomen. The kidney may stay in the pelvis or in the case of a
diaphragmatic hernia, the kidney has been found in the thorax. When the kidney moves
rostrally, it changes blood supply (this is different to the testes that take their blood supply).
Horseshoe kidney: situation where the kidneys are joined together at their lower poles. The
ureters emerge from its anterior surface. The inferior mesenteric artery stops the ascent of
the kidney.
Horseshoe kidney.
B.
Primordial germ cells:
Become specialised almost as soon as the inner cell mass is formed. They are found in the
mesenchyme surrounding the yolk sac.
TS Embryo.
The posterior abdominal wall has a projection called the urogenital ridge, the medial part
being the GONADAL RIDGE.
1. The germ cells migrate inwards around the gut inside the mesenchyme and end up in the
gonadal ridge.
2. Next, the germ cells need to be covered in epithelium to form the PRIMORDIAL
FOLLICLES and need to form a duct system to form the PRIMORDIAL SEMINIFEROUS
TUBULES in males. Ingrowths of mesenchyme called SEX CORDS grow inwards
surrounding the germ cells. There is single sheet of epithelial cells covering this mass
around the opening to the uterine tube.
Enlargement of the gonadal ridge.
3. In the male, there is the need for a duct to transport the germ cells to the seminal vesicle.
Gonadal ridge in male
4. The sex cords surround the germ cells and become the future tubules. Where does the
duct come from? It is actually already there. The MESONEPHRIC DUCT (WOLFFIAN
DUCT). This duct sits in the nephric ridge unused and the seminiferous ducts open into
it. This becomes the DUCTUS DEFERENS. The paramesonephric duct opens into the
VESICOURETHRAL CANAL, the future prostatic urethra.
Lateral view UGS, ductus deferens, and testes.
5. Under the influence of dihydroxytestosterone in particular, glands form around the
junction of the ductus deferens to form the prostate. The seminal vesicle grows
posterosuperiorly from the ductus deferens.
6. The Leydig cells differentiate from the mesenchyme. The fibrous tissue coat of the
testes, the TUNICA ALBUGINEA, forms from the differentiation of the mesenchyme
surrounding the gonadal ridge.
C.
This occurs at the level of L2 and the testes exist lower in the adult. This process is called the
DESCENT OF THE TESTES. What happens is the distance between the lower pole of the
testes to the deep inguinal ring stays constant but the rest of the body grows around it.
Oblique transverse section ant body wall.
On the surface is epithelium, the ectoderm. Deep to that is mesoderm that forms the bulk of
the body wall (the myotome-derived muscles.). Then deep to that is the second layer of
mesoderm that is the peritoneum. There is an opening in the muscle layer; the testes lies in
the tissue between the muscle and the peritoneum. At the same time, a collection of
mesenchyme called the SCROTAL SWELLING forms.
Then, a sac called the PROCESSUS VAGINALIS forms as an outpouching of peritoneum that
enters the scrotum at around six months. The process that causes the testes to move into
the scrotum may involve increased intra-abdominal pressure pushing the testes out a
preformed pathway. The tissue surrounding it at this time is a loose, mucoid tissue called the
GUBERNACULUM.
The seminiferous tubules retain their solid structure until puberty (age 12-14) when they are
cannulised.
D.
Defects of process:
Cryptorchidism: undescended testes may occur anywhere along the line (usually just at the
deep inguinal ring, may be high scrotal or midscrotal). Usually unilateral. Children usually
have highly active CREMASTER REFLEXES, which draw the testes into the abdomen. This
may lead to misdiagnosis.
Ectopic testes: occur when the testes descends incorrectly. May end up in the anterior
abdominal wall or in the penis.
E.
The ovary is simple in its development, but the rest of the female is not.
The definitive urogenital sinus is the approximate equivalent of the VESTIBULE of the vagina.
The swellings that produce the labia majora are called the LABIOSCROTAL SWELLINGS,
later the LABIAL SWELLINGS.
The female produces a separate duct, the PARAMESONEPHRIC DUCT (MULLERIAN
DUCT). This duct appears in the epithelium of the nephric ridge and forms in a process
similar to the development of the neural tube. The paramesonephric duct has an opening at
its upper end that never closes; it is the opening of the uterine tube.
Anterior view of paramesonephric ducts.
The inferior projections of these tubes meet in the midline and grow to meet the superior
recess of the vestibule of the vagina. The ducts join and undergo cavitation. This duct is the
fore-runner of the uterine cavity and uterine tube.
Yet to explain where the vagina comes from. The vagina is formed from the epithelium of the
vestibule. The tissue of the urogenital sinus grows posterosuperiorly, pushing the uterus
before it. This collection of cells is called the VAGINAL PLATE. It undergoes cavitation but
leaves a thin membrane at the opening, the HYMEN. It is usually not a complete partition.
F.
Congenital anomalies of the formation of the uterus.
The partition of the two ducts that form the uterus may be complete or not.
There may be two uteri.
There may be partial fusion of the two uteri, forming a CORNUATE UTERUS.
There may be partition of the vagina, forming a vaginal septum.
There may be a septum within the uterus that divides the cavity into parts.
There may be atresia of the uterine cavity.
Uterine abnormalities.
Malformations of the uterus are associated with persistent abortion.