DEVELOPMENT OF THE UROGENITAL SYSTEM:

M.A.KAI-KAI

Learning ObjectivesLearning ObjectivesTo understand the nephrogenic platenephrogenic plate and the gonadal ridgegonadal ridge and their derivativesderivatives.To understand the originorigin, topography and structure of the urogenital ridge.urogenital ridge.Understand formation of the pronephrospronephros,the induction induction and formation of the mesonephrosmesonephros and formation of the collecting ductsTo know development of the from primordial metanephric structuresl;the metanephric diverticulummetanephric diverticulum and the metanephrogenic massmetanephrogenic mass.To describe nephrogenesisnephrogenesis. Brief description of the main genesgenes regulating development of the kidneys.Describe the cloacacloaca, urogenital sinusurogenital sinus and derivativesderivatives.Understanding primary and secondary sex determinationsex determinationgenetic sex, gonadal sex, phenotypic sex.Describe primordial germ cellsgerm cells and the migrationmigration to the gonadal ridge.Describe stagesstages in gonadogenesisBrief understanding of the molecularmolecular aspects of gonadogenesisDescribe differentiation of the WolffianWolffian and MullerianMullerian ducts into male and female ducts respectively.

Overview of development of the urogenital organsOverview of development of the urogenital organsThe urinary urinary and reproductivereproductive/genital systems are closely associated in topography,function and development.The two systemssystems have a common originorigin from intermediate mesodermintermediate mesoderm of the urogenital ridgeurogenital ridge and have homologous structures.The internal genitalinternal genital duct system is derived from the fetal urinary systemurinary system. Malformation of one system affects the other.Intermediate mesodermIntermediate mesoderm is stimulated by interactionsinteractions with adjacent paraxial mesodermparaxial mesoderm.UGRUGR is longitudinal swelling in dorsolateral side of the abdomenUGRUGR--> formed from:

-- non-segmented intermediate mesoderm --proliferation of surrounding mesodermal coelomic epithelium

LateralLateral UGR, the nephrogenic plate nephrogenic plate forms forms urinary organsurinary organs and internal genitalinternal genital ducts. VentromedialVentromedial UGR is genital ridgegenital ridge, forms gonadsgonads.Paraxial mesodermParaxial mesoderminteracts with intermediateintermediate mesoderm to form urogenitalurogenital organs.Paraxial mesoderm message is Pax2,Pax8Pax2,Pax8 (transcription factors).

Development of the kidney.Development of the kidney.DevelopmentDevelopment progresses through three progresses through three major stages. Two major stages. Two transient stagestransient stages and and third third premanentpremanent stage. stage.ThreeThree kidneys develop,pronephros, kidneys develop,pronephros, mesonephros and metanephros.mesonephros and metanephros.Three kidneys similar in structure and Three kidneys similar in structure and function.function.PronephrosPronephros forms. forms. Pronephric tubulesPronephric tubules degenerate, middle part induce degenerate, middle part induce mesonephros,mesonephros,induces induces metanephrosmetanephros.. Similarities of Similarities of nephric tubulesnephric tubules in in structure and function in foetal and adult structure and function in foetal and adult kidneyskidneys

EfficiencyEfficiency in function. in function.

--pronephros--pronephros

--mesonephros--mesonephros

--metanephros--metanephros

Development of the mesonephros(fig.1- 4)Development of the mesonephros(fig.1- 4) Sequence of development.Sequence of development.

MesonephrosMesonephros develops at level of develops at level of somites 9-26 somites 9-26 Pronephros induces formation of Pronephros induces formation of mesonephros (mesonephros (MNMN). ). Mesonephric tubules(Mesonephric tubules(MTMT) ) form from form from nephrogenic platenephrogenic plate as pronephros as pronephros degenerates.degenerates.MT bud off, hollows into the renal MT bud off, hollows into the renal vesicle and elongates forming the vesicle and elongates forming the mesonephric tubule. Proximal end mesonephric tubule. Proximal end enlarges into a double-walled enlarges into a double-walled Bowman's Bowman's capsule.capsule. (BC). (BC).BC becomes invaded with a rich vascular internal glomerulusinternal glomerulus,forming mesonephric corpusclemesonephric corpuscle (MC)Mesonephric tubulesMesonephric tubules differentiate into a thick,darkdark-staining proximal secretorysecretory segment and a thin,lightlight-staining distal collectingcollecting segment MT drain into collecting tubule,extend caudally and join existing PD.This mesonephricmesonephric/WolffianWolffian duct opens in thecloacacloaca.

