Development of the Reproductive System

Question 1

Which system develops closely with the reproductive system?

Answer 1

The renal system develops closely with the reproductive system.

Question 2

List the components of the early mesoderm.

Answer 2

1 - The paraxial mesoderm is the section of mesoderm closest to the axis of the embryo, surrounding the neural tube.

2 - The lateral plate mesoderm consists of two plates of outward-stretching mesoderm located at the most lateral sides of the embryo.

3 - The intermediate mesoderm is the section of mesoderm between the paraxial and lateral mesoderm.

Question 3

What are the components of the lateral plate mesoderm?

Answer 3

The lateral plate mesoderm is divided into an outer somatic mesoderm and an inner splanchnic mesoderm.

Question 4

From which embryological tissue does the reproductive system develop?

From which embryological tissue does the renal system develop?

Answer 4

• The reproductive system is derived from the intermediate mesoderm.
• The renal system is derived from the same tissue.

Question 5

List the sequences of renal development in the embryo.

When does each sequence develop?

Where is each sequence located during development?

Answer 5

1 - Pronephros.

• The pronephros arises at week 3 of gestation.
• The pronephros is located in the cervical region.

2 - Mesonephros.

• The pronephros degrades and gives rise to the mesonephros at week 4.
• The mesonephros is located in the thoracic and lumbar regions.

3 - Metanephros.

• The mesonephros degrades and gives rise to the metanephros at week 5, which is the permanent renal system.

Question 6

Describe the structure of the mesonephros.

Answer 6

• The mesonephros contains many mesonephric tubules.
• The mesonephric tubules all drain into one mesonephric duct.
• The mesonephric duct lies adjacent to the paramesonephric duct (important in female gonadal development - explained later).
• The mesonephros is contained within a mesenchymal tissue known as the nephrogenic cord.

Question 7

List the components of the metanephros.

Answer 7

The metanephros is composed of:

1 - Ureteric bud.

2 - Metanephric blastema.

Question 8

What does the ureteric bud give rise to?

Answer 8

The ureteric bud gives rise to:

1 - Collecting ducts.

2 - Minor and major calyces.

3 - Renal pelvis.

4 - Ureters.

Question 9

How does the metanephric blastema form?

What does the metanephric blastema give rise to?

Answer 9

• The metanephric blastema is formed from the lower 1/3 of the nephrogenic cord wrapping around the ureteric bud.
• The metanephric blastema gives rise to the nephrons of the kidney.

Question 10

How do the calyces of the kidney form?

Answer 10

The calyces of the kidney form when the ureteric bud bifurcates into branches, forming numerous cavities in the metanephric blastema.

Question 11

What happens to the remnants of the mesonephros?

Answer 11

The remnants of the mesonephros give rise to the genital ridge, which is the precursor to the gonads and genitals.

Question 12

What is the cloaca?

Answer 12

The cloaca is the common chamber into which the intestinal, urinary (metanephros), and genital tracts (remnants of the mesonephros and paramesonephros) open.

Question 13

What is the urogenital sinus?

Answer 13

• The urogenital sinus is the ventral part of the cloaca, formed after the cloaca separates from the anal canal.
• It is the region of the future bladder and lower genital tracts (urethra in males, urethra and lower vagina in females).

Question 14

How do the metanephros and remnants of the mesonephros move during development?

Answer 14

• The metanephros moves up from the pelvic region to the lumbar region.
• The remnants of the mesonephros move down from the lumbar region to the pelvic region.

Question 15

Until which point in gestation is the male reproductive system the same as the female reproductive system?

Answer 15

The male reproductive system is the same as the female reproductive system until week 7 of gestation.

Question 16

Describe the genes involved in the regulation of gonadal development.

Answer 16

Males:

• The SRY gene is a gene found on the Y chromosome. It is also known as the testis-determining factor.
• Expression of SRY upregulates the expression of the SOX9 gene.
• The SOX9 gene upregulates the expression of the SF1 gene, among others.
• These genes will contribute to the formation of the testis by stimulating the differentiation of bipotential gonads to male phenotypes and inhibiting the differentiation to female phenotypes

Females:

The WNT4 gene is a gene found on chromosome 1.

• The WNT4 upregulates the expression of the DAX1 gene, among others.
• These genes will contribute to the formation of the ovaries by stimulating the differentiation of bipotential gonads to female phenotypes and inhibiting the differentiation to male phenotypes.

Interactions between male and female genes:

• SOX9 inhibits WNT4.
• DAX1 inhibits SOX9.
• This is to ensure that only one pathway is expressed.

Question 17

List the components of the genital ridge.

