#13362 - Kidney Development Essay - Organisation of the Body

How do the kidney and urinary tract develop? What common anomalies can occur?

The main function of the urinary system is to maintain electrolyte and water balance and also excrete toxic metabolites such as urea and ammonia. Before birth the fetus excretes copious amounts of dilute urine but after birth the loops of Henle lengthen and the urine becomes more concentrated. The urinary system develops from 2 different germ layers; the kidney and the ureter develops from the intermediate mesoderm which also gives rise to structures such as the suprarenal glands and gonads whereas the bladder and the urethra arises from the endoderm of the hindgut.

1) Development of the three nephric systems

The development of the functional kidney relies on the formation of 3 embryonic kidney systems that develop in the intermediate mesoderm in a craniocaudal sequence. These systems are developmentally distinct both temporally, as they grow and regress at different times and spatially as they are confined to different regions along the anterior posterior axis. First is the formation of the pronephric kidney which induces the formation of the mesonpheric kidney and this then induces the development of the metanephric kidney. In lampreys and hagfishes both the pronephros and the mesonephros are active in the adult but in humans the pronephros and mesonephric renal corpusles regress generating a non segmental adult organ in the metanephric region.

The key transcription factor that is needed for the development of the nephrogenic intermediate mesoderm is Pax 2. This was experimentally proven when the Pax 2 gene was expressed in non nephrogenic regions of the intermediate mesoderm by viral transfection it resulted in ectopic nephric structures. Further supporting this evidence is knock out of Pax 2 in transgenic mice which resulted in the absence of the metanephric region resulting in no kidney. The expression of Pax 2 is thought to be due to signalling factors released from both the somites and ectoderm. This is because when the somites and ectoderm were removed it resulted in decreased gene expression of Pax2 in the nephrogenic region.

The anterior posterior patterning of the three embryonic kidney systems relies on gradients of gene expression. This ensures pronephros, mesonephros and metanephros form in the correct sequence and location.

a) Formation of Pronephros

The first stage in the development of the three kidney systems is the formation of the mesonpheric duct which starts to form during 4^th week in the region of C5 to C7 in the intermediate mesoderm. The cells within this region undergo mesenchymal epithelial transformation resulting in the formation of 2 solid longitudinal ducts on the left and right sides of the embryo. Through proliferation and recruitment of cells to the caudal ends the mesonpheric ducts grow in a caudal direction and eventually leads to the duct fusing with the cloaca. The point at which the duct makes contact with the cloaca, apoptosis occurs which leads to canalisation and this proceeds in a cranial direction and forms the wolffian duct.

The next stage is the formation of the pronephros which occurs as the cervical region of the wolffian duct induces the adjacent mesoderm to undergo mesenchymal epithelial transformation. However unlike animals such as lamphreys and amphibians, in humans the pronephric buds become hollow and regress in a cranio caudal sequence and are nonexistent by day 28.

b) Formation of the mesenophros

In the region inferior to the pronephric region, the mesonpheric duct initially induces the formation of 40 mesonpheric tubules which also develop in a craniocaudal sucession from the upper thoracic region to L3. However as the more caudal tubules begin to differentiate the cranial tubules regress so by the end of mesonpheros formation there are only 20 mesonephroi in the region of L1 to L3. These mesonephroi then differentiate into functional excretory units known as mesonpheric renal corpuscles. At the medial end each corpuscle is made up of a Bowman’s capsule which encircles a glomerulus and at the lateral end the tubule fuses with the wolffian duct. The structures remain functional between week 6 to 10 but regress soon after. In females they degenerate completely due to that lack of testosterone. However in males, the caudal most mesonpheric renal corpusles lead to the formation of the efferent ductules in the genital duct system.

