Uropoeitika.doc

20.1 Formation of the urinary organs 

Development of the urinary organs (3rd - 4th week)

Development of the urinary organs (3rd - 4th week) The development of the urinary system is closely related to that of the genital systemwhich will be described, though, in a separate module. The urinary organs consist of the kidneys (which produce urine, among other things), the ureter (transport of the urine from the kidneys to the bladder), the bladder (temporary storage for the urine) and the urethra (transport of the urine from the bladder to the external world).

Quiz Quiz 07

Quiz Quiz 16

In all vertebrates the kidneys and ureters develop out of the intermediate mesoderm, whereas bladder and urethra derive from the urogenital sinus. Fig. 1 - Transverse section and dorsal view of an embryo (trilaminar) (ca. 21 days)

1 2 3 4 5 6 7 8 9 10 11 12

Legenda Fig. 1 Schematic diagram: Transverse section (in the midcephalic region) with a dorsal view of the threelayered embryo towards the end of the 3rd week of development. The intermediate mesoderm lies between the somites (paraxial mesoderm) and the lateral mesoderm (out of which the coelom arises).

Paraxial mesoderm Intermediate mesoderm Lateral mesoderm Notochord Amnion Intraembryonic coelom Endoderm Ectoderm Somatopleural (mesoderm and ectoderm) Splanchnopleural (mesoderm and endoderm) Neural groove Neural ridge

Due to the lateral folding, the intermediate mesoderm is shifted ventrally and loses its connection with the somites and the lateral mesoderm.

Quiz Quiz 01

The nephrogenic cord develops out of the intermediate mesoderm 9 and extends from the cervical to the caudal region. It becomes segmented like the paraxial mesoderm (somites) (Fig. 1). This segmentation is easily seen in the cranial region, and is rudimentary in the middle region. In the caudal region it isno longer present, as we will describe in detail below. Due to the growth of the inner structures of the embryo, the tissue lying most laterally is displaced ventrally. This leads not only to a separation of the nephrogenic cord from the paraxial and lateral mesoderm, but also to a coalescence of certain median structures such as the two dorsal aortas 12 which fuse to form the definitive (median) aorta.

Fig. 2 - Development of the intermediate mesoderm (stage 10)

1 2 3 4 5

Fig. 3 - Isolation of the nephrogenic cord (stage 11)

1 2 3 4

Neural tube Amniotic cavity Dermatomyome Nephrogenic cord that has moved away from the paraxial mesoderm (somite) (black arrow)

Fig. 2 Transverse section of an embryo at the beginning of the folding (ca. 28 days). In this diagram the nephrogenic cord is clearly distinguished in the intermediate mesoderm. The black arrow indicates the border between the paraxial and intermediate mesoderm.

Neural tube Amniotic cavity Paraxial mesoderm Intermediate mesoderm Yolk sac (umbilical vesicle)

Fig. 4 - Isolation of the nephrogenic cord (stage 12)

5 6 7 8

Legenda

Yolk sac (umbilical vesicle) Notochord Aorta Intraembryonic coelom

Legenda

Fig. 3, Fig. 4 The intermediate mesoderm moves ventrally and loses its connection (black arrow) to the somites and the lateral mesoderm. Also observe its approach towards the intraembryonic coelom

20.2 Development of the renal anlage   



Inleiding Pronephros (forekidney): transitory structure Mesonephros: transitory kidney o Mesonephros: first excretory organ

Metanephros: definitive kidney

Inleiding The kidneys develop along a cranio-caudal gradient. Typically, the development passes through three stages:

  

Pronephros Mesonephros

Color code for the urinary system module Urine excreting system: yellow-beigeorange

Metanephros

In principle, for all three developmental stages with similar designations, the same embryonic anlage - the intermediate mesoderm - is involved. The term "holonephros" can thus be employed to designate all the parts of the urinary system that arise from it. The first two developmental stages have a transitory character and the definitive kidneys actually develop from the metanephros stage.