Answer 17

The genital ridge is composed of:

1 - Coelomic epithelium.

2 - Condensed mesenchyme.

Question 18

Which other tissue that is involved in the development of the gonads is not derived from the genital ridge?

Where do they come from?

Answer 18

Gonocytes, which are derived from the yolk sac

Question 19

How do sex cord cells form?

What is their function?

Answer 19

• Sex cord cells form when coelomic epithelium penetrate the mesenchyme of the genital ridge.
• The surround gonocytes at the genital ridge.
• In females, sex cords form the ovarian follicles.
• In males, sex cords form testis cords, which help to develop and nourish Sertoli cells.

Question 20

What is the tunica albuginea?

Answer 20

The tunica albuginea is the structure formed from the outer genital ridge, which becomes fibrous once the sex cords (ovarian follicles or testis cords) form.

• In males, it forms the outer layer of the testis.
• In females, it forms the outer layer of the ovaries.

Question 21

Describe the development of the testis.

Answer 21

Structural changes:

• The coelomic epithelium penetrate the genital ridge. As they penetrate deeper into the testis, they form the testis cords. These are solid cords that have not yet undergone lumenisation (this occurs at puberty).
• The testis cords arrange into numerous horseshoe-shaped arrangements within the tunica albuginea.
• The testis cords become more convoluted, and are now known as convoluted tubules.
• A network of tubules known as the rete testis form from the convoluted tubules, bridging the seminiferous tubules and the remnants of the mesonephric tubules. Their future function will be to carry sperm from the convoluted tubules to the efferent ducts.
• The mesonephric tubules are now known as the epididymis, and the mesonephric duct is now known as the vas deferens. This continues caudally as the urethra.
• The tunica albuginea sends fibrous septae through the testis, separating it into lobules.

Introduction of functional cells:

• Gonocytes migrate from the yolk sac to the genital ridge.
• The gonocytes migrate to the basement membrane of the seminiferous tubules, and differentiate into spermatogonial stem cells.
• Sertoli cells, which are derived from differentiated mesoderm, surround the spermatogonial stem cells.
• Sertoli cells secrete Mullerian inhibiting factor, causing the paramesonephric duct to degenerate.
• Partial degeneration of the paramesonephric duct can result in the formation of a prostatic utricle.
• Leydig cells form between the arrangements of spermatogonial stem cells and surrounding sertoli cells. They secrete testosterone which assists in the differentiation of the mesonephric duct.

Question 22

Describe the development of the ovaries.

Answer 22

Structural changes:

• The coelomic epithelium penetrate the genital ridge. As they penetrate deeper into the ovary, they form the medullary cords, which degenerate in females.
• Since there is no communication with the mesonephric duct in the females, it partially degenerates, leaving behind the paroophoron.
• However, since there is no Mullerian inhibiting factor (as there are no Sertoli cells), the paramesonephric duct does not degenerate, and opens at the cranial end. In the future, this will become the fallopian tube.
• The paramesonephric ducts from either side fuse at the centre of the embryo, and the wall separating them degenerates. In the future, this will become the uterus.

Introduction of functional cells:

• Gonocytes migrate from the yolk sac to the genital ridge. In the future, the gonocytes will form the oocytes.
• The coelomic epithelium penetrate the genital ridge again, proliferate and differentiate to form cortical cords (so called as they remain close to the cortex). In the future, the cortical cords will form the follicular cells.
• Theca cells are derived from both the mesonephros and from mesenchyme in the ovaries.
• Granulosa cells are derived from ovarian surface epithelium.

Question 23

Describe the formation of the bladder and ducts of the reproductive system in males.

Answer 23

• The mesonephric ducts from the testes on both sides of the embryo move medially and caudally as the vas deferens.
• They end at the inferior edge of the future bladder, forming the lower edge of the trigone. Here, they merge to form prostatic part of the urethra.
• Meanwhile, the ureteric buds continue medially as the two ureters.
• They end at the superolateral ends of the future bladder, forming the other two superior edges of the trigone.
• The bladder itself is formed from the (metanephric) ureteric bud.
• The prostate is derived from the endoderm.

Question 24

Describe the formation of the ducts of the reproductive system in females.

Answer 24

• The uterine canal forms from the fusing and continuation of the paramesonephric ducts.
• At the end of this structure, the sinovaginal bulb forms. This is a solid cord of cells which attaches to the urogenital sinus. It later undergoes lumenisation to form the cervix, fornix and vagina.
• The urethra wasn’t covered for females in the lecture, so presumably its the same for females as it is for males (the ureteric buds move medially as the ureters and continue as the bladder but there are no vas deferens merging at the inferior edge of the trigone).

