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.
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.