Development of the metanephros(1)Development of the metanephros(1)Sequence of development.Sequence of development.MetanephrosMetanephros develops at somites 26-28 develops at somites 26-28 from 2 precursors-->from 2 precursors-->metanephric metanephric diverticulumdiverticulum (MD)/ureteric bud (MD)/ureteric budand and metanephrogenic massmetanephrogenic mass(MM)(MM)Formation Formation MDMD and and MMMM is by is by reciprocal inductivereciprocal inductive interactionsinteractions between between the tissues.the tissues.The diverticulum forms caudal of the The diverticulum forms caudal of the existing existing mesonephricmesonephric ductduct,grows,growscraniodorsally into the nephrogenic craniodorsally into the nephrogenic plate(NP), which aggregates and plate(NP), which aggregates and proliferates into proliferates into MMMM..MDMDurinary duct systemurinary duct systemMMMMnephronsnephronsThe The metanephricmetanephric diverticulumdiverticulum branches branches as it grows towards the as it grows towards the metanephrogenic metanephrogenic massmass..

Development of the metanephros(2). The Duct SystemDevelopment of the metanephros(2). The Duct System

Morphogenesis of the metanephric diverticulum.Morphogenesis of the metanephric diverticulum.The dichotomous dichotomous branching pattern is species specific. Simple,unilobar/unipyramidalunipyramidal kidneyskidneys(horse) the proximal end dilates into a renal pelvisrenal pelvispapillary/collecting ducts (CD)collecting tubules (CT)In multipyramidal kidneysmultipyramidal kidneys e.g. the MD branches; bifurcates into 2 major calycesmajor calyces and several minorminor calycescalycespapillary ducts.

Development of the metanephros(3).Development of the metanephros(3).The The MMMM forms the forms the nephronsnephrons by by nephrogenesisnephrogenesis..MD MD forms the forms the duct systemduct systemNephrogenesisNephrogenesisNephrogenesis Nephrogenesis and differentiation of the and differentiation of the metanephric diverticulummetanephric diverticulum are co-ordinated. are co-ordinated.Each collecting tubule grows into a solid Each collecting tubule grows into a solid cluster of metanephrogenic mass.cluster of metanephrogenic mass.The MM has an internal dense medulla The MM has an internal dense medulla which forms the nephrons, and an external, which forms the nephrons, and an external, light loose layer which forms the interstitial light loose layer which forms the interstitial connective tissue and capsule.connective tissue and capsule.

Development of the metanephros(4)Development of the metanephros(4)

Stages in nephrogenesis.Stages in nephrogenesis.Solid cluster of metanephrogenic mass forms and hollows into a Solid cluster of metanephrogenic mass forms and hollows into a renal renal vesiclevesicle(RV) with epithelial lining(RV) with epithelial liningelongate into elongate into metanephric tubules metanephric tubules (MT).(MT).Proliferation,elongation of the MM dependent upon Proliferation,elongation of the MM dependent upon reciprocalreciprocal tissue tissue interactions using several interactions using several molecularmolecular signals signalstubules grow tubules grow centrifugallycentrifugally and form and form nephronsnephrons..

