#13351 - Urinary Development - Organisation of the Body

Urinary development

Function: excretion of metabolic waste, especially nitrogenous waste-10% ammonia, urea (less toxic and concentrated-less water)

-Before birth- copious dilute urine, after birth loops of Henle lengthen and urine becomes more concentrated.

-intermediate mesoderm gives rise to nephric lineage- suprarenal glands, gonads, genital duct

1) Development of the three nephric systems

-Intermediate mesoderm gives rise to 3 embryonic kidney structures which develop in a craniocaudal sequence

-Pronephros, mesonephros, metanephros

-These systems are developmentally distinct in two ways: temporally (as they grow and regress at different times) and spatially (as they are confined to different regions along the AP axis)

- Regression of the pronephros and mesonephric renal corpuscles generates the non-segmental adult organ

Experimental

-Pax 2-transcription factor- expressed in nephrogenic intermediate mesoderm - potent initiator of nephron development- ectopic nephric structures induced in non nephrogenic regions, anywhere in the intermediate mesoderm when expressed by viral transfection of mesoderm in chick embryos

-expression of Pax2 could be due to signalling factors released from somites and ectoderm- when separated decreased gene expression

- KO of Pax2 in mice still form a mesonephric duct in pronephric and mesonephric region but doesn’t grow into the metanephric region-so metanephric kidney doesn’t develop. However when there is double KO out of both Pax2 and Pax8, There is no urinary system-redundancy in the system

Pax 2 mutations lead to renal coloboma, oligomeganephronia (fewer larger nephrons)

The hypothesis that the above genes are essential in matching the development of the urinary system to the position on the AP axis is further supported by the finding that they are all found in concentration gradients along the axis.

Gradients of gene expression along the AP axis specify positional identity, allowing renal structures to form topically. This ensures that the pronephros, mesonephros and metanephros form in the correct sequence and location. Regression of the pronephros and mesonephros result in the non-segmental adult organ, which develops from the metanephros. Therefore although the adult kidney does not appear segmented like the spine, this does not reflect its developmental origin.

a) Formation of Pronephros

-4^th week- intermediate mesoderm along the 5^th to 7^th cervical axial levels- undergo mesenchymal TO epithelial transformation which give rise to the mesonephric ROD -appears as a pair of 2 solid longitudinal RODS

-rods grow in a caudal direction due to the proliferation and recruitment of cells at the caudal ends

-The mesonpheric rod unites with the cloaca in the lumbar region- at which point regulated apoptosis creates a lumen (canalises) which progresses cranially forming the Mesonephric duct

-in the cervical region the rods induce adjacent intermediate mesoderm to condense and form epithelial buds/balls (pronephros)- not functional, as they don’t fuse with the mesonephric duct- become hollow and regress in a cranio caudal sequence and are absent by day 28

b) Formation of the mesenophros

-Caudal to the pronephric region the mesonpheric duct-induces formation of about 40 mesonpheric tubules in craniocaudal succession- upper thoracic region to L3- leads to mesonephros-as more caudal tubules differentiate, the more cranial ones regress- so only about 20 mesonephroi-L1 to L3

-the mesoneprhic tubules differentiate into excretory units-Mesonpheric renal corpuscles similar to a shortened version of the adult metanephric nephron with medial end forming bowman’s capsule- wraps around a knot of capillaries called a glomerulus to form the renal capsule-lateral tip of each developing mesonephric tubule fuses with Mesonephric duct- allows a passage from the excretory units to the cloaca

-functional between 6-10 wks-blood supply from the paired dorsal aortae, after 10 weeks they cease to function and regress- in females they degenerate completely but in males the caudal most corpuscles contribute to the genital duct system (Efferent ductules?)

c) Formation of the metanephros

-Development of metanephros begins at end of Wk 4 while mesonephros is functional

-Definitive kidneys/Metanephroi- 2 functional components- excretory & collecting- derived from different sources of intermediate mesoderm-Reciprocal induction

-at the caudal end the mesonpheric duct is induced to form the ureteric bud-forms in the intermediate mesoderm of the sacral region- becomes the URETER

-each ureteric bud penetrates sacral intermediate mesoderm metanephric blastema- metanephric mesenchyme induces the urteric bud to grow and birfucate

