Development of Genito-Urinary System

Development of Genital System 

The Intermediate mesoderm is the Germ layer that contributes to the Genito-urinary system. External genitalia are fully differentiated at approximately 12 weeks. Female external genital differentiation is complete by 11 weeks. Male external genital differentiation is completed by 14 weeks.

Male development begins earlier than female development, i.e., the testes develop earlier than the ovary. However, female external genital development reaches terminal differentiation before male. The Intermediate mesoderm gives rise to the genital ridge, which forms at week 5. The genital ridge can develop into either the testes or the ovary. 

SRY gene is responsible for the sex of the baby. It has the sex region Y and is Found on the short arm of the Y chromosome. It Codes for protein testis-determining factor (TDF). TDF determines whether a testis is formed or not. Males have a Y chromosome and an SRY gene. Testis development starts at week 7, and Ovary development starts from week 8 onwards. Sex determination is not possible before week 6. The embryo is recognized as an indeterminate embryo at week 6. Sex determination can be done by 7 weeks, and well differentiation begins from week 11.

Ultrasonographic sex determination is done by looking at the external genitalia. This is Possible after week 11.

The trigone of the urinary bladder develops from the Mesoderm. Genital and renal development occur simultaneously. The urinary bladder develops from the endoderm of the urogenital sinus. The epiblast forms the primordial germ cells and primitive streak. By the end of week 2, the primordial germ cell is isolated from the primitive streak. Germ cells migrate to the endodermal wall of the yolk sac. These germ cells later migrate to the urogenital ridge. The urogenital ridge has an indifferent gonad until week 6. Mesonephric or Wolffian duct forms. Paramesonephric or Mullerian duct forms. Paramesonephric duct lies to the side of the mesonephric duct.

Factors for Sex Differentiation 

• Males have factors that prevent the growth of ovaries.
• In males, the indifferent gonad is exposed to the SRY gene that encodes the testis-determining factor (TDF).
• Leydig cells produce testosterone.
• Sertoli cells produce an inhibin known as the Mullerian inhibiting factor (MIF).
• The presence of these factors promotes testis formation.
• The absence of the factors promotes ovary formation.

Transverse Section of Embryo Showing Notochord

The notochord is also known as the axial mesoderm. The paraxial mesoderm lies lateral to it, and the lateral plate mesoderm lies lateral to the paraxial mesoderm.

Intermediate Mesoderm

The Intermediate mesoderm contributes to the major portion of the Genito-urinary system. It Forms ridges and ducts. Genito-urinary ridges from the following:

• Testis
• Ovary
• Kidney

There are two types of ducts: Mullerian and Wolffian. Females have a functional Mullerian duct only. Males have a Mullerian inhibiting substance (MIS). Males have a functional Wolffian duct only. External genitalia develop from the dorsal somatic lateral plate mesoderm.

Development of Renal System

Three systems develop in the urogenital ridge:

• Pronephros: The Pronephros will degenerate
• Mesonephros: The mesonephros is degenerated, leaving behind a duct known as the mesonephric duct. The right and left mesonephric ducts give rise to the ureteric bud.
• Metanephros: Metanephros of the urogenital ridge develops into the kidney. The ureteric bud stimulates metanephros to form the kidney. The Urogenital sinus forms the:
  • Bladder
  • Urethra
  • Lower vagina
• The definitive kidney becomes functional by the 12th week.
• The kidney begins excreting urine by the 12th week.

Development of the Female Genito-Urinary System

Mullerian/Paramesonephric ducts fuse in the midline to form the uterus. The unfused part becomes the uterine tube. Mullerian duct develops into the:

• Uterus
• Uterine tube
• Upper one-third of the vagina: Upper one-third of the vagina develops from the fused part of the mullerian duct.

Lower two-thirds of the vagina develops from the endodermal derivative of the urogenital sinus. Endoderm of urogenital sinus develops into the:

• Urinary bladder
• Urethra
• Lower two-thirds of the vagina.

The Mullerian duct is mesodermal. The Urogenital sinus is endodermal. Mesonephros leaves behind two mesonephric ducts that open into the posterior wall of the urogenital sinus. The Urogenital sinus develops in the cloaca region. Mesonephric duct gives rise to the ureteric bud that penetrates the metanephros. As mentioned earlier, Metanephros forms the kidney.

Renal agenesis is a congenital abnormality in which Kidney is not formed as the ureteric bud does not penetrate the metanephros and Ureteric bud forms the ureter. Therefore, the mesonephric duct forms the ureter. Trigone of urinary bladder derived from the mesoderm. Tip of the mesonephric duct absorbed into the posterior wall of the urogenital sinus, forming the trigone of the urinary bladder. After forming the trigone, the mullerian duct can be referred to as the wolffian duct. Wolffian duct is found in males only. In females, this duct regresses and leaves vestigial remnants:

• Epoophoron – found above the ovary.
• Paroophoron – found around the ovary.
• Gartner’s duct.