Development of the metanephros (5).Development of the metanephros (5).The The proximal endproximal end of the PCT dilates and of the PCT dilates and invaginates to form the invaginates to form the Bowman's capsuleBowman's capsule The DCT attaches to the collecting tubule.The DCT attaches to the collecting tubule.The BC becomes invaded by the glomerulus and The BC becomes invaded by the glomerulus and the the renal corpusclerenal corpuscle (RC) is formed. (RC) is formed.Early RC form in the Early RC form in the corticomedullary junctioncorticomedullary junction, , some degenerate, later nephrons form in the some degenerate, later nephrons form in the cortex.cortex. About 20K(cat) 300-500K(dog) nephrons form. About 20K(cat) 300-500K(dog) nephrons form. Interstitial connective tissue forms between Interstitial connective tissue forms between nephrons, a thick fibrous capsule surrounds the nephrons, a thick fibrous capsule surrounds the kidney.kidney.NephrogenesisNephrogenesis ceases at birth continue for 1-3 wks ceases at birth continue for 1-3 wks in the dog and pig.in the dog and pig.

Reciprocal interaction of kidney tissues.Reciprocal interaction of kidney tissues.The renal precursors; The renal precursors; metanephric metanephric diverticulumdiverticulum(MD) and (MD) and metanephrogenic metanephrogenic mesenchymemesenchyme(MM) interact and (MM) interact and reciprocally inducereciprocally induce each other to form the kidney. each other to form the kidney. Only the Only the MMMM has the has the competencecompetence to respond to the to respond to the MDMD to form kidney tubules and if induced by other to form kidney tubules and if induced by other tissues, the MM will not respondtissues, the MM will not respond

Glial derived neurotrophic factor(GDNF) regulates growth and branching of ureteric bud

A. Wild type mouse has normalbranching of ureteric bud

B. GDNF knock out mouse no branching

Gonadogenesis Gonadogenesis

Sex determinationSex determination-->sequence of interdependent events.-->sequence of interdependent events.Primary Primary sex determination at fertilisationsex determination at fertilisationgeneticgenetic sex: XY, XX. sex: XY, XX.BipotentialBipotential gonad gonadY chromosomes encodes testis-determining Y chromosomes encodes testis-determining factorfactorSRYSRYgonadalgonadal sex sextestis secretes testis secretes testosteronetestosteroneSecondarySecondary sex determinationsex determinationphenotypicphenotypic sex. sex.AbsenceAbsence of of YY chromosome forms chromosome forms ovariesovaries

--anti-Mullerian hormone(AMH)--anti-Mullerian hormone(AMH)regression of paramesonephric ductregression of paramesonephric duct

Wolffian duct forms male genital ducts.Wolffian duct forms male genital ducts.

GonadogenesisGonadogenesisGonadogenesis occurs at the Gonadogenesis occurs at the genital genital ridgeridge initiated by 2 simultaneous events:- initiated by 2 simultaneous events:-

(i).Formation of (i).Formation of gonadal cordsgonadal cordsDegeneration of the Degeneration of the mesonephricmesonephric corpuscles corpuscles form epithelial cords and form epithelial cords and invade genital ridge.invade genital ridge.ProliferationProliferation of coelomic epithelium of coelomic epithelium invade mesonephric epithelial and resident invade mesonephric epithelial and resident mesodermal cells.These proliferate mesodermal cells.These proliferate forming network of epithelial forming network of epithelial cords/cords/blastemablastema..PGCPGC are large,endodermal and migrate are large,endodermal and migrate from the yolk sac into the developing from the yolk sac into the developing gonadgonad

The The indifferentindifferent gonad consist of a central core and a peripheral cortex, surrounded gonad consist of a central core and a peripheral cortex, surrounded by coelomic epithelium. by coelomic epithelium. PGCPGC arrive at the genital ridge at 7,21 days in rodents and cat arrive at the genital ridge at 7,21 days in rodents and catproliferateproliferateepithelial components incorporate PGC, forming gonadal cords.epithelial components incorporate PGC, forming gonadal cords.Gonadal differentiation begins. Gonadal differentiation begins.

