EMBRYOLOGY

Question 1

From what embryonic tissues to the urogenital system arise?

Answer 1

Intermediate mesoderm (located btween lateral plate mesoderm and paraxial mesoderm)

Question 2

What are the three ‘urinary’ structures that develop in the embryo?

Answer 2

Cranially to caudally:

1. Pronephros
2. Mesonephros
3. Metanephros (produces adult kidney)

Question 3

What is the urogenital ridge, and where is it?

Answer 3

It forms on either side of the aorta and bulges into the forming peritoneal cavity. It contains the developing urogenital system. This is where the pronephros, mesonephros and metanephros develop.

Question 4

What is the pronephros?

Answer 4

It is the first, rudimentary kidney (develops in week 4 but almost completely gone by week 6). Doesn’t produce any urine at all.

Question 5

What is the mesonephros?

Answer 5

It is the ‘second’ kidney which forms in week 4 and degenerates by week 6. Develops in thoraco-lumbar region. It produces a small amount of urine. The mesonephric duct [Wolffian duct] delivers urine to the cloaca, and does not degenerate.

Question 6

What is the metanephros? What are the two parts, and what do they become?

Answer 6

Gives rise to the definitive adult kidney. Starts developing in week 6 in the lower lumbar sacral regions. It has two components:

1. Ureteric duct (metanephric diverticulum) which develops as an outgrowth FROM the Wolffian/mesonephric duct. [this becomes the ureter –> collecting ducts through branching morphogenesis]
2. Metanephric blastema from the condensation of nearby mesoderm around the ureteric bud. [This becomes the kidney tissue from Bowman’s capsule –> DCT]

Question 7

What are the kidney functions before birth?

Answer 7

Metanephric kidneys produce urine from 3rd month until birth. Urine is expelled from the bladder into the amniotic cavity fluid. Amniotic fluid is swallowed and inspired.

Question 8

Prior to ascent, which way do the kidneys hilum face?

Answer 8

Anteriorly. During ascent, they rotate medially.

Question 9

How do accessory renal arteries develop?

Answer 9

Superior migration of kidneys causes new arteries to form and old ones to regress; if the old ones do not regress you are left with extras.

Question 10

What are common congenital errors of development with the kidney?

Answer 10

1. Polycystic kidney disease - ureteric buds & metanephric blastema connections, resulting in cysts which progressively enlarge.
2. Agenesis - no kidneys developed (oligohydramnios) or hypoplasia. Unilateral or bilateral.
3. Extra kidney or branched/duplicated ureter.
4. Ectopic kidney - extra production or abnormal migration.
5. Horseshoe/fusion of inferior poles (caught on inferior mesenteric artery or aortic bifurcation so lower than normal)

Question 11

What is the cloaca?

Answer 11

The single exit point for the digestive and urogenital systems during early development.

Question 12

What divides the cloaca, and into what does it divide?

Answer 12

The urorectal septum divides the cloaca during the development so anteriorly/ventrally you have the urogenital sinus, and posteriorly/dorsally you have the rectal & anal canal.

Question 13

What forms the bladder & urethra?

Answer 13

Bladder = upper part of urogenital sinus
Urethra = lower part of urogenital sinus

Question 14

What is the bladder & urethra epithelium derived from? And underlying tissues?

Answer 14

Epithelium = endoderm

Underlying tissues = mesoderm

Question 15

What is the fate of the mesonephric duct (from which the ureteric bud initially developed)?

Answer 15

In the male it becomes the vas deferens (it shifts downwards and enters urethra in prostatic part, this is why you have the ‘bridge over waters’ as the vas deferens passes over the ureters during development of bladder & urethra.

In the female it degenerates.

Question 16

What is the allantois, and what does it become?

Answer 16

The allantois was the outpouching of the yolk sack, which become the ‘connecting stalk’ (umbilical cord). It was continous with the cloaca, then with the urogenital sinus, and then with the bladder through the allantoic duct. It eventually degenerates and becomes the median ligament (urachus) after birth.

Question 17

During embryogenesis, both mesonephric ducts and paramesonephric ducts form. Which degenerates in males, and which in females. What do the remaining ducts become?

Answer 17

Mesonephros remains in males (paramesonephric degenerates due to anti-paramesonephric hormone). Mesonephros [Wolffian ducts] becomes the vas deferens, seminal vesicles and epididymis.

Mesonephric ducts degenerate in females (due to low testosterone), but Paramesonephric ducts [Mullerian] remain in females. These become the uterine tubes, and fuse inferiorly to give rise to the uterovaginal canal. (lower vagina is from sinovaginal bulb from lower urogenital sinus).

Question 18

From what do the prostate & bulbourethral glands develop?

Answer 18

Outgrows of the urethra.

Question 19

From what does the vagina develop?

Answer 19

Upper vagina from the paramesonphric ducts, lower vagina from the sinovaginal bulb which is from the lower urogenital sinus)

Question 20

What fistulas can occur during development of the urogenital tract?

Answer 20

Rectovesical (bladder to rectum), recto-urethral, rectovaginal.

Question 21

At which week to male and female genitalia differentiate during development? Up until then, what are they composed of?

Answer 21

Week 8

labioscrotal swellings lateral to the urogenital folds which surround the urogenital groove on the midline.

Question 22

Describe the development of the external genitalia in males.

Answer 22

urogenital folds fuse over the midline in males (enclosing urogenital groove which becomes the lumen of the urethra)

Laboscrotal folds fuse in the midline too and become the scrotum.

Question 23

Describe the development of the external genitalia in females.

Answer 23

The urogenital groove becomes the vestibule which contains the openings for the vagina and urethra.
The urogenital folds remain and become labia minora.
The labioscrotal folds thicken and become labia majora.

Question 24

What are the homologues in male/female anatomy?

Answer 24

Penis/clitoris
Scrotum/labia majora
Bulbourethral glands/greater vestibular glands

Question 25

Define Hypospadias & Epispadias?

Answer 25

Hypospadias = urethra opens onto ventral surface of penis (urogenital folds fail to fuse on midline)
Epispadias = Urethra opens onto dorsal surface of penis (associated with bladder extrophy)