Urine forming system: pink-lilac-violet

Pronephros (forekidney): transitory structure The pronephros develops during the 4th week, beginning in the cranial part of the nephrogenic cord and it atrophies during the 5th week. Three characteristic pronephros structures can be distinguished:

 

The pronephros duct The pronephros tubules

10

in the neck region 11

 The external or coelom glomeruli 13 that have been shown to exist in humans (1) Beginning with the 4th week, conforming to the cranioFig. 5 - Early pronephros development caudal gradient, the pronephros in the neck region (stages 10-12) divides into independent masses of cells, the nephrotomes 11 . Each nephrotome develops into anepithelialized pronephros glomerulus. Laterally, they form the pronephros tubules that can partly bind with the coelom. Via the fusion of these tubules between two nephrotomes the hollow pronephros duct arises that is the anlage of the pronephric collecting duct. In humans, this pronephros system corresponds more to a primitive and transient structure that is functionally of no importance. According to the classical view, the pronephros duct stops in the caudal region at the level of the 13th -14th somite and then goes over into themesonephric duct (Wolffian duct).

1 2 1+2 3 4 5 6 7 8

Nephrogenic cord Mesonephric duct (Wolff) Mesonephros Intestinal tube Cloaca Atrophying nephrotomes Yolk sac (umbilical vesicle) Allantois Outflow of the mesonephric duct into the cloaca

Meer info The origin of the mesonephric duct (Wolffian duct): At present it is not sure whether the mesonephric duct (Wolffian duct) arises from the joining up of these pronephros tubules or from the first mesonephric vesicles. (2)

Legenda Fig. 5 Schematic sagittal section through a 4-week-old embryo. One can recognize the segmented part of the nephrogenic cord, the pronephros and the unsegmented part, the mesonephros. In this stage 12 the mesonephric duct already discharges into the cloaca.

Mesonephros: transitory kidney The mesonephros differentiates itself during the 4th week and after the 8th week it degenerates. It replaces the pronephros and develops from three structures:

 

Nephrogenic cord

10

Mesonephric duct

11

Fig. 6 - Development of the mesonephros

1 2 1+2 3 4 5 6 7 8 9

Legenda Fig. 6 Sagittal section of a 5-weekold embryo. The pronephros atrophies while the mesonephric duct grows caudally and fuses with the cloaca wall. In this stage, it goes through a mesenchymal- epithelial transformation and forms a central lumen. Only the caudal part remains mesenchymal. Observe the ureter bud at its caudal end.

Nephrogenic cord Mesonephric duct Mesonephros Intestine Cloaca Atrophied nephrotome Yolk sac (umbilical vesicle) Allantois Outflow of the mesonephric duct into the cloaca Ureter bud (anlage)

Fig. 7 - Development of the mesonephros as the first excretory organ

Neural tube Notochord Aorta dorsalis Dorsal mesentery Intestinal tube Ectoderm Somite Inferior cardinal vein

Quiz Quiz 17

in the dorso-lumbal region

 Glomerular capillary network 13 The mesonephric duct forms on the dorsal side of the nephrogenic cord at the level of the 9th somite. Initially it consists of a solid mesenchymal cord of cells 11 . It releases itself from the nephrogenic cord and is finally localized under the ectoderm, which probably plays an inductive role in its formation (3). Released from the nephrogenic cord, it develops in the caudal direction andcanalizes itself at the same time 12 , in order to finally end in the cloaca. As soon as it is canalized one calls it themesonephric duct (Wolffian duct). At the site where the mesonephric duct (Wolffian duct) discharges into the cloaca, the rear wall of the bladderforms.

