05.28.09(b): development of the urinary system

Author(s): Matthew Velkey, 2009

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Development of the Urinary System

Matt Velkey

Spring 2008

The Origin of the Kidney

In the vertebrate embryo, the first stage of kidney development occurs after gastrulation, within a region called the intermediate mesoderm.

Epiblast

Ectoderm

Mesoderm

Endoderm

Neural tubeNeural crest

Paraxial Mesoderm

Intermediate Mesoderm

Lateral plate Mesoderm

Gut, pancreas, liver, lung, etc.

Muscle, Bone, Skin

Mesenteries, HemangioblasticHeart, limbs, etc.

Kidney, Gonads, Reproductive organs

M. Velkey

Intermediate Mesoderm

The midline NOTOCHORD secretes factors that organize the dorsal-ventral and medio-lateral axes of the embryo.

Gilbert, Scott. Developmental Biology. 2006.

The first epithelial component of the urogenital system is the NEPHRIC DUCT.It arises within the intermediate mesoderm adjacent to the 10-12th somitesand extends posteriorly.

DiI lineage tracing in the chick embryo reveals that the Nephric Duct formsby extension rather than by recruitment of mesoderm to the epithelial duct.

Obara-Ishihara et al., Develop. 126, 1103 (1999)

As the Nephric duct forms and grows caudally, Lim1 and Pax2 expression mark the epithelium of the duct.

Lim1Pax2

Lim1 is essential for nephric duct formationPax2 and its related gene Pax8 are also essential for nephric duct formation

Carlson. Human Embryology and Developmental Biology. (Both Images)

Overview of early Pro-, Meso-, and Metanephric patterning

The outgrowth of the Ureteric bud, or metanephricdiverticulum is the first critical step in the developmentof the adult kidney.

The ureteric bud epithelia and the adjacent metanephricblastema, or metanephric mesenchyme, are the twoprimordial cell types of the adult kidney.

Reciprocal inductive interactions between the uretericbud epithelia and the metanephric mesenchyme generate the nephrons, collecting ducts and the three dimensional architecture of the kidney.

Errors in these interactions may result in renal dysfunction/agenesis

Effects of renal dysfunction: oligohydramnios and “Potter sequence”

• Amniotic fluid produced via activity of the kidneys; renal dysfunction therefore causes decrease in amniotic fluid production

• Reduction in amniotic fluid results in increased mechanical pressure on fetus (e.g. from uterine musculature) thus affecting limbs, face, and even growth of internal organs, particularly the lungs.

Overview of metanephric kidney induction

• Mesenchyme in blastema expresses WT1 (makes tissue competent to respond to signaling from epithelium of ureteric bud), and also produces glial-derived neurotrophic factor (GDNF) and hepatocyte growth factor (HGF )

• Ureteric bud expresses a GDNF receptor (aka RET) and an HGF receptor (aka MET) and responds by secreting BMP7 and FGF2, which stimulates proliferation of mesenchyme

• Mesenchyme undergoes mesenchymal-to-epithelial transition in response to WNT4 and PAX2

• Mutations in any one of these may cause inhibition of ureteric bud growth and renal agenesis.

Langman’s Medical Embryology, 9th ed. 2004.

The GDNF protein isSufficient to induceUreteric bud outgrowth Within the posteriorAspect of the nephric Duct.

Source Undetermined

RET continues to be expressed in the tips of the branching ureteric budsThis is where proliferation and induction are ongoing

RET

Induced mesenchyme (Nephron Precursors)

Thus, the number of induced nephrons is proportional to the number of branch points of the developing ureteric bud.

M. Velkey

The expression of RET is regulated by retinoids through cell-cell signaling.Stromal cells express retinoic acid receptors and relay a signal to the ureteric bud that maintains RET expression.

RA receptors

Retinoic Acid

RET

*Vitamin A deficiencyleads to smaller kidneys with fewer nephrons.

M. Velkey

Summary of branching morphogenesis and metanephric induction

•The RET/GFR1/GDNF signaling complex regulates ureteric bud outgrowth and branching morphogenesis.

•Expression of RET in the ureteric bud tip requires signals from the stromal cells.

•These signals are controlled, at least in part, by retinoic acid receptors.

•The number of branches and branch points directly determine the number of nephrons in the adult kidney.

Next steps: transforming mesenchyme into epithelial tubes

Cytokeratin Pax2 Laminin Cytokeratin

The Pax2 gene is a critical factor in the mesenchyme that controls the

Response to inductive signals

This response is commonly called the Mesenchyme-to-Epithelial Transition (MET)

Source Undetermined Source Undetermined

In Pax2 null embryos, the Nephric Duct forms but there are no Pro- or mesonephric Tubules, there is no ureteric bud and there is no metanephros. Pax2 is required for the Mesenchyme to respond to inductive signals.

mesonephros

Metanephric mesenchyme

Ureteric bud

cloaca

Antibody staining Pax is red Epithelial cytokeratin is green

Nephric duct

Source Undetermined

Loss of Pax2 Function:

Mice homozygous for a Pax2 null allele have complete renal agenesis

Humans carrying one mutant Pax2 allele (heterozygotes) exhibit -Renal-Coloboma Syndrome

In both mice and humans, Pax2 is haplo-insufficient*, i.e. a reduction in Gene dosage results in a phenotype of varying penetrance.

