FORMATION OF THE UNDIFFERENTIATED URO-GENITAL SYSTEM 1) The gonads The undifferentiated gonads become individualized at the sixth week of development; theyare made up of a set of cells, the somatic blastema, which is organized into sections, theprimitive sex cords, colonized by germ cells or primordial gonocytes; From the fourth week of development, proliferation cell of the ventral face of themesonephros raises the dorsal wall of the coelom which thickens at this level determiningthe formation of the genital ridges which project on either side of the dorsal meso. Thesomatic blastema brings together mesenchymal elements, cells derived from themesonephros and those of the coelomic wall. The primordial gonocytes appeared from the third week in the extra-embryonic mesenchymeof 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 withinthe primitive sex cords. 2) Undifferentiated genital tracts The Wolffian ducts or mesonephrotic ducts drain the contents of the mesonephrotic vesiclestowards the primary urogenital sinus. During regression of the mesonephros, some tubuleslocated opposite the gonadal outline persist and connect each side the gonad to theWolffian canal which becomes a genital tract. The Müller ducts are formed from a longitudinal fold of the posterior wall of the coelomewhose 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 alsocalled paramesonephrotic canals. In the caudal part of their course, they cross the Wolffiancanals and meet at the midline. The conjoined ends of the two Müller ducts come intocontact together with the posterior wall of the primary urogenital sinus on the midline at thelevel of the junction of the Wolff ducts. This abutment determines a small projection of thewall 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 theWolffian canal, the ureteral bud develops laterally towards the metanephros. When the budreaches the blastema, it gives two branches, the future large calyxes. Each of these initialbranches 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 4thorder correspond to the small calyxes, beyond, they will give the collecting tubes or Bellinitubes. Initially located near the bladder, the renal anlage, at the same time as it increases involume, moves between the 6th and 9th week in a cranial direction to come into contact withthe adrenal gland.
The lowering of the lower part of the vagina causes a stretching of the vesicourethral canalwhich becomes a narrow channel, the urethra. This conduit, only urinary, allows the flow ofurine 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 remainsseparated 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 metanephroticblastema which will determine the formation of the nephrons and the upper part of theexcretory system. The first will be formed by transformation of metanephrotic vesicles fromthe 8th week of development and, according to the same process, new nephrons will formbeyond birth allowing the renewal and growth of the definitive kidneys. Each metanephroticvesicle elongates and takes on an italic “S” shape. The part in contact with the excretory canal lengthens and ends at this excretory canal, it willbecome the distal convoluted tubule (DCT) of the nephron. The other end comes intocontact with and surrounds a capillary platoon irrigated by a branch of the renal artery thusforming a glomerulus (combining glomerular chamber and platoon). The intermediate zoneof each metanephrotic vesicle elongates and presents three segments: the proximalconvoluted tubule (PCT) which follows the glomerular chamber, the loop of Henle and astraight segment which joins the TCD. In the contact zone between the TCD and theglomerular platoon, the wall of the tubule modifies and forms a particular zone, the maculadensa (MD). The first nephrons become functional around the 8th week and the first drops ofurine appear around the 10th week. in the excretory system and, until birth, they will bepoured 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 ofdevelopment. The cloaca is divided into two zones, one dorsal, the anorectal canal, terminalpart of the digestive tract, the other ventral, the primary urogenital sinus, where the Wolffianducts end. This junction divides the primary urogenital sinus into two parts, one locatedabove, 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. Thefurther development of the definitive urogenital sinus is linked to sex. The upper part of the vesicourethral canal widens to give the bladder, it remains provisionallyconnected at its top to the Allantoid canal. After the reintegration of the digestive loops andthe closure of the umbilical region, the segment located between the umbilicus and thebladder becomes the urachal canal which obliterates around the 5-6th month and becomesfibrous. At the junction of the Wolffian ducts, the posterior bladder wall and duct wall activelyproliferate such that the proliferation encloses the end of the Wolffian ducts and the proximalportion of the ureters derived from the ureteral buds. This proliferation laterally pushes theorifices of the ureters which will now be located above and outside those of the Wolffianducts.