Mechanisms of mammalian primary sex determinationMechanisms of mammalian primary sex determination Several genes essential for sexual differentiationSeveral genes essential for sexual differentiationfemale is default gonadfemale is default gonad----SRYSRY located on located on YY-chromosome-chromosome--sex determination requires interaction between SRY and another factor: --sex determination requires interaction between SRY and another factor: SOX9SOX9?.?. Mode of action of SRYMode of action of SRY--SRY could activate SOX9(transcription factor)--SRY could activate SOX9(transcription factor)--SRY mediates expression of cell-surface male-specific antigen --SRY mediates expression of cell-surface male-specific antigen Y-histocompatibilityY-histocompatibility

antigen converting epithelial to antigen converting epithelial to Sertoli cellsSertoli cells..

--SC secrete glycoprotein;--SC secrete glycoprotein;clusterinclusterin which initiates which initiates testicular differentiationtesticular differentiation. SOX9SOX9--migrates into the nucleus, binds to promoter site of gene for --migrates into the nucleus, binds to promoter site of gene for AMHAMH..--activate --activate FGF9FGF9? ? Steroidogenic factorSteroidogenic factor((Sf1Sf1) activated by ) activated by SRYSRY??--Sf1 could activate --Sf1 could activate LeydigLeydig cells and cells and Sertoli Sertoli cellscellssecreting testosterone and AMH respectively.secreting testosterone and AMH respectively. Wnt4Wnt4 and and Dax1Dax1 involved in involved in ovaryovary formation formation--Dax1 suppresses testis formation and activates ovary development.--Dax1 suppresses testis formation and activates ovary development.

Sex DeterminationSex Determination

(Modified from Gilbert 2006)

Definitive gonads:TestisDefinitive gonads:TestisY-chromosome produces Y-chromosome produces Y-HY-H which which mediates transformation of the sexually mediates transformation of the sexually indifferent gonad into testis.indifferent gonad into testis.The Y-H antigen reacts with gonadal cords The Y-H antigen reacts with gonadal cords to form to form SertoliSertoli cells. cells.Gonadal cords hollow and anastomose into Gonadal cords hollow and anastomose into testicular cords enclosing mitotic testicular cords enclosing mitotic gonocytesgonocytes/prespermatogonia/prespermatogoniaClusterinClusterin regulates aggregation of regulates aggregation of prespermatogonia in the medullaprespermatogonia in the medullaThe cortex develops a thick tunica The cortex develops a thick tunica albuginea, separates the testicular tubules albuginea, separates the testicular tubules from the germinal epithelium. from the germinal epithelium. Testicular cords Testicular cords (TC) form loops and (TC) form loops and interconnect with mesonephric tubules(MT).interconnect with mesonephric tubules(MT).MTMT differentiate into peripheral differentiate into peripheral seminiferous tubules, central rete testis and seminiferous tubules, central rete testis and efferent ductuli.efferent ductuli.WD form WD form epididymisepididymis and and ductus deferensductus deferens..

Interstitial/Sertoli cellsSCs are derived from the epithelial of SCs are derived from the epithelial of mesonephric corpuscles and coelomic mesonephric corpuscles and coelomic epithelium.cells proliferate in fetus and epithelium.cells proliferate in fetus and postpartum.Mitosis ceases at postpartum.Mitosis ceases at spermatogenesis.spermatogenesis.Fetal SC secrete anti-Mullerian Fetal SC secrete anti-Mullerian hormone(AMH)--> regress hormone(AMH)--> regress paramesonephricparamesonephric ducts. ducts. LeydigLeydig cells(LC) derived from all cells(LC) derived from all mesodermal components of the gonadal mesodermal components of the gonadal ridge,proliferate and differentiate,mitosis at ridge,proliferate and differentiate,mitosis at birthbirthTesticular cord formation precedes and is Testicular cord formation precedes and is essential for differentiation of Leydig cells.essential for differentiation of Leydig cells.LC cytodifferentiate and secrete LC cytodifferentiate and secrete testosteronetestosterone in fetus. in fetus.

Definitive Ovary formation.Definitive Ovary formation. In the absence of the Y chromosome in PGC.In the absence of the Y chromosome in PGC.