Thus, the mutations are dominant because both normal Pax2 alleles are required for normal kidney development

*This is also true for other Pax genes, such as Pax6 (aniridia) in the developing eye

CLINICAL CORRELATES

Renal-Coloboma Syndrome

Patients typically exhibit at least these 3 symptoms:

1- Renal hypoplasia - due to reduced proliferation of the mesenchyme derived epithelia during development.

2- Vesicouretral Reflux - most likely due to improper connection of the ureter to the bladder or possibly due to inherent defects in epithelial cells of the mature ureter. 3- Optic Nerve Colobomas - due to failure of the optic fissure to fuse. Expression of Pax2 is observed in part of the optic cup and optic stalk.

Pax2 Marks Proliferating Epithelia in a Variety of Renal Diseases.

These include: Renal Cell CarcinomaWilms’ TumorJuvenile Cystic and Dysplastic KidneysPolycystic Kidney Disease

Also, Pax2 is reactivated in regenerating proximal tubules after ischemia or toxic injury.

Pax2 Gain of Function

Thus, failure to suppress Pax2 during development or reactivation of Pax2 in the adult can lead to aberrant proliferation of the renal epithelia.

Eya1 is expressed in the metanephric mesenchyme and is a transcriptionalco-factor that interacts with a variety of DNA binding proteins, including Pax2.

Homozygous Eya1 mice have no kidneys because the ureteric bud fails to grow and the metanephric mesenchyme undergoes apoptosis.

In humans, mutations in Six genes are also associated with Branchio-Oto-Renal Syndrome, underscoring the importance of the Eya-Six genetic interactions.

Source Undetermined

Branchio-Oto-Renal Syndrome (BOR)

Renal anomalities include:Unilateral or bilateral hypoplasia, dysplasia, and aplasia,duplicated or absent ureters, hydroureter, distorted calyces.

Hearing Impairment can be mild to severe, with defects found in:atresia or stenosis of auditory canal, absent or underdevelopedcochlea, absent or abnormal semi-circular canals.

Cranio-Facial abnormalities include cervical cysts and fistulas

BOR is caused by mutations in the Eya1or Six1 genes.

As with Pax2, BOR is a dominantly inherited disease becauseEya1 is haploinsufficient.

Other critical regulators of the Mesenchymal-to-epithelial transitionInclude:

WT1- Wilms tumor suppressor gene

Wnt-4 - secreted signaling molecule

BMP7 - secreted signal

BF-4 - stromal transcription factor

Pax2 WT1

Expression of WT1 is dynamic, with low levels of protein in the mesenchyme and increasing amounts in the podocyte precursors of the glomerulus.

The WT1 gene is essential for early kidney development - mesenchyme induction

WT1 also plays an essential role in differentiation and maintenance of thePodocytes.

Source Undetermined

Source Undetermined

Wilms’ Tumor (Nephroblastoma)

• Found in infants from 0-24 months of age• Consists of blastemal, epithelial, and stromal cell types• Given the age of onset and the differentiation potential of tumors, it is

likely that they arise from mutations in one or more genes that function during kidney development.

• There are both familial and sporadic forms of Wilms’ tumor• Deletions or mutations of WT1 are associated with 10-20% of sporadic

Wilms’ tumors• Additional tumor suppressor genes may map to 11p13, IGF2, H19 etc.

Source Undetermined

Source Undetermined

Polarization of the metanephricmesenchyme is a step-wise processthat requires changes in cell-adhesion molecule expression, theformation of adherends junctions,the deposition of a laminin containingbasement membrane, and the formationof tight junctions.

This establishes the apical and baso-lateral sides of the epithelial cell

Failure to make or maintainpolarized epithelia results inrenal cysts.

Abnormalities of Epithelial Cell Differentiation

Cystic Diseases

- Cysts are derived from epithelial cells- Are due to Proliferation and Accumulation of Fluid- Are associated with a Loss of Epithelial Cell Polarity- Commonly occur in multiple tissues, such as kidney pancreas and liver

Polystic Kidney Disease (PKD) is the most common single gene Disorder in the adult population. About 1/3000 individuals Have a mutation in the associated genes. Patients have bothRenal and pancreatic cysts.

There are dominant and recessive forms of PKD.

The most common is autosomal dominant PKD (ADPKD)

PKD

PKD is characterized by the progressive accumulation of renal cysts

Cysts are large fluid filled epithelial spheres that ultimately can displace the normal tissue and lead to end stage renal disease.


egf

Proliferation

Blockage & budding

Loss of Polarity

EGF receptorNa,K ATPase

Mechanism of Cyst Formation

basal

apical

M. Velkey

Polycystic kidney disease and PKD1 (polycystin-1)

Polycystin-1 is an essential protein that maintains polarity and/or fluidtransport across the renal epithelia. Mechanism of disease is probably similar in humans.

