Genitourinary Embryology

EMBRYOLOGY:   GENITOURINARY SYSTEM

• Begin to develop during 4th week of gestation.
• Include the URINARY SYSTEM and GENITAL SYSTEM.

These develop from:

A. Intermediate mesoderm:

1. Nephrostomes: Takes part in the formation of non-functioning kidneys.
2. Urogenital ridge: Caudal to segmental nephrostomes the intermediate mesoderm forms urogenital ridge, 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.

B. Cloaca (endodermal cloaca):

• It is the dilated post-allantoic part of hindgut. It is divided into two parts by urorectal septum:

1. Primitive rectum (dorsal part): It forms rectum (lower part) and upper part of anal canal.
2. Primitive urogenital sinus (ventral part) : lt plays important role in development of urogenital sinus.

C. Coelomic mesothelium:  It proliferates to form genital ridge from which develops male and female gonads.

 O/B of embryonic phases discussed under 5 subdivisions:

1. Nephric system
2. Vesicourethral unit
3. The gonads
4. Genital duct system
5. External genitalia

1. NEPHRIC SYSTEM

• Developed as: Pronephros – Earliest stage: Vestigial structure
  
•  Metanephros -Final phase

KIDNEY develop from:    

• METANEPHROS (Metanephric mesoderm or Blastema) derived from intermediate mesoderm.

Forms :

1. GLOMERULUS 
2. PROXIMAL CONVOLUTED TUBULE 
3. LOOP OF HENLE
4. DISTAL CONVOLUTED TUBULE

• URETERIC BUD (MESONEPHROS) arises from lower part of mesonephric duct.

Forms:  

1. PELVIS
2. MAJOR CALYCES
3. COLLECTING TUBULES
4. URETER 

 2.VESICOURETHRAL UNIT 

• 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 :    

 A. PROSTATIC URETHRA & MEMBRANOUS URETHRA

• Anterior wall of prostatic urethra above the opening of prostatic utricle–> from vesicouretral canal(endoderm).
• Anterior wall of prostatic urethra below the opening of prostatic utricle & whole of membranous urethra → from definite urogenital sinus(endoderm).
• Posterior wall of prostatic urethra → from mesonephric/wolffian duct(mesodermal).

 B. PENILE URETHRA

• Upto the glans → from phallic part of Urogenital sinus (endoderm).
• In the glans →from ectodermal urethral groove (ectoderm).     

 3. GONADS (Testis & Ovaries)

Derived from 3 sources:

1. Mesothelium  (mesoderm epithelium)
2. Underlying mesenchyme   (embryonic connective tissue)
3. Primordial germ cells developed in the 4th week by proliferation of endodermal cells of dorsal wall of hindgut (part of yolk sac). 
4. Primordium germ cells migrate into genital ridge, where proliferation of both germinal & nongerminal cells leads to formation of gonads.

• At the 6th week, the gonad consists of superficial germinal epithelium and internal blastema.
• Differentiation of genital ridge into female or male gonads (ovary & testes) occurs at 6-10 weeks.
• By 12 weeks this differentiation has occured in all fetuses.

Development of testes

• Genital ridge is covered by germinal epithelium (previous coelomic epithelium), which proliferates and forms sex cords (prirnitive seminiferous cords).
• Large number of sertoli cells  are derived from these sex cords.
• Lyedig cells are also derived from sex cord, which in turn is derived from germinal epithelium.
• Spermatocytes (sperms) are derived from primordial germ cells, which in turn develop from endodermal cells of hind-gut (part ofyolk sac).

Development of ovary

• Coelomic epithelium on medial side of the mesonephros becomes thickened to form genital ridge, the site where ovary develops.
• Genital ridge is covered by germinal epithelium, from these germinal epithelium, cords of cells (sex cords or medullary cords) proliferate and grow into the underlying mesoderm.
• Primordial germ cells  migrate to region of developing ovary (genital ridge area) and give rise to oocytes.
• The sex cords become broken up into small masses. The cells of each mass surround one oocyte to form primordial follicles.
• At birth each ovary contains about 2 million primary follicles.

4. GENITAL DUCT SYSTEM

• During 5th & 6th weeks, both male & females have two genital duct systems, derived from mesoderm:
  

1. WOLFFIAN DUCT  – mesonephric duct ; male genital system
2. MULLERIAN DUCT –  paramesonephric duct ; female genital system

5. EXTERNAL GENITALIA