1. DEVELOPMENT OF THE UROGENITAL SYSTEM:
M.A.KAI-KAI

2. Learning Objectives
To understand the nephrogenic plate and the gonadal ridge and their derivatives.
To understand the origin, topography and structure of the urogenital ridge.
Understand formation of the pronephros,the induction and formation of the mesonephros and formation of the collecting ducts
To know development of the from primordial metanephric structuresl;the metanephric diverticulum and the metanephrogenic mass.
To describe nephrogenesis.
Brief description of the main genes regulating development of the kidneys.
Describe the cloaca, urogenital sinus and derivatives.
Understanding primary and secondary sex determinationgenetic sex, gonadal sex, phenotypic sex.
Describe primordial germ cells and the migration to the gonadal ridge.
Describe stages in gonadogenesis
Brief understanding of the molecular aspects of gonadogenesis
Describe differentiation of the Wolffian and Mullerian ducts into male and female ducts respectively.

3. Overview of development of the urogenital organs
The urinary and reproductive/genital systems are closely associated in topography,function and development.
The two systems have a common origin from intermediate mesoderm of the urogenital ridge and have homologous structures.
The internal genital duct system is derived from the fetal urinary system.
Malformation of one system affects the other.
Intermediate mesoderm is stimulated by interactions with adjacent paraxial mesoderm.
UGR is longitudinal swelling in dorsolateral side of the abdomen
UGR--> formed from:
-- non-segmented intermediate mesoderm
--proliferation of surrounding mesodermal coelomic epithelium
Lateral UGR, the nephrogenic plate forms urinary organs and internal genital ducts.
Ventromedial UGR is genital ridge, forms gonads.
Paraxial mesoderm
interacts with intermediate mesoderm to form urogenital organs.
Paraxial mesoderm message is Pax2,Pax8 (transcription factors).

4. Development of the kidney.
Development progresses through three major stages. Two transient stages and third premanent stage.
Three kidneys develop,pronephros, mesonephros and metanephros.
Three kidneys similar in structure and function.
Pronephros forms. Pronephric tubules degenerate, middle part induce mesonephros,induces metanephros.
Similarities of nephric tubules in structure and function in foetal and adult kidneys
Efficiency in function.
--pronephros
--mesonephros
--metanephros

5. Development of the mesonephros(fig.1- 4)
Sequence of development.
Mesonephros develops at level of somites 9-26
Pronephros induces formation of mesonephros (MN).
Mesonephric tubules(MT) form from nephrogenic plate as pronephros degenerates.
MT bud off, hollows into the renal vesicle and elongates forming the mesonephric tubule. Proximal end enlarges into a double-walled Bowman's capsule. (BC).
BC becomes invaded with a rich vascular internal glomerulus,forming mesonephric corpuscle (MC)
Mesonephric tubules differentiate into a thick,dark-staining proximal secretory segment and a thin,light-staining distal collecting segment
MT drain into collecting tubule,extend caudally and join existing PD.This
mesonephric/Wolffian duct opens in the
cloaca.

6. Development of the metanephros(1)
Sequence of development.
Metanephros develops at somites from 2 precursors-->metanephric diverticulum (MD)/ureteric bud
and metanephrogenic mass
(MM)
Formation MD and MM is by reciprocal inductive interactions between the tissues.
The diverticulum forms caudal of the existing mesonephric duct,grows
craniodorsally into the nephrogenic plate(NP), which aggregates and proliferates into MM.
MDurinary duct system
MMnephrons
The metanephric diverticulum branches as it grows towards the metanephrogenic mass.

7. Development of the metanephros(2). The Duct System
Morphogenesis of the metanephric diverticulum.
The dichotomous branching pattern is species specific.
Simple,unilobar/unipyramidal kidneys(horse) the proximal end dilates into a renal pelvispapillary/
collecting ducts (CD)collecting tubules (CT)
In multipyramidal kidneys e.g. the MD branches; bifurcates into 2 major calyces and several minor calycespapillary ducts.