E18 8 days

Heterozygous mice show some cystogenesis later in life, i.e. 18-24 months


Glomerular Development

Source Undetermined

Source Undetermined

afferentarteriole

efferentarteriole

podocytesperietalglomerularepithelium

thick ascendinglimb

distaltubule

collectingduct

proximaltubule

Henle'sloop

renalvesicle

uretericbuds

Collagen IV2 x 1 & 2

Collagen IV

During development of the GBM, there is a switch in Collagen gene expression from the 1 and 2 chains to 3, 4, 5 chains. There is a switch from laminin 1, 1 to 5, 2

The glomerular basement membrane (GBM)

M. Velkey

Normal GBM Alport’s Syndrome

- thickening of GBM- separation of layers- electron dense granules- fusion of foot processes

Failure to Switch Collagen IV Chains Results in Alport’s Syndrome

Source UndeterminedSource Undetermined

Interdigitated Podocyte footprocesses form the pores that allow small molecules to flow into the urinary space but retain larger proteins in the blood stream.

Source Undetermined

Source Undetermined

Slit DiaphragmFoot Processes

Endothelial Fenestrae

Basement Membrane

The Mature Filtration Barrier

Source Undetermined

Development of the filtration barrier depends on endothelial, podocyte, andGBM interactions.

a- Podocyte and endothelial precursor cells align on opposite sides of GBM

b- Cells begin to secrete GBM specific extracellular matrix proteins

c- Podocytes extend primary, secondary, and tertiary foot processes that inter- digitate to make slit diaphragms. Endothelial cells become fenestrated.

Source Undetermined

Human Congenital Nephrotic Syndrome (CNF)

• hereditary, with different degrees of severity• early onset, from birth to adolescence• characterized by high proteinuria, leakage of protein through the GBM filter• microcysts and expanded Bowman’s spaces• foot process effacement• associated with mutations in nephrin (NPHS1)


Nephrin and Podocin interact at the slit diaphragm to maintain the filter pore and/or the integrity of the foot processes.

Source Undetermined

Conclusions

Mutations in nephrin or podocin cause congenital nephrotic syndromes

Foot process effacement is a common feature among CNS and leads to high proteinurea

Both proteins are localized to the slit diaphragm and are membrane associated.

The extracellular domains of Nephrin may help maintain the pore size

Other malformations: pelvic kidneys and horseshoe kidney

• Kidneys form near the tail of the embryo.• Growth of the embryo in length causes

the kidneys to “ascend” to their final position in the lumbar region.

• One kidney might fail to ascend• Fusion of the two kidneys into a single

horseshoe kidney impedes ascent beyond the inferior mesenteric artery




Other malformations: abnormal ureter attachment sites

Relationship of mesonephric (Wolffian), paramesonephric (Müllerian), and metanephric (ureteric) ducts is such that the ureters attach at sites other than the bladder.


Image of abnormal ureter attachment

sites removed.

Original image: Carlson - Human Embryology and Developmental Biology, 4th Edition.

Other malformations: urachal cysts and fistulas


M. Velkey

Slide 5: Gilbert, Scott. Developmental Biology. 2006.

Slide 6: Obara-Ishihara et al., Develop. 126, 1103 (1999)

Slide 7: Carlson. Human Embryology and Developmental Biology. (Both Images)

Slide 8: Carlson: Human Embryology and Developemental Biology, 4th Edition. Copyright 2009 by Mosby, an imprint of Elsevier, Inc.


Slide 11: Langman’s Medical Embryology, 9th ed. 2004.

Slide 12: Source Undetermined

Slide 13: M. Velkey

Slide 14: M. Velkey


Slide 17: Source Undetermined; Source Undetermined






Slide 27: Carlson: Human Embryology and Developemental Biology, 4th Edition. Copyright 2009 by Mosby, an imprint of Elsevier, Inc..


Slide 31: M. Velkey




Additional Source Information for more information see: http://open.umich.edu/wiki/CitationPolicy

M. VelkeySlide 36: Source Undetermined; Source UndeterminedSlide 37: Source Undetermined; Source UndeterminedSlide 38: Source UndeterminedSlide 39: Source UndeterminedSlide 40: Source Undetermined; Source UndeterminedSlide 41: Source UndeterminedSlide 43: Langman’s Medical Embryology, 9th ed. 2004.; Langman’s Medical Embryology, 9th ed. 2004.; Langman’s Medical Embryology, 9th ed. 2004.Slide 44: Langman’s Medical Embryology, 9th ed. 2004; Original Image Carlson: Human Embryology and Developemental Biology,4th Edition. Copyright 2009 by Mosby, an imprint of Elsevier, Inc. Slide 45: Langman’s Medical Embryology, 9th ed. 2004; Carlson: Human Embryology and Developemental Biology, 4th Edition. Copyright 2009 by Mosby, an imprint of Elsevier, Inc.

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