Formation of the Undifferentiated Uro-genital System

FORMATION OF THE UNDIFFERENTIATED URO-GENITAL SYSTEM 1) The gonads The undifferentiated gonads become individualized at the sixth week of development; they are made up of a set of cells, the somatic blastema, which is organized into sections, the primitive sex cords, colonized by germ cells or primordial gonocytes; From the fourth week of development, proliferation cell of the ventral face of the mesonephros raises the dorsal wall of the coelom which thickens at this level determining the formation of the genital ridges which project on either side of the dorsal meso. The somatic blastema brings together mesenchymal elements, cells derived from the mesonephros and those of the coelomic wall. The primordial gonocytes appeared from the third week in the extra-embryonic mesenchyme of the wall of the lecithocele on either side of the Allantoid diverticulum. During the 5th week, they migrate laterally from the dorsal meso towards the genital ridges and are placed within the primitive sex cords. 2) Undifferentiated genital tracts The Wolffian ducts or mesonephrotic ducts drain the contents of the mesonephrotic vesicles towards the primary urogenital sinus. During regression of the mesonephros, some tubules located opposite the gonadal outline persist and connect each side the gonad to the Wolffian canal which becomes a genital tract. The Müller ducts are formed from a longitudinal fold of the posterior wall of the coelome whose edges come together constituting a channel which is isolated in the mesenchyme. Located at their cephalic part between the mesonephros and the lateral wall, they are also called paramesonephrotic canals. In the caudal part of their course, they cross the Wolffian canals and meet at the midline. The conjoined ends of the two Müller ducts come into contact together with the posterior wall of the primary urogenital sinus on the midline at the level of the junction of the Wolff ducts. This abutment determines a small projection of the wall of the primary urogenital sinus, the müllerian tubercle. 3) The excretory system Appearing during the fifth week of development from an outgrowth of the lower part of the Wolffian canal, the ureteral bud develops laterally towards the metanephros. When the bud reaches the blastema, it gives two branches, the future large calyxes. Each of these initial branches develops within the blastema and gives successive divisions in dichotomous mode (into two secondary branches) up to the thirteenth order. The bifurcations of the 3rd and 4th order correspond to the small calyxes, beyond, they will give the collecting tubes or Bellini tubes. Initially located near the bladder, the renal anlage, at the same time as it increases in volume, moves between the 6th and 9th week in a cranial direction to come into contact with the adrenal gland. The lowering of the lower part of the vagina causes a stretching of the vesicourethral canal which becomes a narrow channel, the urethra. This conduit, only urinary, allows the flow of urine from the bladder to the urinary meatus. It opens into the flared area, the vestibule, which overlooks the urogenital slit. Only this vestibular zone is urogenital, it remains separated from the lower part of the vagina by the hymen. 4) Nephrons It is the interactions between the derivatives of the ureteral bud and the metanephrotic blastema which will determine the formation of the nephrons and the upper part of the excretory system. The first will be formed by transformation of metanephrotic vesicles from the 8th week of development and, according to the same process, new nephrons will form beyond birth allowing the renewal and growth of the definitive kidneys. Each metanephrotic vesicle elongates and takes on an italic “S” shape. The part in contact with the excretory canal lengthens and ends at this excretory canal, it will become the distal convoluted tubule (DCT) of the nephron. The other end comes into contact with and surrounds a capillary platoon irrigated by a branch of the renal artery thus forming a glomerulus (combining glomerular chamber and platoon). The intermediate zone of each metanephrotic vesicle elongates and presents three segments: the proximal convoluted tubule (PCT) which follows the glomerular chamber, the loop of Henle and a straight segment which joins the TCD. In the contact zone between the TCD and the glomerular platoon, the wall of the tubule modifies and forms a particular zone, the macula densa (MD). The first nephrons become functional around the 8th week and the first drops of urine appear around the 10th week. in the excretory system and, until birth, they will be poured into the amniotic fluid, the renewal of which they ensure. 5) The urogenital sinus The primary urogenital sinus results from septation of the cloaca at the 7th week of development. The cloaca is divided into two zones, one dorsal, the anorectal canal, terminal part of the digestive tract, the other ventral, the primary urogenital sinus, where the Wolffian ducts end. This junction divides the primary urogenital sinus into two parts, one located above, the vesicourethral canal, the other located below, the definitive urogenital sinus, closed by the urogenital membrane which is resorbed at the 8th week of development. The further development of the definitive urogenital sinus is linked to sex. The upper part of the vesicourethral canal widens to give the bladder, it remains provisionally connected at its top to the Allantoid canal. After the reintegration of the digestive loops and the closure of the umbilical region, the segment located between the umbilicus and the bladder becomes the urachal canal which obliterates around the 5-6th month and becomes fibrous. At the junction of the Wolffian ducts, the posterior bladder wall and duct wall actively proliferate such that the proliferation encloses the end of the Wolffian ducts and the proximal portion of the ureters derived from the ureteral buds. This proliferation laterally pushes the orifices of the ureters which will now be located above and outside those of the Wolffian ducts.