c) Formation of the metanephros

While the mesonephros is still functional, the metanephros, which gives rise to the definitive kidneys in humans, begin to develop. The caudal end of the mesonpheric duct is induced by the nearby sacral mesenchyme in the intermediate mesoderm, known as the metanephric blastmea, to evaginate and form another branch, known as the ureteric bud. This ureteric bud gives rise to the ureter in the developed urinary system. As the ureteric bud grows it penetrates the metanephric blastema which again induces the bud to bifurcate. At the first contact, the ureteric bud enlarges to form the initial ampulla which gives rise to the renal pelvis. During 6^th week the ureteric bud birfucates 4 times giving rise to 16 branches when then coalesce to form the 4 major calyces. In a similar pattern the major calyces birfucate four times and then fuse to form the minor calyces. Finally during the 32^nd week there are 11 additional generations of birfucations and these become the future collecting ducts. The branching of the ureteric bud was experimentally seen through the use of Hoxb7/GFP transfene which proved the branching morphogenesis. One of the key problems that can arise from the branching of the ureteric bud is the formation of a Y shaped bifid ureter which occurs if the ureteric bud bifurcates prematurely. As the muscular walls contract asynchronously in both ureters the major problem is the reflux of urine into the other causing stagnation of the urine which increases the likelihood of ureter infections.

One of the key signalling factors released from the metanephric mesenchyme that induces the budding and bifurcation of the ureteric bud is GDNF which acts as a ligand to the receptor RET and co-receptor GFRA both of which are expressed along the mesonpheric duct. The actions of GDNF were shown when misexpression of this signalling factor elsewhere in the intermediate mesoderm led to the formation of ectopic ureteric buds. Similarly the receptors where shown to have a role in the ureteric bud birfucations as transgenic mice that were deficient in RET and GRFA had bilateral renal agenesis. The caudal positioning of the ureteric bud is due to inhibitory factors such as BMP4 released elsewhere in the mesoderm that suppresses RET signalling. This was shown as BMP deficient mice developed ectopic ureteric buds or had double ureters.

As the ureteric bud grows and bifurcates in the metanephric blasteme it also releases signalling factors that induce the surrounding mesenchyme to condense around the tips of the branches and form a metanephric blastemal cap. The cap which initially consists of mesenchyme surrounded by extracellular matrix is induced to increase the gene expression of E CADHERIN which causes it to become reoorganised into an epithelial vesicle which is also known as the nephric vesicle. When viewed under a microscope, the vesicle is first seen as a comma that then develops into an S shaped tubule as it proliferates. Then at the collecting tubules stage it fuses with the ureteric duct to form a continuous lumina and is known as the urinefrous tubule. This tubule becomes the definitive nephron, which is the functional metanephric excretory unit and differentiates into the bownman’s capsule, proximal and distal convoluted tubules and loops of Henle.

The key signalling factor released by the ureteric bud that induces the mesenchymal epithelial transformation is Wnt9b. In transgenic mice with knock out of Wnt9b, the ureteric bud undergoes initial branching but the surrounding mesenchyme fails to condense. In the presence of Wnt9b, there is induced expression of Wnt4 in the metanephric blastema. Knock out of Wnt4 results in the mesenchyme undergoing apoptosis which also results in the inhibition of branching of the ureteric bud. The condensation of the nephrogenic mesoderm is essential as it maintains the reciprocal induction between the ureteric bud and the metanephric blastema. Whilst the mesenchyme is condensing the expression of WT1 is upregulated which is important in the formation of the nephric vesicle but also ensures continuous bifurcation of the ureteric tube. Inactivating mutations in the WT1 gene, which is a tumour suppressor gene leads to the formation of a kidney tumour, also known as the Wilms tumour.

In summary the definitive kidney is made up of 2 functional components, excretory and collecting which are derived from the ureteric bud and the metanephric blastema respectively. The origins of the different parts of the kidney is shown in the table below. The key evidence that the ureteric bud and mesenchyme contribute to separate lineages and the role of reciprocal induction between these 2 distinct regions of the intermediate mesoderm was shown by Saxen in 1987. Through microsurgery Saxen found that when the mesenchyme and ureter bud were separated and grown in vitro cultivation there was a standstill in the morphogenesis of both components. But when they were recombined in vitro with a thin membranous filter between them, it triggered both epithelial conversion of the mesenchyme and branching of the ureter derived...