-When the ureteric bud first contacts the metanepheric blastema- tip expands to form the initial ampulla that gives rise to the renal pelvis. 6^TH WEEK- the ureteric bud bifurcates 4 times yielding 16 branches- these branches coalesce to form 4 major calyces. 7^TH WEEK- next 4 generations of bifurcations fuse forming the minor calyces- 32^nd WEEK-11 addittional generations of bifurcation- become the future collecting ducts

-Branching of the ureter duct can be visualised by the use of Hoxb7/GFP transgene to visualise UB branching morphogenesis

If the ureteric bud bifurcates prematurely (could be because blastema is further away)- Y shaped bifid ureter- muscular walls contract asynchronously- so urine may reflux from one branch into the other- stagnation of the urine-infections of the ureter

Factors expressed in the metanephric mesoderm that regulate induction of budding and branching of the ureteric bud-Key signalling factor is Gdnf found in METANEPHRIC MESENCHYME is a ligand of the receptor Ret, Gfra expressed in the mesonephric duct

Evidence: Misexpression of GDNF elsewhere in the intermediate mesoderm is sufficient to induce ectopic ureteric buds, mice deficient in RET/GDNF- bilateral renal agenesis

Caudal positioning of the ureteric duct- due to inhibitory factors released elsewhere in the mesoderm, repressing induction in the more cranial regions- E.g BMP4 inhibits RET signalling. BMP4 signalling molecule deficient mice develop ectopic ureteric buds and double ureters

-As ureteric bud grows and branches into metanephric mesoderm it induces mesenchyme to condense around the tips of the ureteric branches/ampulla and form a metanephric blastemal cap

- the ampulla of the collecting tubule induces the metanephric blastemal cap-which initially is mesenchyme surrounded by extracellular matrix to becomes reoorganised into an epithelial vesicle- increased gene expression of E-CADHERIN

-cap differentiates into a nephric vesicle.

First develops as a comma S shaped tubule fuses with the uretic duct urinefrous tubule (continuous lumina) The tubule ultimately forms a definitive nephron (metanephric excretory unit) bowman’s capsule, proximal and distal convoluted tubules and loopes of Henle

-Mesenchymal-epithelial transformation relies on ureteric bud induced expression of Wnt4 in the blastema-Evidence: KO of WNT4 results in the mesnchyme undergoing apoptosis which leads to the inhibition of branching of ureteric bud.

condensation of the nephrogenic mesoderm ensures continual ureteric branching and growth-WT1 is upregulated in metanephric mesenchyme as it condenses during MESENCHYMAL-EPITHELIAL transformation-responsible for forming the nephric epithelial vesicles-ensures continuous birfacation

-Wilms tumour- mutations in WT1 gene- kidney tumour-inactivating mutations in tumour suppressor gene

-Due to reciprocal induction- number of nephrons depends on growth and branching of the ureteric bud

Saxen proposed ureteric bud and mesenchyme contribute to separate lineages- dissociation of the ureteric bud from the mesenchyme showed different structures- variation of the pore size in the filter- showed the prescence of reciprocal interactions

Functional metanephroi: Blood plasma from the glomerular filtrate is concentrated and converted into urine by the convultated tubules and loop of Henle- urine passes down the collecting system into the ureters-into bladder

-Fetal kidneys: Main function is not to clear waste products (that is done by the placenta)- but is important as it supplements the production of the amniotic fluid-maintains hydrostatic pressure and acts as a support cushion

a) Rengal agenesis: Insufficient amniotic fluid/oligohydroaminos- range of abnormalities-Potter sequence E.g deformed limbs, wrinkly, dry skin, abnormal facial appearence

Bilateral renal agnesis: infants are still born/die withing a few days of being born- potter sequence

-unilateral renal agenesis- babies often survive as the remaining kidney undergoes compensatory hypetrophy

-Due to: Faulty tissue interactions between ureteric bud and nephrogenic mesenchyme (Ureteric bud fails to contact metanephric mesenchyme 1/10000 births), complete failure of formation of kidney or can result in hypoplasia-smaller number of nephrons-inadequate branching of ureteric bud

b) Nephron pathologies

-Mutations in defects in podocyte processes surrounding capillaries/defects in basement membrane sepearting the two usually result in excessive protein loss into the urine Proteinuria

c) Polycystic kidney disease

Congenital

-Formation of cysts in the kidney- can be due to mutations in polycytin1...