Gonadal cords concentrate in the periphery forming Gonadal cords concentrate in the periphery forming the primary ovarian cortex(POC)the primary ovarian cortex(POC)The POC degenerates The POC degenerates and more epithelial cords proliferate and secondary and more epithelial cords proliferate and secondary cortex forms.cortex forms.The gonadal cords and germ cells in the medulla The gonadal cords and germ cells in the medulla degenerate. degenerate. The medulla becomes stroma of vascular, lymphatic The medulla becomes stroma of vascular, lymphatic and nervous tissues and nervous tissues Surviving gondal cords form rete ovarii which serves Surviving gondal cords form rete ovarii which serves as general morphogenic organiser.as general morphogenic organiser.Gonocytes continue mitosis fetal life and in early Gonocytes continue mitosis fetal life and in early postnatal period.postnatal period.Some gonocytes begin meiosis and progress to Some gonocytes begin meiosis and progress to diplotene of prophase I, these become surrounded by diplotene of prophase I, these become surrounded by single layer of follicular cells then meiosis is arrested single layer of follicular cells then meiosis is arrested till pubertytill pubertyGonadal cords break up Gonadal cords break up into primordial follicles containing meiotic and into primordial follicles containing meiotic and premeiotic germ cells (GC)premeiotic germ cells (GC)Meiosis in the fetal GC is triggered by meiosis-Meiosis in the fetal GC is triggered by meiosis-inducing substances in the ovary which are not species inducing substances in the ovary which are not species specificspecific

Gilbert 2006)

Formation of the urogenital sinus[(UGS)fig.8]Formation of the urogenital sinus[(UGS)fig.8]The The cloacacloaca forms the common opening forms the common opening for the urinary and digestive systems.for the urinary and digestive systems.The cloaca is bounded caudally by the The cloaca is bounded caudally by the the the cloacalcloacal membranemembrane,lying in the ,lying in the proctodeum. proctodeum. The The mesonephric ductsmesonephric ducts enter the cloaca enter the cloaca laterally.laterally.Formation of the Formation of the UGSUGS begins with begins with division of the cloaca by the mesodermaldivision of the cloaca by the mesodermal uroectal septumuroectal septum The UGS separates the cloaca into The UGS separates the cloaca into dorsal and ventral parts bounded by anal dorsal and ventral parts bounded by anal and and urogenital urogenital membranes.membranes.The membranes degenerate forming the The membranes degenerate forming the anal and urogenital anal and urogenital orificesorifices..

Formation of the urogenital sinus[(UGS)fig.8]Formation of the urogenital sinus[(UGS)fig.8]The UGS is divisible into a cranial The UGS is divisible into a cranial urethrovesicularurethrovesicular and the pelvic UGS and and the pelvic UGS and caudal caudal phallicphallic UGS. UGS.The The UGSUGS opens caudally into the opens caudally into the amniotic cavity and cranially into the amniotic cavity and cranially into the allantoic cavity via the allantoic cavity via the urachusurachus andandallantoic stalk.allantoic stalk.With With growth growth and expansion of the UGS and expansion of the UGS the the mesonephricmesonephric and and metanephricmetanephric ducts ducts open separately on dorsal wall of UGS at open separately on dorsal wall of UGS at the the trigonetrigone of bladder. of bladder.The urachus extends into the umbilical The urachus extends into the umbilical cord as the allantoic stalk.cord as the allantoic stalk.

Differentiation of the UGS.Differentiation of the UGS.In the In the malemale,the ,the proximal proximal UVUGS UVUGS expands into the expands into the bladderbladder, the , the distaldistal part part forms the forms the pelvicpelvic urethra and the urethra and the phallicphallic UGS forms the UGS forms the penile penile urethra.urethra.In the In the femalefemale the short the short urethraurethra forms from forms from the the proximalproximal UVUGS, the UVUGS, the distaldistal UVUGS UVUGS and the and the phallicphallic form the form the vestibulevestibule and part and part of the vagina.of the vagina.

