The urogenital system includes two entirely different functional units, the urinary system and genital system.
However, embryologically and anatomically they are closely related. These develop from:-
- Intermediate mesoderm: Which is divided into two parts:-
- Nephrostomes: Takes part in the formation of non-functioning kidneys.
- Urogenital ridge: Caudal to segmental nephrostomes the intermediate mesoderm forms urogenital ridge, which is divided into;
- Nephrogenic cord (metanephric blastema or metanephric mesoderm): Takes part in formation of excretory part of kidney.
- Gonadal ridge: Takes part in development of genital system.
- Cloaca (endodermal cloaca): It is the dilated post-allantoic part of hindgut. It is divided into two parts by urorectal septum:
- Primitive rectum (dorsal part): It forms rectum (lower part) and upper part of anal canal.
- Primitive urogenital sinus (ventral part): It plays important role in development of urogenital sinus.
- Coelomic mesothelium: It proliferates to form genital ridge from which develops male and female gonads.
Kidney and ureterKidneys develop from two sources;
- Metanephros (metanephric mesoderm or blastema): It is the lowest part of nephrogenic cord which is derived from intermediate mesoderm. It forms the excretory unit of kidney i.e. glomerulus, proximal convoluted tubule, loop of henle and distal convoluted tubule.
- Ureteric bud: It arises from lower part of mesonephric duct. It forms collecting part of kidney (pelvis, major calyces, minor calyces, collecting tubules and ureter.
Urinary bladder and urethraThe urinary bladder (except trigone) is derived from vesicourethral canal, cranial part of urogenital sinus (endodermal). Trigone is derived from absorbed part of Wolffian duct/mesonephric duct (mesodermal).
Development of urethra is as follows:-
- Prostatic urethra and membranous urethra
- Anterior wall of prostatic urethra above the opening of prostatic utricle - from vesicourethral canal (endoderm).
- Anterior wall of prostatic urethra below the opening of prostatic utricle and whole of the membranous urethra- from definite urogenital sinus (endoderm)
- Posterior wall of prostatic urethra - from mesonephric/Wolffian duct (mesodermal).
- Penile urethra
- Upto the glans - from phallic part of urogenital sinus (endoderm).
- In the glans - from ectodermal urethral groove (ectoderm).
- Allantois is a vestigial structure and initially bladder is continuous with it. Most of the allantois soon constricts to form a thick fibrous cord called the urachus, which extends from the apex of bladder to the umbilicus. In the adults, the median umbilical ligament represents the urachus. Incomplete closure of urachus results in urachal cyst, sinus or fistula:
- Urachal cyst: Middle part of allantois remains patent (proximal and distal parts are obliterated).
- Urachal sinus: Patent inferior end of urachus dilate to form urachal sinus.
- Urachal fistula - Entire urachus remains patent.
Gonads and genital duct system
GonadsGonadogenesis refers formation of gonads, i.e. testes in males and ovaries in females. Genital ridge (urogenital ridge) is the site where gonads develop. 'Primordial germ cells' are developed in the 4th week by proliferation of endodermal cells of the dorsal wall of hindgut (part of yolk sac. The primordium germ cells migrate into genital ridge, where proliferation of both germinal and non-germinal cells leads to formation of gonads.
Genital duct system
- During 5th and 6th weeks, both male and females have two genital duct systems, derived from mesoderm:
- Mesonephric duct (Wolffian duct) and mesonephric tubules.
- Paramesonephric duct (Mullerian duct).
- Mesonephric duct is the main genital duct in males as it gives rise to mainly male genital system:
|MESONEPHRIC DUCT (WOLFFIAN DUCT) : Main Genital duct in males|
|In males||In females (mostly degenerate except)|
|Posterior wall of prostatic urethra proximal to opening of ejaculatory duct||Posterior wall of urethra|
|Ureteric bud gives rise to ureter, pelvis, calyces and collecting tubules||Ureteric bud gives rise to ureter, pelvis, calyces and collecting tubules|
|Trigone of bladder||Trigone of bladder|
|Vas deferens, ejaculatory duct||Duct of epophoron (Gartner's duct)|
|Epididymis, seminal vesicle|
|Appendix of epididymis|
|Mesodermal part of prostate|
|Ductus aberrant superior||Epoophoron|
|Efferent ductules of testis||Paraphoron|
|Ductus aberrant inferior|
- Epoophoron is a remnant of mesonephric tubules, while duct of epoophoron is derived from mesonephric duct.
- Paramesonephric duct is the main genital duct in females as it gives rise to mainly female genital system.
|PARAMESONEPHRIC DUCT (MULLERIAN DUCT): Main Genital duct in females|
|Fallopian tubes||Appendix of testis (Hydatid of Morgagni)|
|Upper 2/3 of vagina|
- Lower 1/3 of vagina is derived from sinovaginal bulb derived from urogenital sinus. Therefore is endodermal in origin (upper 2/3 is mesodermal in origin as derived from Paramesonephric duct).
|Embryogical structure||Fate in Female||Fate in Male|
|Genital swelling||Labia majora||Scrotum|
|Genital fold||Labia minora||Ventral aspect of penis, penile urethra|
|Genital tubercle||Clitoris||Glans penis|
Development of diaphragm
Diaphragm develops from 4 structures derived from mesoderm which unite to form a sheet.
- Septum transversum: Gives origin to sternal part and central tendinous region.
- Pleuroperitoneal membrane: Dorsal paired portion.
- Dorsal mesentery of oesophagus: Gives origin to crural part (Dorsal unpaired portion).
- Lateral body walls: Gives origin to peripheral costal portion of diaphragm (Cervical Myotome).