Urogenital system

• The urogenital system develops from the intermediate mesoderm that dissociates from the paraxial and the lateral plate mesoderm during the folding process of the embryo. This intermediate mesoderm forms a left and right cord of cells (also known as the nephrogenic cords), which run along the entire dorsal side of the human embryo.
• Differentiation of intermediate mesoderm starts with the formation of a segmental series of epithelial buds in the most cranial region and with the development of a primary nephric duct in the lateral region at each side of the embryo.
• These segmental buds represent the earliest form of the kidney, the pronephros, while the primary nephric ducts organize the development of much of the adult urogenital system
• The mesonephroi form simple nephrons that filter and remove body wastes of the embryo. They are drained by the mesonephric (Wolffian) ducts, which are the continuation of the primary nephric ducts and open into the posterior wall of the primitive urogenital sinus.
• The mesonephroi remain prominent until a pair of ureteric buds arises from the distal mesonephric ducts to induce the overlying sacral intermediate mesoderm to develop into the metanephroi by the 5th week. Most of the metanephroi become the functional definitive kidneys but in male, some of them persist together with the mesonephric ducts, and integrate in the genital duct system.  

   
• The development of the metanephroi begins with the induction and formation of a pair of uretic buds that penetrates in a portion of the sacral intermediate mesoderm called the metanephric blastema.
• Differentiation of this blastema gives rise to the urine-forming units of the kidneys, the nephrons, while the ureters and the collecting duct system of the kidneys differentiate from the ureteric buds
• The nephronic tubuli in the definitive kidney are formed by metanephrogenic mesoderm
• The collecting ducts are formed by the uretic buds
• The cloaca is divided into the rectum and the urogenital sinus.
• The superior portion of the urogenital sinus becomes the bladder, whereas its inferior portion gives rise to the pelvic (membranous and prostatic) urethra and penile urethra in males or the pelvic (membranous) urethra and vestibule of the vagina in females.
• Development of the genital system starts during the 5th week with the indifferent phase. During that stage, the germ cells migrate from the yolk sac to the area just medial to the mesonephroi at the 10th thoracic segment. After arrival, the germ cells induce the formation of the genital ridges which consist of two major types of cells: one derived from the mesonephros and the other arising from the adjacent coelomic epithelium. 
• The cells of genital ridges become somatic support cells that will differentiate into Sertoli cells in the male or into granulosa cells in the female when the early indifferent stage is finished
• he indifferent stage is furthermore characterized by the formation of a pair of Müllerian ducts in the dorsal body wall just lateral to the mesonephric (Wolffian) ducts.
• Differentiation of the testes starts with the development of pre-Sertoli cells that recruit mesonephric cells into the gonadal ridge to give rise to Leydig cells, myoepithelial cells, interstitial cells and endothelial cells. The differentiated Sertoli cells join the migrated germ cells and, together with the myoepithelial cells, give rise to the future seminiferous tubules.
• Somatic support cells, which do not contain germ cells, differentiate into the testes, which will connect with a limited number of mesonephric tubules and will canalize the seminiferous tubules to the mesonephric (Wolffian) duct. 
• The mesonephric tubules become the efferent ductules while the mesonephric ducts become the vasa deferentia. The Müllerian ducts degenerate in males. During the 3rd month, the distal vas deferens gives rise to the seminal vesicle while the prostate and bulbourethral glands develop from the adjacent pelvic urethra. Simultaneously, the indifferent external genitalia differentiate into the penis and scrotum. Late in fetal development, the testes descend into the scrotum.
• In females, the somatic support cells differentiate into granulosa cells that also surround the migrated germ cells to form the primordial follicles of the ovary. The mesonephric (Wolffian) ducts degenerate but now, the Müllerian ducts become the genital ducts. Their proximal portions form the oviducts while fusion of their distal portions gives rise to the uterus and contributes to the formation of the vagina. The indifferent external genitalia develop into the clitoris and the paired labia majora and minora.
• Which embryonic kidney system develop during time (in order) →

  • pronephros
  • mesonephros
  • metanephros
• which structures give rise tot the pronephros:

  • pronephrogenic tube
  • progenitor
• Which bloodvessels lead blood to and from the mesonepros → lead to aorta, lead from vena cardinalis posterior
• Which germlayer gives rise to the urogenital system? → intermediate mesoderm
• into which coelom do the genital ridges protrude? → intra embryonic
• Which type of gonad is present during the early indifferent stage? → the indifferent glands
• Which blood vessel lead blood to and from the metanephros? → Lead to renal artery and lead from vena renalis
• Which structures originate from the metanephric blastema? → nephrons and parts of the collecting duct