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MBS 232
Embryological Development of
the Urogenital System
K. Gamieldien (PhD)
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Developing Embryo:
3 layers:
Ectoderm outmost Mesoderm middle
Endoderm inner
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Development of Urinary System
The developmentofthe kidney proceedsthrough a
seriesofsuccessive phases, each marked by the
developmentofa more advanced kidney:
Pronephros Mesonephros
Metanephros
Only metanephros persiststo become the adult
kidneys Metanephric kidney producesurine by fetal month
three
Contributestothe volume ofamniotic fluid
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Mesonephros
The developmentofthe pronephric ductproceeds in
acranial-to-caudal direction.
As it elongatescaudally,the pronephric duct inducesnearby
intermediate mesoderm in the thoracolumbar
area to become epithelial tubulescalled mesonephrictubules.
Each mesonephrictubule receives a blood supply from abranch
ofthe aorta, ending in a capillary tuft analogous
tothe glomerulus ofthe definitive nephron.
The mesonephrictubule forms a capsule around thecapillary tuft,
allowingforfiltration ofblood. Thisfiltrateflowsthrough the
mesonephrictubule and is drained intothe continuation ofthe
pronephric duct, nowcalled themesonephric duct orWolffian duct.
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Mesonephros cont
The nephrotomesofthe pronephros degenerate whilethe mesonephric
duct extendstowardsthe mostcaudalend ofthe embryo,ultimately
attachingtothe cloaca
The cloaca is defined as a posterioropeningthatservesasthe only
such openingforthe intestinal, reproductive,and urinary tracts
ofcertain animal species ie commonpassageway forfeces,urine and
reproduction ).
The mammalian mesonephros issimilartothe kidneysofaquatic
amphibians and fishes.
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Metanephros
Duringthe fifth week ofgestation,the mesonephric ductdevelops an
outpouching,the ureteric bud, near itsattachmenttothe cloaca.
This bud, alsocalled the metanephrogenic
diverticulum,growsposteriorly and towardsthe head ofthe embryo.The
elongated stalk ofthe ureteric bud,called themetanephric duct,
laterformsthe ureter.
Asthe cranial end ofthe bud extends intotheintermediate
mesoderm, itundergoes a seriesofbranchingstoform the collecting
ductsystem ofthekidney. It alsoformsthe major and minorcalyces
andthe renal pelvis.
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Developmentofthe Urinary
Organs
Figure 23.18c, d
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Metanephros cont
The portion ofundifferentiated intermediate mesoderm in
contactwith the tipsofthe branchingureteric bud is known asthe
metanephrogenic blastema.
Signals released from the ureteric bud induce the
differentiationofthe metanephrogenic blastema intothe renal
tubules.
Asthe renal tubulesgrow,they come intocontact and join
withconnectingtubules ofthe collecting ductsystem,forming
acontinuouspassage forflowfrom the renal tubule tothecollecting
duct.
Simultaneously,precursorsofvascular endothelial cells begin
totake theirposition atthe tipsofthe renal tubules. These cells
differentiate intothe cellsofthe definitive glomerulus. In
humans, all ofthe branchesofthe ureteric bud and the
nephronicunits have been formed by 32 to 36 weeksofgestation.
However,these structures are not yet mature, and willcontinue to
mature after birth. Once matured, humans have anestimated one
million nephrons (approximately 500,000 perkidney) or more
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