University:
Stanford University
Course:
MED 101 | Human Anatomy
Academic year:

2023
Views:

236

Pages:

2
Author:

larbi43100

DEVELOPMENT OF THE URO-GENITAL SYSTEM 1) Introduction The definitive urinary system is made up of secretory elements and excretory ducts. The secretory structures of the definitive kidney derive from the caudal part of the nephrogenic cord, the metanephros. The excretory pathways derive from the mesonephric duct and the ventral part of the urogenital sinus; in their caudal part, their development is linked to that of the genital apparatus and depends on its differentiation. The genital tract remains the same in both sexes until the 8th week of development. At this undifferentiated stage, it is made up of three parts: the genital glands, the genital tracts and the outline of the external genitalia. From the 8th week, all of these urogenital structures evolve differently depending on the sex. These morphological transformations depend on genetic factors, 2) THE NEPHROGENIC CORD It comes from the segmentation, from the fourth week of development, of the intermediate mesoblast into small cell clusters located in the same horizontal plane as the somites, the nephrotomes. 1. The pronephros From the level of the 2nd occipital pair to that of the 5th cervical pair, these nephrotomes will be at the origin of the pronephros. Each of the nephrotomes is hollowed out by a small rudimentary tubule which opens into the coelomic cavity. The lateral ends of these tubules join and confluence to form a common canal, the pronephrotic duct. After regression of the pronephros, only the caudal part of this canal persists. 2. The mesonephros From the level of the 6th cervical pair to that of the 4th lumbar pair, even before regression of the pronephros, one or two nephrotomes per metamere are formed; this set will constitute the mesonephros. Each of these nephrotomes elongates to give a cellular cord which deepens and constitutes a mesonephrotic tubule. The axial end of these tubules is in contact with vascular primordia supplied by a segmental branch of the aorta. Their lateral ends join to form, with the rest of the pronephrotic duct, a single canal, the mesonephrotic duct or Wolffian duct. During the 5th week, part of the mesonephros cells are involved in the constitution of the undifferentiated gonad. The further evolution of mesonephrotic structures is linked to sex. 3. The metanephros The most caudal part of the nephrogenic cord does not metamerize; At the 5th week, it constitutes an undivided cell mass, the blastema, at the origin of the metanephros. The differentiation of this blastema requires its penetration by the primordia of the excretory ducts. The latter derive, from the 5th week, from an outgrowth of the caudal part of the Wolffian canal, the ureteral bud, the first branches of which form the beginnings of the future large chalices. On contact with the excretory ducts, the cells of the blastema densify and form a cap whose ends hollow out to form meta nephrotic vesicles, each of which will be the origin of a nephron.