Question 25

Which hormone stimulates the growth of the penis, scrotum and prostate?

Answer 25

Testosterone stimulates the growth of the penis, scrotum and prostate.

Question 26

Describe the indifferent stage of development of the external genitalia (the stage that is common between males and females).

Until which stage in gestation is the male genitalia able to be told apart from the female genitalia?

Answer 26

• At the surface of the perineal surface of the embryo, the mesoderm surrounding the cloaca differentiates to form two cloacal folds.
• The two cloacal folds meet at the superior end to form the genital tubercle. The folds now encircle a gap in the perineum.
• A septum known as the urorectal septum divides the cloacal folds into two - the urogenital fold and the anal fold.
• The urorectal septum disappears, and the urogenital fold separates from the anal fold completely. This leaves a gap separating the urogenital fold from the anal fold, which is known as the perineum.
• Deep to the perineum is the perineal body, formed by the remnants of the urorectal septum.
• Swellings lateral to the urogenital fold appear as a result of further differentiation of the mesoderm surrounding the cloaca. These are known as genital swellings. In the future, these will form the labia majora in females and the scrotum in males.
• From week 14, the male genitalia is able to be told apart from the female genitalia.

Question 27

Describe the development of the male external genitalia from week 14.

Answer 27

• The genital tubercle elongates outwards, pulling the urethra with it.
• The urogenital fold closes completely, forming the shaft of the penis.
• The genital swellings move medially and fuse to form the scrotum.
• A ligament known as the gubernaculum bridges the testis and abdominal wall with the scrotum.
• The gubernaculum shortens, pulling the testis into the scrotum, and along with it, the lower layers of the abdominal wall.
• The skin of the scrotum is formed from a continuation of the abdominal wall:
• The internal oblique forms the cremasteric fascia and the external oblique forms the spermatic fascia.
• The descent of the testis forms the inguinal canal.

Question 28

What is the vaginal tunic?

Answer 28

• The peritoneum descends with the testis, forming a pouch open at the abdominal end. This pouch is known as the processus vaginalis.
• As the processus vaginalis enters the scrotum, it closes and forms a closed pouch in the scrotum known as the vaginal tunic.

Question 29

Describe the development of the female external genitalia from week 14.

Answer 29

• The genital tubercle remains as the clitoris.
• The urogenital fold remains open to form the labia minora.
• The lateral swellings form the labia majora.

Question 30

List 6 congenital defects of the female genital tract.

What are they, and how do they form?

Answer 30

1 - Uterus didelphys with double vagina is the formation of two uteruses and two vaginas.

• It occurs when the paramesonephric ducts fuse, but the wall separating them fails to degenerate in the developing female. The ducts attach to different points on the urogenital sinus, so the two paramesonephric ducts descend separately.

2 - Uterus arcuatus is the formation of a bulging indentation in the uterus.

• It occurs when the wall separating the paramesonephric ducts only partially degenerates.

3 - Uterus bicornis is the presence of two uteruses but only one vagina.

• It occurs when the paramesonephric ducts fuse, but the wall separating them fails to degenerate (creating two uteruses). As the uteruses descend, they both open into the same vagina because the paramesonephric ducts still attach to the same point on the urogenital sinus before opening.

4 - Uterus bicornis unicollis is the formation of one uterus and one vagina derived only from one paramesonephric duct.

• It occurs when the other paramesonephric duct fails to fuse with the other paramesonephric duct in development.

5 - Cervical atresia is complete closure of the cervix.

• It occurs when the sinovaginal bulb fails to lumenise during development, causing the cervix to remain closed.
• The vagina is still lumenised in cervical atresia.

6 - Vaginal atresia is complete closure of the vagina.

• It occurs when the sinovaginal bulb fails to lumenise during development, causing the vagina to remain closed.
• The cervix is still lumenised in vaginal atresia.

Question 31

List 6 congenital defects of the male genital tract.

What are they, and how do they form?

Answer 31

1 - Hypospadias is the formation of orifices in the shaft of the penis.

• It is caused by incomplete closure of the urogenital fold.

2 - Cryptorchidism is failure of one or both testis to descend into the scrotum.

• It is caused by failure of the gubernaculum to shorten.

Question 32

List 4 risks of cryptorchidism.

Answer 32

1 - Increased risk of testicular torsion.

2 - Increased risk of inguinal hernia.

3 - Decreased fertility.

4 - Increased risk of testicular cancer.

Question 33

What are Wolffian ducts?

Answer 33

Wolffian ducts are another term for mesonephric ducts.

Question 34

What are Mullerian ducts?

Answer 34

Mullerian ducts are another term for paramesonephric ducts.