Embryology

Question 0

What are the important features of the mesonephros?

Answer 0

Second kidney system

• primitive renal function but not able to conserve water
• mesonephric tubules develop caudal to the pronephric region
• mesonephric tubules + mesonephric duct = embryonic duct
• mesonephric duct sprouts ureteric bud & is important to the development of the male reproductive system
• regress or form another opening

Question 1

What are the important features of the pronephros?

Answer 1

First kidney system

Never functions in humans
Pronephric duct extends from cervical region to the cloaca and drives the development of the next stage

Question 2

What are the important features of the metanephros?

Answer 2

Definitive kidney

• ureteric bud induces development
• collecting duct derived from ureteric bud
• excretory component derived from intermediate mesoderm (under the influence of the ureteric bud)

Functions normally except for the ability to conserve water

Question 3

What is the urogenital ridge?

Answer 3

Region of intermediate giving rise to both the embryonic kidney & the gonad

Question 4

Outline the ascent of the kidney during development.

Answer 4

Metanephric kidney seems to shift cranially, but this is more to do with the elongation of the trunk of the embryo

Kidneys cross the arterial fork formed by the vessels returning blood from the foetus to the placenta

New blood supply created sequentially (each time the old blood supply regresses - common to have multiple accessory renal arteries)

Question 5

What is the histiological composition of the ureter and bladder?

Answer 5

URETER = 2 layers of smooth muscle (third appears in lower third of ureter), lined by transitional epithelium

BLADDER = 3 layers of smooth muscle (outer adventitia), lined by transitional epithelium (“umbrella cells” on surface make impermeable)

Question 6

Give some examples of kidney defects and what the embryological basis is.

Answer 6

Renal agenesis: ureteric bud does not appear/fails to interact with the metanephric cap/blastema

• unilateral = compatible with life
• bilateral = can cause oligohydramnios, low amniotic fluid, which squashes the foetus (part of Potter’s syndrome/sequence)

Ectopic kidney: failed migration of kidney
- renal artery did not regress

Accessory/supernumerary renal arteries (common)
- failed regression of embryonic vessels formed during ascent of the kidney

Horseshoe kidney: lower poles fuse during ascent (inferior mesenteric artery prevents further ascent)
- kidneys pushed together during passage through arterial fork

Wilm’s tumour (nephroblastoma): 0-3yrs, possibly bilateral

Duplication defects: ureteric bud splitting at mesonephric duct can cause an ectopic ureteral opening -> ureters don’t enter the bladder in the correct place -> possible incontinence (if inferior to internal urethral sphincter in females)

Cystic kidney: multicystic (developmental, e.g. atresia of ureter) or polycystic (genetic; recessive, presents early, poor prognosis)

Question 7

How does the hindgut separate from the urinary tract during development?

Answer 7

Urorectal septum: mesodermal wedge degrades, leaving the urogenital sinus

Also forms future bladder and parts of the future urethra

Question 8

What does the allantois develop into?

Answer 8

Allantois -> urachus -> median umbilical ligament

Question 9

Outline the development of the male urogenital sinus.

Answer 9

1. Mesonephric ducts reach urogenital sinus
2. Ureteric buds sprout from mesonephric ducts
3. Ureteric buds open into bladder; smooth musculature begin to appear & urogenital sinus expands
4. Ureteric buds & mesonephric ducts make independent openings in the urogenital sinus
5. Mesonephric ducts maintained (androgens signal) & the prostate and prostatic urethra are formed

Question 10

Outline the development of the female urogenital sinus.

Answer 10

1. Mesonephric ducts reach urogenital sinus
2. Ureteric buds sprout from mesonephric ducts
3. Mesonephric ducts begin to regress (no androgen signal) & urogenital sinus expands
4. Ureteric buds open into the urogenital sinus & mesonephric duct regression continues

Question 11

What are the components of the male urethra and the female urethra?

Answer 11

MALE:

• Pelvic: pre-prostatic, prostatic, membranous
• Phallic: Spongy

FEMALE: pelvic part of urogenital sinus

Question 12

Outline the development of the male and female external genitalia.

Answer 12

INDIFFERENT STAGE (7 weeks)

MALE (12 weeks)

• phallus elongates and becomes the glans penis
• urogenital sinus becomes the urethral groove
• genital swellings become scrotum

FEMALE (12 weeks)

• phallus becomes the clitoris
• urogenital sinus becomes the vaginal and urethral orifices, and the labia minora
• genital swellings become the labia majora

Question 13

Give some examples of defects involving the bladder and urinary tract.

Answer 13

Fistulae between tracts
- malformation of cloaca/urorectal septum

Exstrophy of bladder: bladder opens onto abdominal wall (associated with syndromes with other defects)

Ectopic urethral orifices

Hypospadias (common): defect in fusion of urethral folds -> urethra opens onto ventral surface of the penis instead of on the glans

Question 14

What embryonic tissue are the kidneys derived from?

Answer 14

Intermediate mesoderm

Question 15

What are the origins of the collecting duct and the excretory system of the nephron?

Answer 15

Collecting duct = ureteric bud

Excretory system = metanephric cap

Question 16

What is the embryological derivation of the bladder muscle?

Answer 16

Hindgut (therefore is smooth muscle)