Conginetal Malformations.Conginetal Malformations.Renal agenesisRenal agenesislack of development. lack of development. DysplasiaDysplasiawide range of abnormal development of nephrons and collecting ducts, wide range of abnormal development of nephrons and collecting ducts, form cystsform cysts HypoplasiaHypoplasiasgenosis of uretersgenosis of ureterhydronephrosis.hydronephrosis.Renal hyperparathyroidism.Renal hyperparathyroidism.Ectopic kidneys and ureters.Ectopic kidneys and ureters.Polycystic kidneysPolycystic kidneysfailure of nephrons to join collecting ducts?failure of nephrons to join collecting ducts?Rectovaginal constrictionRectovaginal constrictionhereditary, anovestibular stenosis.hereditary, anovestibular stenosis.Vesicourachal diverticulum--<outpockecting of bladder when urachus fails to close.Vesicourachal diverticulum--<outpockecting of bladder when urachus fails to close.CryptorchidismCryptorchidismfailure of normal descent of testis.failure of normal descent of testis.Mesonephric or paramesonephric duct abnormalitiesMesonephric or paramesonephric duct abnormalitiesstenotic/aplasia of duct.stenotic/aplasia of duct.PseudohermaphroditePseudohermaphroditegonads of one sex and external gentalia of the opposite sex, gonads of one sex and external gentalia of the opposite sex, hormonal factors.hormonal factors.True hermaphroditesTrue hermaphroditesgonadal defects at early stage of development, separate gonads gonadal defects at early stage of development, separate gonads or ovo-testis, a chromosomal abnormality.or ovo-testis, a chromosomal abnormality.Freemartin cattleFreemartin cattle intersexuality, a genotypic female(XX) born with normal male co- intersexuality, a genotypic female(XX) born with normal male co-twin. Fused placental vesselstwin. Fused placental vesselstransport of foetal androgens to female.transport of foetal androgens to female.

SummarySummary

The The urinary system urinary system andand genital system genital system are derived from the intermediate are derived from the intermediate mesoderm,the nephrogenic plate and genital ridge of the urogenital ridge.mesoderm,the nephrogenic plate and genital ridge of the urogenital ridge.The kidney develops in three stages in rapid successionThe kidney develops in three stages in rapid successionpronephrospronephros, , mesonephrosmesonephros andand metanephrosmetanephros in cranial-caudal direction. in cranial-caudal direction.The The pronephrospronephros is of limited function in mammals but acts as is of limited function in mammals but acts as inducerinducer of the of the mesonephros.mesonephros.The The metanephrosmetanephros forms from two precursors forms from two precursorsthe the metanephric diverticulummetanephric diverticulum and and the the metanephrogenic massmetanephrogenic mass.Development of the metanephros requires reciprocal .Development of the metanephros requires reciprocal tissue interactiontissue interaction using several molecular signals. using several molecular signals.Sex determination is dependent upon interaction of several factors__. Genetic sex, Sex determination is dependent upon interaction of several factors__. Genetic sex, gonadal sex, hormonal action and phenotypic sex.gonadal sex, hormonal action and phenotypic sex.Germ cells are endodermal, the other tissues are mesodermal. Early in Germ cells are endodermal, the other tissues are mesodermal. Early in development, germ cells migrate from the yolk sac to the genital ridge.development, germ cells migrate from the yolk sac to the genital ridge.Development of the urogenital system is dependent on molecular signalsDevelopment of the urogenital system is dependent on molecular signals..The external urinary and genital organs are formed from the The external urinary and genital organs are formed from the cloacacloaca and and urogenital urogenital sinussinus..The The WolffianWolffian and and MullerianMullerian ducts form the adult ducts form the adult male male and and female female ducts ducts respectively.respectively.

ReferencesReferences

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pp447-461, 523-540, 548-561, Sinauer Associate. Massachusetts.pp447-461, 523-540, 548-561, Sinauer Associate. Massachusetts.

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3. Noden, M. and de Lahunta(1985). The Embryology of Domestic Animals, pp 312-3. Noden, M. and de Lahunta(1985). The Embryology of Domestic Animals, pp 312-341, 349-355 Williams and Wilkins, London. 341, 349-355 Williams and Wilkins, London.