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

9. Development of the metanephros(4)
Stages in nephrogenesis.
Solid cluster of metanephrogenic mass forms and hollows into a renal vesicle(RV) with epithelial liningelongate into metanephric tubules (MT).
Proliferation,elongation of the MM dependent upon reciprocal tissue interactions using several molecular signalstubules grow centrifugally and form nephrons.

10. Development of the metanephros (5).
The proximal end of the PCT dilates and invaginates to form the Bowman's capsule
The DCT attaches to the collecting tubule.
The BC becomes invaded by the glomerulus and the renal corpuscle (RC) is formed.
Early RC form in the corticomedullary junction, some degenerate, later nephrons form in the cortex.
About 20K(cat) K(dog) nephrons form.
Interstitial connective tissue forms between nephrons, a thick fibrous capsule surrounds the kidney.
Nephrogenesis ceases at birth continue for 1-3 wks in the dog and pig.
Reciprocal interaction of kidney tissues.
The renal precursors; metanephric diverticulum(MD) and metanephrogenic mesenchyme(MM) interact and reciprocally induce each other to form the kidney.
Only the MM has the competence to respond to the MD to form kidney tubules and if induced by other tissues, the MM will not respond

11. Glial derived neurotrophic factor(GDNF) regulates
growth and branching of ureteric bud
Wild type mouse has normal
branching of ureteric bud
B. GDNF knock out mouse
no branching

13. Gonadogenesis
Sex determination-->sequence of interdependent events.
Primary sex determination at fertilisationgenetic sex: XY, XX.
Bipotential gonadY chromosomes encodes testis-determining factorSRYgonadal sextestis secretes testosterone
Secondary sex determinationphenotypic sex.
Absence of Y chromosome forms ovaries
--anti-Mullerian hormone(AMH)regression of paramesonephric duct
Wolffian duct forms male genital ducts.

14. Gonadogenesis
Gonadogenesis occurs at the genital ridge initiated by 2 simultaneous events:-
(i).Formation of gonadal cords
Degeneration of the mesonephric corpuscles form epithelial cords and invade genital ridge.
Proliferation of coelomic epithelium invade mesonephric epithelial and resident mesodermal cells.These proliferate forming network of epithelial cords/blastema.
PGC are large,endodermal and migrate from the yolk sac into the developing gonad

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

16. Mechanisms of mammalian primary sex determination
Several genes essential for sexual differentiationfemale is default gonad
--SRY located on Y-chromosome
--sex determination requires interaction between SRY and another factor: SOX9?.
Mode of action of SRY
--SRY could activate SOX9(transcription factor)
--SRY mediates expression of cell-surface male-specific antigen Y-histocompatibility antigen converting epithelial to Sertoli cells.
--SC secrete glycoprotein;clusterin which initiates testicular differentiation.
SOX9
--migrates into the nucleus, binds to promoter site of gene for AMH.
--activate FGF9?
Steroidogenic factor(Sf1) activated by SRY?
--Sf1 could activate Leydig cells and Sertoli cells
secreting testosterone and AMH respectively.
Wnt4 and Dax1 involved in ovary formation
--Dax1 suppresses testis formation and activates ovary development.

17. Sex Determination
(Modified from Gilbert 2006)

18. Definitive gonads:Testis
Y-chromosome produces Y-H which mediates transformation of the sexually indifferent gonad into testis.
The Y-H antigen reacts with gonadal cords to form Sertoli cells.
Gonadal cords hollow and anastomose into testicular cords enclosing mitotic gonocytes/prespermatogonia
Clusterin regulates aggregation of prespermatogonia in the medulla
The cortex develops a thick tunica albuginea, separates the testicular tubules from the germinal epithelium.
Testicular cords (TC) form loops and interconnect with mesonephric tubules(MT).
MT differentiate into peripheral seminiferous tubules, central rete testis and efferent ductuli.
WD form epididymis and ductus deferens.