Development of Trigone

The Tip of mesonephric duct absorbed into the posterior wall of the urogenital sinus to form the trigone of the urinary bladder. Trigone is mesodermal in origin. Urinary bladder is endodermal.

After forming the trigone, the proximal part of the mesonephric duct is referred to as the wolffian duct. The Wolffian duct is vestigial in females. Wolffian duct forms the reproductive tubes in males known as the Ductus deferens

In females, the ductus deferens is vestigial remnant and  known as Gartner’s duct. Endoderm lies anteriorly while the intermediate mesoderm lies posteriorly. Endoderm forms the gut tube, while intermediate mesoderm forms the kidney system. Gut tube has three parts:

• Foregut
• Midgut
• Hindgut

Hindgut

Hindgut has a cloaca region. Uro-rectal septum develops in the cloacal region. Urogenital sinus lies in front of the septum. Rectum develops behind the septum. There are 3 derivatives of the urogenital sinus:

• Urinary bladder
• Urethra
• Lower two-thirds of the vagina.

These structures and their epithelium are endodermal in origin.

Structures that are Mesodermal in origin:

• Trigone of urinary bladder.
• Posterior wall of the proximal urethra.

Intermediate Mesoderm 

Intermediate mesoderm is Divided into:

• Pronephros
• Mesonephros
• Metanephros: Metanephros forms the true kidney.

Pronephros and mesonephros degenerate. Mesonephros leaves behind the right and left mesonephric ducts. Mesonephric ducts open into the posterior wall of the urogenital sinus. These ducts give rise to the ureteric bud. Ureteric bud penetrates the metanephros, stimulating kidney development. Renal agenesis occurs if no penetration occurs. After penetration, the metanephros forms the metanephric blastema. Metanephric blastema forms the excretory portion of the kidney. Ureteric bud has 5 derivatives that make up the kidney’s collecting system. The kidney has two parts: the excretory system (nephrons) and the collecting system (ureter system). The Kidneys are derived from the metanephros. The ureter is derived from the mesonephric duct. The bladder is derived from the endoderm of the urogenital sinus. The posterior wall of the urinary bladder and urethra derived from the tip of the mesonephric duct (mesoderm). The Proximal part of mesonephric duct known as wolffian duct after forming the trigone. Wolffian duct develops into the male reproductive system. This duct is vestigial in females.

Development of Nephron

The Ureteric bud stimulates the metanephros to form the excretory portion of the kidney. The Excretory portion constitutes the nephron.the Parts of the nephron are:

• Glomerulus
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule

Urine is collected in the collecting duct. Collecting duct derived from the ureteric bud. The 5 derivatives of the ureteric bud:

• Ureter
• Renal pelvis
• Major calyx
• Minor calyx
• Collecting ducts

These structures form the collecting system.

Anatomy of the Nephron

The renal pyramid lies within the medulla, and the cortex lies at the periphery. Part of the cortex extends into the medulla to form the renal columns of Bertini. The Renal medulla contains the nephron system, which projects into the papilla, which projects into the minor calyx.

Nephron Embryology

The nephron system is referred to as the true kidney. The SNephron system develops from a metanephric blastema. The 5 derivatives of the ureteric bud collect urine formed in the nephron. The collecting tube is part of the nephron and develops from the metanephric blastema.

Nephron Structure

The glomerulus is a capillary plexus derived from the metanephric blastema. Glomerulus projects into the Bowman’s capsule, providing a filter of blood called urine. Urine follows this order in the nephron:

Glomerulus → Proximal convoluted tubule → Loop of Henle → Distal convoluted tubule → Collecting system.

The collecting system's components include the Collecting duct, Minor calyx, Major calyx, Renal pelvis, and Ureter. These structures are derived from the ureteric bud.

Mullerian and Wolffian Derivatives

At week 6, the gonadal ridge forms the testis or ovary in the Indeterminate embryo. At that time, both ducts are Present. The paramesonephric duct lies lateral to the mesonephric duct and is the precursor to the Mullerian duct.

The right and left mullerian ducts fuse in the midline, forming an opening in the posterior wall of the urogenital sinus. The right and left mesonephric ducts open separately into the urogenital sinus. Three openings in the urogenital sinus are found in the cloaca region. The Testis development starts at week 7. The Ovary development starts at week 8.

Ovary Development 

Ovaries are Derived from the genital ridge. Paramesonephric ducts fuse in the midline to form the uterus and upper one-third of the vagina. The unfused part of these ducts forms the uterine tube. The Endoderm of urogenital sinus gives rise to the lower two-thirds of the vagina, urinary bladder and urethra. Mesonephric duct forms the kidney system and degenerates in females.

