Icon module 20

Development of the urinary organs (3rd - 4th week)

The development of the urinary system is closely related to that of the genital system which will be described, though, in a separate module.
The urinary organs consist of the kidneys (which produce urine, among other things), the ureter (transport of the urine from the kidneys to the bladder), the bladder (temporary storage for the urine) and the urethra (transport of the urine from the bladder to the external world).

In all vertebrates the kidneys and ureters develop out of the intermediate mesoderm, whereas bladder and urethra derive from the urogenital sinus.

Fig. 1 - Transverse section and dorsal view of an embryo (trilaminar)
(ca. 21 days)

  1. Paraxial mesoderm
  2. Intermediate mesoderm
  3. Lateral mesoderm
  4. Notochord
  5. Amnion
  6. Intraembryonic coelom
  7. Endoderm
  8. Ectoderm
  9. Somatopleura (mesoderm and ectoderm)
  10. Splanchnopleura (mesoderm and endoderm)
  11. Neural groove
  12. Neural ridge

Fig. 1

Schematic diagram: Transverse section (in the midcephalic region) with a dorsal view of the three-layered embryo towards the end of the 3rd week of development. The intermediate mesoderm lies between the somites (paraxial mesoderm) and the lateral mesoderm (out of which the coelom arises).

Lors de la lateral folding, the intermediate mesoderm is shifted ventrally and loses its connection with the somites and the lateral mesoderm.

The nephrogenic cord develops out of the intermediate mesoderm (stage 9) and extends from the cervical to the caudal region. It becomes segmented like the paraxial mesoderm (somites) (Fig. 1). This segmentation is easily seen in the cranial region, and is rudimentary in the middle region. In the caudal region it is no longer present, as we will describe in detail below.

Due to the growth of the inner structures of the embryo, the tissue lying most laterally is displaced ventrally. This leads not only to a separation of the nephrogenic cord from the paraxial and lateral mesoderm, but also to a coalescence of certain median structures such as the two dorsal aortas (stage 12) which fuse to form the definitive (median) aorta.

Fig. 2 - Development of the intermediate mesoderm (stage 10)

  1. Neural tube
  2. Amniotic cavity
  3. Paraxial mesoderm
  4. Intermediate mesoderm
  5. Yolk sac (umbilical vesicle)

Fig. 2

Transverse section of an embryo at the beginning of the folding (ca. 28 days). In this diagram the nephrogenic cord is clearly distinguished in the intermediate mesoderm.

The black arrow indicates the border between the paraxial and intermediate mesoderm.

Fig. 3 - Isolation of the nephrogenic cord (stage 11)

  1. Neural tube
  2. Amniotic cavity
  3. Dermatomyome
  4. Nephrogenic cord that has moved away from the paraxial mesoderm (somite) (black arrow)

Fig. 4 - Isolation of the nephrogenic cord (stage 12)

Yolk sac (umbilical vesicle)
Intraembryonic coelom

Fig. 3, 4

The intermediate mesoderm moves ventrally and loses its connection (black arrow) to the somites and the lateral mesoderm.

Also observe its approach towards the intraembryonic coelom

Fig. 4