Development of the Reproductive Tract

Question 1

Which mesoderm does the urogenital tract originate from?

Answer 1

Intermediate mesoderm.

Question 2

Name the 3 sets of kidney structures and when they appear/regress.

Answer 2

1. Pronephros: Regresses by week 4.
2. Mesonephros
3. Metanephros: Appears in week 5, is functional by week 11.

NB: form in a cranial to caudal sequence.

Question 3

Describe the pronephros?

Answer 3

In cervical region.
7-10 solid cell groups, rudimentry and non-functional.
Regress by week 4.

Question 4

Describe the Mesonephros?

Answer 4

Derived from thoracic and upper lumbar segments.
After regression of the pronephros, the first excretory tubules of the mesonephros appear.
Also contributes supporting cells to the genital ridge.

Question 5

Describe the metanephros?

Answer 5

The definative kidney.
Appears in week 5, functional by week 11.
Excretory units form from metanephric mesoderm.
Has 2 parts;
- Ureteric bud: Outgrowth of mesonephric duct - source of all the duct system.
- Metanephric cap: From metanephric mesenchyme. Surrounds the bud and together these form the kidney. Provides the excretory units.

Question 6

How is the anterior urogenital sinus and anal canal formed?

Answer 6

The cloaca is hindgut to begin with (endoderm).
Urorectal septum divides cloaca by fusion with cloacal membrane to form the anterior urogenital sinus and posterior anal canal. This happens in week 4-7.

Question 7

How is the bladder formed?

Answer 7

The bladder forms from the urogenital sinus and caudal mesonephric duct.

Question 8

What forms the lining of the bladder?

Answer 8

Endoderm from cloaca.

Question 9

When is genetic sex determined?

Answer 9

Fertilisation.

Question 10

When do gonads acquire male/female features?

Answer 10

Week 7.

Question 11

Describe the indifferent stage of genital duct development.

Answer 11

2 pairs of ducts develop in weeks 5-6.
The Paramesonephric duct (Mullerian).
The Mesonephric duct (Wolffian).

Question 12

What is the role of the Mesonephric/Wolffian ducts in males/females?

Answer 12

Drains urin from the mesonephric kidney.
In males, under the influence of testosterone, it forms the ductus deferens and ejaculatory duct when mesonephros vanishes.

In females it completely disappears except a few non-functional remnants.

Question 13

What is the role of the Paramesonephric/Mullerian ducts?

Answer 13

Form funnel shaped cranial ends which open to the peritoneal cavity. They migrate caudally until they reach the pelvic region.
They approach each other in the midline and the cranial part forms the uterine tubes and the caudal part fuses to form the uterovaginal primordium (uterus and superior vagina).

In the male they degenerate due to the action of anti-mullerian hormone made by Sertoli cells of the testes.

Question 14

When do the gonads initially appear? As what structure?

Answer 14

A pair of longitudinal ridges called the urogenital ridges in week 5.

Question 15

Which is more lateral, paramesonephric or mesonephric ducts?

Answer 15

paramesonephric.

Question 16

Describe indifferent gonad development?

Answer 16

Urogenital ridges are a mesodermal structure projecting into the coelomic cavity.
Primordial germ cells originate in the yolk sac and move to the genital ridge via dorsal mesentery to form the primitive gonad.
Primordial germ cells form a cord called the primitive sex cords.

Question 17

Describe how the gonads differentiate to be male.

Answer 17

If the embryo is male, the Y chromosome encodes testes determining factor SRY which acts on all somatic cells.
SRY causes proliferation of the sex cords. They become horse-shoe shaped and break into tubules.
Leydig cells begin producing testosterone and Sertoli cells produce AMH.
Testes cords are solid until puberty when they;
- Acquire a lumen forming the seminiferous tubule.
- Join with rete testes and efferent tubules.
- rete testes and mesonephric duct link to form the ductus deferens.

Question 18

How do gonads develops from the indifferent stage in females?

Answer 18

Wnt4 is the ‘ovary determining gene’.
PGC’s reach genital ridge by week 6 and somatic cells grow from coelomic epithelium.
PGC’s divide by mitosis to give a pool of oogonia. These enter meiotic arrest at month 4 and are now oocytes.
Oocytes become associated with follicular cells - primordial follicles.

Question 19

Describe how external genitalia are formed in males and females?

Answer 19

A pair of cloacal fold develop around the cloacal membrane.
They join to form the genital tubercle cranially.
Caudally they are subdivided into;
- Urethral folds anteriorly: Form labia minora in female, and fuse to form the urethral raphe.
- Anal folds posteriorly.
- Genital swellings appear on either side of the urethral folds to form the scrotal swellings in the male, and labia majora in the female.
- Genital tubercle becomes phallus in males.

Question 20

Where do the prostate and bulbourethral glands form from?

Answer 20

Prostate as outgrowth from the prostatic urethra.

Bulbourethral as outgrowths from the penile urethra.