Mesonephric duct forms 3 vestigial remnants: epoophoron, paroophoron and Gartner’s duct.

To read more about the testis development, log on to the Preppladder app and watch the anatomy videos made by our esteemed faculty. 

Mullerian Agenesis 

The Mullerian duct forms the uterus, uterine tube, and upper one-third of the vagina. The Upper one-third of the vagina is mesodermal in origin, and the Lower two-thirds of the vagina is endodermal in origin – urogenital sinus.

The endodermal vaginal epithelium of the lower vagina encroaches on the upper third. Therefore, the entire vagina has endodermal epithelium. Mullerian agenesis involves the absence of the uterus, uterine tube, and upper one-third of the vagina. A shallow vagina and ovaries are present in Mullerian agenesis. Ovaries are derived from the genital ridge. A shallow vagina is the lower two-thirds of the vagina. Cases of Mullerian agenesis are referred to as Mayer-Rokitansky-Kuster-Hauser syndrome.

Multiple variations of this syndrome exist. None of the three derivatives are present. A Small, functional uterus is present.

Wolffian Duct: Vestigial Remnants 

• Oophoron/Epiophoron above the ovary.
• Paraophoron around the ovary.
• Gartner’s duct near the vagina
• Ovary is also known as oophoron.
• Wolffian duct regresses in females after the formation of the trigone by the mesonephric duct.
• Gartner’s duct found on the anterior lateral wall of the vagina and broad ligament.
• Gartner’s duct is the vestigial remnant ductus deferens found in females.
• Uterus covered by double-folded peritoneum forming the broad ligament.

Wolffian Duct Derivatives

Testis develops from the genital ridge. The Mesonephric ducts in males give rise to mesonephric tubules. Mesonephric tubules form the collecting system for the semen/sperm. The mesonephric duct is also known as the wolffian duct. The Derivative of the mesonephric tubules is Efferent ductules.

Derivatives of the mesonephric duct:

• Epididymis
• Ductus deferens
• Seminal vesicle
• Common ejaculatory duct

The Urogenital sinus in males forms the:

• Urinary bladder
• The inner part of the prostate gland
• Urethra

The outer prostate is mesodermal as a derivative of the mesonephric duct. The Prostate gland has two parts:

• Deeper endodermal prostate near the urethra, i.e. the ductular/glandular prostate, derived from the urogenital sinus.
• Peripheral mesodermal prostate; derived from the wolffian duct.

Mullerian duct is inhibited in males by the mullerian inhibiting substance. The Vestigial structures of mullerian duct in males: Small uterus known as the prostate utricle and Appendix of the testis.

Mesonephric duct is concerned with kidney and trigone formation. After this, it is referred to as the wolffian duct and forms the male reproductive system.

External Genitalia

Develop from the dorsal somatic portion of lateral plate mesoderm. The External genitalia includes:

• Labia majora.
• Labia minora.
• Clitoris.
• Scrotum.
• Glans penis.

The derivatives of the genital tubercle are Clitoris in female and Glans penis in male

Derivatives of genital/urethral fold are Labia minora in females and Penile-scrotal raphe/Penoscrotal raphe in males.

The Derivatives of genital swelling are Labia majora in female and Scrotum in male.

Frequently Asked Questions

Q. WRONG about the genital system development is:

1. Develop from mesoderm.
2. Genital ridge forms at week 5.
3. Testes develop earlier than the ovary.
4. External genitalia are fully differentiated at week 10

Answer: External genitalia are fully differentiated at week 10

Q: Why is ovary development delayed?

Answer: All individuals should become female by a default mechanism. The development of female features is prevented by the SRY gene

Q: Trigone of urinary bladder develops from-

1. Mesoderm
2. Ectoderm
3. Endoderm of urachus
4. Endoderm of urogenital sinus

Answer: Mesoderm

Q: All are derivatives of ureteric bud EXCEPT:

1. Ureter
2. Renal pelvis
3. Minor calyx
4. Collecting tubule

Answer: Collecting duct

Q. Development of labia majora is from:

1. Urogenital sinus
2. Urethral fold
3. Genital ridge
4. Genital swelling

Answer: Genital Swelling

Q. Ejaculatory duct develops from-

1. Ureteric bud
2. Mesonephric duct
3. Paramesonephric duct
4. Wolffian duct

Answer: Wolffian duct

Sign up to our PrepLadder app today to learn more about this. Access Video Lectures, digital notes, QBank, and Mock Tests for FREE to ace your NEET PG preparation. Elevate your study experience and gear up for success. Start your journey with PrepLadder today!