21.2 Embryo with undifferentiated gonads



Anatomic relationship between the genital and urinary systems

Quiz

Quiz 20

The development of the genital apparatus accompanies that of the urinary system (formation of the urinary apparatus). It also has its origin in the intermediate mesoderm 9 and urogenital sinus 15 . The primordial germ cells 7 share in the formation of the gonads, but have an ectodermal origin 3. In males, the development of the testes is closely tied with that of the mesonephros. In females, on the other hand, the mesonephros plays no role at all.

The intermediate mesoblast is the origin of an elongated structure, the urogenital ridge (crista urogenitalis), which lies on both sides of the midline between the lateral mesoderm and the root of the dorsal mesenterium of the embryo. It consists of two main components, the nephrogenic cord, out of which the urinary apparatus arises and the genital ridge 14 as the origin of the gonads. The genital ridge extends from the upper thorax region to the level of the cloaca. The true gonad anlage develops, though, out of only the middle area. The cranial and caudal parts of the urogenital ridge form the upper and lower gonadal bands, respectively, that as embryonic structures secure the gonads cranially and caudally.

Color code
for the
genital tract module

Paramesonephros
(Müller):
blue
Mesonephros
(Wolff):
fuchsia-violet
Urogenital sinus
(endoderm):
yellow
Ligaments:
brown


Fig. 3 - Sexually indifferent embryo:
Migration of the primordial germ cells into the genital ridge		  Legend

1
2

3
4
5
6
7		 Coelomic epithelium
Local mesenchyma
(in proliferation)
Gonadal cord
Primordial germ cells (PGC)
Mesenchyma
Allantois
Omphalomesenteric duct
8
9
10
11
12
13
14
 Intestine
Dorsal mesentery
Genital ridge
Nephrogenic cord
Mesonephric duct (Wolff)
Mesonephric tubule
Aorta

Fig. 3
Small picture:
Anlage of the genital ridge at 5 weeks.

Main picture: development of the indifferent gonad with the migration of primordial germ cells (PGC) at around the 6th week.

Note! The module's color code is not used in this diagram.


More info

After the mesonephros has atrophied, the lower gubernaculum inserts at the lower pole of the gonad, extends to the inguinal region and, in a man, thus forms the gubernaculum and, in women, the ovarian ligament and round ligament (ligamentum teres uteri). After the atrophy of the mesonephros, the upper gubernaculum disappears (in men) while in women, the suspensory ligament of ovary is formed, which extends from the upper pole of the ovary towards the back and upwards into the lumbar region.
Upper and lower gubernaculum in an embryo in the indifferent stage, ca. 6 weeks
 Legend


In an embryo with indifferent gonads two bands secure the gonads at the lower and upper poles.

(See: with definitions)

Ovarian ligament and suspensory ligament of ovaries: ca. 4 months
Gubernaculum testis
ca. 3 months
 Legend


In a woman the upper gubernaculum inserts at the upper pole of the ovary and forms the suspensory ligament of ovary. The lower gubernaculum inserts at the lower pole of the ovary and extends as the ovarian ligament to the uterine tube angle, where it continues as the round ligament of uterus (ligamentum teres uteri)
(See with definitions)

In a man the upper gubernaculum disappears, while the gubernaculum testis forms from the lower portion.
(See with definitions)


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