Embryology

Question 1

Which system forms a template for reproductive system?

Answer 1

The urinary system

Question 2

When does the genital ridge form?

Answer 2

Week 5-6

Question 3

What structure thickens to become the genital/gonadal ridge?

Answer 3

Medial part of the mesonephric ridge

Question 4

What has to happen in order to produce Indifferent Gonads (not differentiated into a definitive testis or ovary yet) and in time Gonads?

Answer 4

Germ cells migrate from the yolk sac, via dorsal mesentery and embed into primitive sex cords (these are formed from proliferation of the epithelium in each Genital ridge)

Question 5

How does the para-mesonephric duct form?

Answer 5

• Epithelium of the mesonephros invaginates to form a tube.
• This tube becomes the para-mesonephric duct

Question 6

Where does the cranial end of the paramesonephric duct open into?

Answer 6

The peritoneal cavity

Question 7

True or false: The paramesonephric duct crosses ventral (anteriorly) to the mesonephric duct

Answer 7

True

Question 8

Where does the paramesonephric duct project into at it’s caudal end?

Answer 8

It projects into the dorsal wall of the urogenital sinus

Question 9

What do the paramesonephric ducts devlop into in the female?

Answer 9

The genital tract:

• Fallopian tubes
• Uterus
• Cervix
• The upper one-third of the vagina.

Question 10

What happens to the paramesonephric duct in males?

Answer 10

It degenerates

Question 11

What does the mesonephric duct develop into in males?

Answer 11

• Epididymis
• Vas deferens
• Seminal vesicle

Question 12

In males, what structures does the urogenital sinus develop into? (3)

Answer 12

• Bladder
• Urethra
• Prostate - outgrowth from the urethra

Question 13

In females, what structures does the urogenital sinus develop into? (2)

Answer 13

• Bladder
• Lower part of vagina

Question 14

What happens to the mesonephric duct in females?

Answer 14

It degenerates

Question 15

How does the indifferent gonad (un-differentiated) know whether to develop into a testis?

Answer 15

Sex-determining Region Y (SRY) is a gene found on Y chromosomes that leads to the development of male phenotypes, such as testes.

• The SRY encodes for a protein called TDF (testis determining factor)
• Primordial germ cells (within the sex cords of the indifferent gonad) express the SRY
• TDF makes the indifferent gonad develop into a testis
• The SRY transforms the primordial cells into Sertoli cells which secrete MIS (Mullerian-inhibiting-substance)
• Paramesonephric duct degenerates
• MIS transforms mesenchymal cells between the sex cords to form Leydig cells
• Leydig cells produce testosterone - male external genitalia

Question 16

Testis development

Answer 16

Question 17

The male external genitalia are formed under the influence of what hormone?

Answer 17

Testosterone

Question 18

How does an indifferent gonad develop into an ovary?

Answer 18

• No SRY region
• No TDF (which results in development of testis)

So, by default the indifferent gonad develops into an ovary

Question 19

Postnatal ovarian development

Answer 19

• There are approx 2 million primordial follicles at birth
• Many follicles become atretic before puberty (Degenereated)
• After puberty, hormonal influences stimulate 12-15 follicles, per 28 day cycle, to develop
• Only one follicle (normally) reaches maturity and ovulation
• Continues until the menopause

Question 20

Absence of testosterone causes what to happen in females?

Answer 20

Degeneration of mesonephros and mesonephric duct

Question 21

During development how do the ovaries descend?

Answer 21

• The ovaries originate on the posterior abdominal wall (T10) and move down to within the pelvis.
• The movement is arrested by the gubernaculum (formed in development and connects the ovary to the labum majora)
• Later in pregnancy the gubernaculum becomes the round ligament

Question 22

Descent of the testis

Answer 22

• Week 7 – Level of T10 (Diaphragm undescended)
• Week 12 – Level of deep inguinal ring (remains until 7th month)
• Enters scrotum just prior to birth (39 weeks)
• The gubernaculum guides the testes through the inguinal canal into the scrotum.

Question 23

What is Gartner’s cysts?

Answer 23

• Most common benign cystic lesions of the vagina.
• They represent embryologic remnants of the caudal end of the mesonephric duct (should disintegrate in females)

Question 24

What is Cryptorchidism?

Answer 24

Parents notice absence of testes in scrotum (cryptorchidism). Reasons for this could be:

• Undescended
• Ectopic
• Retractile testes
• Absent testes

Managed by Orchiopexy: Surgery to move an undescended (cryptorchid) testicle into the scrotum and permanently fix it there. Orchiopexy typically also describes the surgery used to resolve testicular torsion.

Question 25

What are some complications associated with undescended testis?

Answer 25

* Infertility * Malignant transformation (germ cell tumours) * Testicular torsion - when tissues around the testis are not attached well causing the testes to twist around the spermatic chord. This cuts off the blood flow to the testicle.

Question 26

What is Orchiopexy?

Question 27

What are some anomalies of the paramesonephric duct that can affect females?

Answer 27

* Complete duplication of the uterus * Bicornuate uterus * Septate uterus (septum in the middle of the uterine cavity) * Arcuate uterus - the uterine anomaly that is least commonly associated with reproductive failure.