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Urinary System – Session 2 1

EmbryologySession 6

Development of the Kidneys and Urinary Tract

LO E6.1 Describe the three sets of excretory organs, the pronephros, mesonephros and its duct and the metanephros (the permanent kidney)

Structure Appears egresses

!unctional"

#omment

$ronephros Start of week4

End of week4

No

%esonephros

End of week4

End of week8

 Yes.(No waterconservingmechanism)

Mesonephric duct has a ver importantro!e in the deve!opment of the ma!ereproductive tract.Mesonephric duct sprouts the ureteric

bud" the primordium of the co!!ectingsstem of the de#nitive kidne.

%etanephros

$eek % No Yes.(&rom end of #rsttrimester)

 'he de#nitive kidne.o!!ecting sstem deve!ops from theureteric ud and the e*cretor sstemdeve!ops from metanephric tissuecap.

 'he emronic kidne andgonad oth originate fromthe urogenital ridge" a

region of intermediatemesoderm.

 'he organisation of this intermediate mesoderm !eads to + sstemsdeve!oping se,uentia!!. 'he disappearance of one sstem marks the startof the ne*t (see ta!e). 'he #rst sstemappears in the cervica! region" thepronephros.

$ronephros

 'he #rst kidne sstem" never functionsin humans. -owever" it produces thepronephric duct" which e*tends from thecervica! region to the c!oaca and drives thedeve!opment of the ne*t stage (ecomingthe Mesonephric duct)

%esonephros

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Urinary System – Session 2

 'he mesonephros sprouts tubules that deve!opcaudal to the pronephric region. 'hesetuu!es p!us the %esonephric duct makes upthe embryonic &idney.

 'he Mesonephric duct a!so sprouts the uretericbud" which induces deve!opment of thede#nitive kidne. 'he Mesonephric duct a!so hasan important ro!e in the deve!opment of thereproductive sstem in the ma!e.%etanephros

 'he Ureteric bud sprouts from the Mesonephricduct. /t induces the development of thede'nitive &idney within the intermediatemesoderm of the cauda! region of the emro

that !ies c!oset to it.

 'he ureteric ud then e*pands and ranches intothis di0erentiated intermediate mesoderm" themetanephric !astema" forming the de#nitivekidnes structure.

 'he ureteric ud drives thedevelopment of thede'nitive &idney. 'heco!!ective sstem is derived

from the ureteric ud itse!f.

 'he e*cretor component isderived from the intermediatemesoderm under the in2uenceof the ureteric ud.

LO E6. Describe the positiona! changes of the kidneys and

understand ho" congenita! anoma!ies of position and rena! #esse!soccur 

The (ascent) of the &idney

 'he metanephric kidne #rst appears in the pe!vic region. /t thenundergoes an apparent cauda! to crania! shift" crossing the arterial for& formed by vessels returning blood from the foetus to theplacenta*

-owever the kidnes don+t actually move. 3eve!opment is crania!  cauda!" and the trunk ust e*tends downwards" making it appear as thoughthe kidnes

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Urinary System – Session 2 +

enal Agenesis

 'he ureteric ud fails to interact ,ith the intermediate mesoderm.an a0ect one (uni!atera!) or oth (i!atera!) kidnes.

%igration Defect

/f a kidne fai!s to cross the arteria!fork (as the kidnes dont actua!!move its more the fork snags thekidne as it deve!ops" pu!!ing itdown)" it ends up much !ower thanit shou!d e (5).

3uring their 6ascent the kidnes !iee*treme! c!ose to one another. /f the oth get caught on the arteria!fork the can fuse and form a horseshoe kidne (7).

Duplication defects

Splitting of the ureteric bud" eitherpartia! or comp!ete can !ead toanorma!ities. 'he sstemic conse,uence isan ectopic opening" for e*amp!e into thevagina or urethra" passing the !adderand causing incontinence.

#ystic Kidney Disease

%ulticystic Kidney Disease  5tresia of ureter$olycystic Kidney Disease  9ecessive" presents ear!" poor prognosis.

Abnormal enal -essels

5s the kidnes ascend the re,uire new arteria! supp!" and the previoussupp! disappears. /f it remains" the are accessor" or supernumeraryarteries. 'hese arteria!s are end arteries" as the main rena! arter wi!!

not ranch to supp! that area of the kidne if an accessor arter ispresent. 'his means there is no co!!atera! supp!.

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4 Urinary System – Session 2

LO E6.$ Describe the formation of the urinary b!adder and urethra

.ladder

 'he !adder is a hindgut derivate" meaning it is derivedfrom the cauda! portion of the primitive guttube formed during emronic fo!ding in thefourth ,ee&  of deve!opment. 'he cauda!portion is a di!ated" !ind pouch ca!!ed thecloaca. 'he c!oaca is separated from the outside the cloacal membrane" one of the twomesoderm:!ess regions !eft present aftergastru!ation.

 'he c!oaca is divided the urorectal septum into the urogenital sinus(future !adder and urethra) and anorecta! cana! (future rectum and ana!cana!).

5!so invo!ved in the deve!opment of the !adder is the allantois" which isa superoventra! diverticu!um of the hindgut and e*tends into the umi!ica!cord. 'he !umen on the a!!antois ecomes o!iterated to ecome theurachus/ ,hich is the median umbilical ligament in adults.

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%ale .ladder

o %esonephric ducts

(M3) reach the urogenitalsinus (;

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> Urinary System – Session 2

o $rostatic

o %embranouso Spongy

 'he 'rst three parts are analogous to the female urethra. 'he

spongy urethra is the phallic part.

LO E6.% Describe and exp!ain the embryo!ogica! basis for commoncongenita! disorders of the urinary tract, e.g. horseshoe kidney,ectopic urethra! ori&ces, exstrophy of the b!adder and uracha!anoma!ies'or orseshoe kidney and ectopic urethra! ori&ces, see abo#e.

E1strophy of the .ladder Urachal Anomalies5 congenita! anoma! in which part of the urinary bladder is present outside the body. /t occurs due tomaldevelopment of the lo,er abdominal ,all"!eading to a rupture that causes the !adder tocommunicate with the amniotic 2uid.

E*stroph of the !adder ma e due to a urachal'stula. 'his is a patent urachus" which norma!! ecomes the medianumbilical ligament. /f it remains as a duct" it wi!! connectthe !adder to the umi!icus.

3ypospadias5 defect in fusion of urethra! fo!ds. 'he urethra opens

onto the ventral surface/ rather than at the end of the glans. 'he incidence of this is increasing.