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

20. Definitive Ovary formation.
In the absence of the Y chromosome in PGC.
Gonadal cords concentrate in the periphery forming the primary ovarian cortex(POC)
The POC degenerates
and more epithelial cords proliferate and secondary cortex forms.
The gonadal cords and germ cells in the medulla degenerate.
The medulla becomes stroma of vascular, lymphatic and nervous tissues
Surviving gondal cords form rete ovarii which serves as general morphogenic organiser.
Gonocytes continue mitosis fetal life and in early postnatal period.
Some gonocytes begin meiosis and progress to diplotene of prophase I, these become surrounded by single layer of follicular cells then meiosis is arrested till puberty
Gonadal cords break up
into primordial follicles containing meiotic and premeiotic germ cells (GC)
Meiosis in the fetal GC is triggered by meiosis-inducing substances in the ovary which are not species specific
Gilbert 2006)

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

22. Formation of the urogenital sinus[(UGS)fig.8]
The UGS is divisible into a cranial urethrovesicular and the pelvic UGS and caudal phallic UGS.
The UGS opens caudally into the amniotic cavity and cranially into the allantoic cavity via the urachus and
allantoic stalk.
With growth and expansion of the UGS the mesonephric and metanephric ducts open separately on dorsal wall of UGS at the trigone of bladder.
The urachus extends into the umbilical cord as the allantoic stalk.
Differentiation of the UGS.
In the male,the proximal UVUGS expands into the bladder, the distal part forms the pelvic urethra and the phallic UGS forms the penile urethra.
In the female the short urethra forms from the proximal UVUGS, the distal UVUGS and the phallic form the vestibule and part of the vagina.

23. Conginetal Malformations.
Renal agenesislack of development.
Dysplasiawide range of abnormal development of nephrons and collecting ducts, form cysts
Hypoplasiasgenosis of ureterhydronephrosis.
Renal hyperparathyroidism.
Ectopic kidneys and ureters.
Polycystic kidneysfailure of nephrons to join collecting ducts?
Rectovaginal constrictionhereditary, anovestibular stenosis.
Vesicourachal diverticulum--<outpockecting of bladder when urachus fails to close.
Cryptorchidismfailure of normal descent of testis.
Mesonephric or paramesonephric duct abnormalitiesstenotic/aplasia of duct.
Pseudohermaphroditegonads of one sex and external gentalia of the opposite sex, hormonal factors.
True hermaphroditesgonadal defects at early stage of development, separate gonads or ovo-testis, a chromosomal abnormality.
Freemartin cattle intersexuality, a genotypic female(XX) born with normal male co-twin. Fused placental vesselstransport of foetal androgens to female.

24. Summary
The urinary system and genital system are derived from the intermediate mesoderm,the nephrogenic plate and genital ridge of the urogenital ridge.
The kidney develops in three stages in rapid successionpronephros, mesonephros and metanephros in cranial-caudal direction.
The pronephros is of limited function in mammals but acts as inducer of the mesonephros.
The metanephros forms from two precursorsthe metanephric diverticulum and the metanephrogenic mass.Development of the metanephros requires reciprocal tissue interaction using several molecular signals.
Sex determination is dependent upon interaction of several factors__. Genetic sex, gonadal sex, hormonal action and phenotypic sex.
Germ cells are endodermal, the other tissues are mesodermal. Early in development, germ cells migrate from the yolk sac to the genital ridge.
Development of the urogenital system is dependent on molecular signals.
The external urinary and genital organs are formed from the cloaca and urogenital sinus.
The Wolffian and Mullerian ducts form the adult male and female ducts respectively.

25. References
1. Gilbert, Scott.F(2003). Developmental Biology. 6th.Edition.
pp , , , Sinauer Associate. Massachusetts.
2. McGeady, T.A., Quinn, P.J., Fitzpatrick, E.S., & Rayan, M.T., (2006).
Veterinary Embryology. Page ;
3. Noden, M. and de Lahunta(1985). The Embryology of Domestic Animals, pp , Williams and Wilkins, London.