Embrology of the urogenital system

1. Embrology of the urogenital
system.
Development of the genital
system.
Dr Mberesero J.

2. Introduction.
• The key to sexual dimorphism is the Y
chromosome which contain the sex
determining region on Y (SRY) short arm
(Yp11).
• Male and female morphological
characteristics begin to develop by
seventh week the initial period of genital
development is the indifferent stage of
sexual development.

3. Development of gonads.
• The gonads (testes and ovaries ) are
derived from three sources.
– Mesothelium (mesoderm epithelium) lining the
posterior abdominal wall.
– Underlying mesenchyme ( embryonic
connective tissue)
– Primordial germ cells.
• Indifferent gonads.
– By fifth week a thickened area of mesothelium
develops on the medial side of the
mesonephros.

4. Indifferent gonads cont:
• Proliferation of this epithelium and the
underlying mesenchyme produce a bulge
on the medial side of the mesonephros
the gonadal ridge.
• Finger like epithelial cords the gonadal
cords grow into the underlying
mesenchyme.
• The indifferent gonads consist of an
external cortex and an internal medulla.

5. Indifferent gonads cont:
• In an embryo with XX the cortex of the
indifferent gonads differentiate into an
ovary and the medulla regresses.
• In an embryo with XY the medulla
differentiate into a testis and the cortex
regresses.

6. Primordial germ cells.
• These are large spherical cells visible in the 4 th
week among the endodermal cells of the
umbilical vesicle near the origin of allantois.
• In the 6th week the primordial germ cells arise
from yolk sac endoderm migrate to the gut and
then through dorsal mesentery to reach the
gonadal ridge.
• Germ cells proliferate and migrate into cellular
cords to become surrounded by supporting cells
(germ cells that fail to enter a cellular cord
undergo degeneration).

8. Sex determination.
• The sex of the embryo depends on whether an
X-bearing sperm or a Y-bearing sperm has
fertilized the X- bearing oocyte.
• Before 7th week the appearance of the two
gonads is identical.
• Development of the male phenotype requires a
Y chromosome.
• The SRY gene for a testis-determining factor
(TDF) has been localized in the sex determing
region in short arm of Y chromosome.

11. Development of the ovary.
• The X chromosome bear genes for ovarian
development.
• Cortical cords extend from the surface
epithelium of the developing ovary ito the
underlying mesenchyme.
• As the cortical cords increase in size the
primordial germ cells are incorporated in them.
• At approx. 16 weeks the cords begin to break
into isolated cell clusters- Primordial follicles
constisting of oogonia.
• Active mitosis of oogonia occur during fetal life.

13. Development of the ovary cont;
• No oogonia form postnataly, although
many degenerate before birth, two milion
or so that remain enlarge to become
primary oocyte before birth.
• primary oocytes commence meiosis, but
remain frozen in prophase of Meiosis I
until ovulation in the reproductively
capable female.

14. Development of the genital ducts.
• During 5th and 6th week there indifferent
gonads.
• The mesonephric ducts ( Wolffian ducts)
male reproductive system.
• The paramesonephric ducts ( Müllerian
ducts) female reproductive system.

15. Female genital ducts and glands
• Female sexual development does not depend on
presence of ovaries or hormones.
• Uterine tubes develop from unfused cranial
parts of the müllerian ducts.
• The caudal fused portion of these ducts form the
uterus and the vagina ( superior part)
• The endometrial stroma and myometrium are
derived from splanchnic mesenchyme. .
• Fusion of the müllerian ducts also brigs together
a peritoneal fold that forms the broad ligament
and two peritoneal compartments– the
rectouterine pouch and vesicouterine pouch.

16. Uterus and vagina.
• The fibromuscular wall of the vagina is derived
fro the surrounding mesenchyme.
• The uterovaginal primordium with the urogenital
sinus form the sinus tubercle induce formation of
paired endodermal growth the sinovaginal bulbs.
• The sinovaginal bulbs fuse to form a vagina
plate whose central cells break down later to
form the lumen of the vagina.
• Until late fetal life, the lumen of the vagina is
separated from the cavity of the urogenital sinus
by a membrane the hymen ( derived from
invagination of the posterior wall of the
urogenital sinus).

18. Development of external genitalia
• The external genitalia are not fully
differentiated until the 12th week.
• At 4th week genital tubercle in both sexes
at the cranial end of the cloaca
membrane.
• Labioscrotal swellings and urogenital folds
develop on each side of the cloaca
membrane.
• The genital tubercle soon elongates to
form the primordial phallus.

19. Development of external genitalia

20. Female External Genitalia.
• The primordial phallus in the female
gradually becomes the clitoris ( relatively
large at 18 weeks).
• The urethral folds do not fuse, except
posteriorly where they join to form the
frenulum of the labia manora.
• The unfused parts of the urogenital folds
forms the labia minora.
• Most parts of the labiaoscrotal folds form
the labia mijora.

21. Female External Genitalia.