1 2 3 4 5 6 7 8

Quiz Quiz 02

Legenda

Fig. 7 Transverse section along A (see previous figure). The urogenital ridge projects into the lumen of the coelom. With the S-shaped mesonephric tubules the mesonephric duct (Wolffian duct) forms a transitory precursor of the adult excretory system. The medial end of the mesonephric tubule is closed and forms a funnel (Bowman’s capsule) that surrounds a tuft of capillaries (the glomerulus). The capillaries come from lateral branches of the dorsal aorta and drain into the inferior cardinal vein. This functional unit is also termed the excretory unit of the mesonephros.

9 10 11

Mesonephric duct (Wolffian duct) Mesonephric tubule Urogenital ridge

Quiz

Meer info The mesonephric duct (Wolffian duct) Animal experiments have shown that the growth and the caudal extension of the mesonephric duct depend on the presence of the extracellular molecule fibronectin (an integrin). The differentiation of the solid cord into a duct depends on the secretion of the BMP4 (bone morphogenetic protein 4) by the ectoderm (4).

Quiz 03

Mesonephros: first excretory organ Shortly after the differentiation of the mesonephric duct, through mesenchymal-epithelial transformation, the mesonephric vesicles 11 arise out of the nephrogenic cord, which represents a mesoderm condensate. Via the mesonephric tubules they connect up with the mesonephric duct 14 . This differentiation takes place bilaterally in the area between the upper thoracic region (Th1) and the lumbar region (L3). A cranio-caudal gradient is also visible here. To the extent in which new mesonephric vesicles and tubules develop caudally, the cranial elements begin to atrophy so that never more than 30 pairs exist in the mesonephros. They form the excretory system that closely resembles the adult nephrons.

Quiz Quiz 15

Medially, the mesonephric vesicle dead-ends in that it forms a funnel (Bowman's capsule). Each of these funnels surrounds a tuft of capillaries (glomerulus), which have also arisen in the nephrogenic tissue and come from lateral (visceral) branches of the dorsal aorta. They drain into the inferior cardinal vein. The capsule with glomerulus together form a renal corpuscle. A renal corpuscle and its associated tubule are called a nephron and the functional unit an excretory mesonephric unit. The production of urine begins in the mesonephros during the 6th week 17 . After the 10th week these nephrons become inactive and atrophy. While in the female all mesonephric tubules atrophy completely, in the male a few that lie caudally remainin order to develop into the testicular efferent ducts.

Metanephros: definitive kidney The metanephros develops from three intermediate mesoderm structures of the sacral region:  Ureter anlage 14  Metanephric vesicle 13 

Glomerular capillary network

13

The ureter anlage 14 is an epithelial diverticulum from the caudal part of

Fig. 9 - Development of the metanephros: 5th week

Legenda

the mesonephric duct (Wolffian duct) in the area of the first sacral vertebra (S1). The anlage intrudes into the metanephric vesicle and forms the extra- and intrarenal excretory passages. The metanephric blastema 13 corresponds to the sacral part of the nephrogenic cord below L3. It is mesenchymal tissue out of which the metanephric vesicles arise. From these originate the nephrons (= functional units of the kidneys). At present it is still not clear whether the glomerular capillary network develops through vasculogenesis (direct development of vessels from the metanephric vesicles) or through angiogenesis (development from existing vessels of the metanephros) (5).

1a Pronephros (atrophying) 1b Mesonephros (atrophying) 2 Mesonephric duct (Wolffian duct) 3 Nephrogenic cord 4 Ureter anlage 5 Metanephric blastema 6 Liver anlage 7 Cloaca

Fig. 9 Sagittal section of a 5week-old embryo development of the metanephros. In the caudal region of the nephrogenic cord one observes the development of the metanephrogenic blastema that is in contact with the ureter anlage. In this stage the pronephros has disappeared almost completely. The mesonephros is also in the process of atrophying.

20.3 The upper urinary tract: development of the metanephros   

Ureter anlage and metanephric blastema: reciprocal induction Morphologic differentiation of the urinary tract o The ureter anlage and the formation of the evacuating urinary tract o The metanephric blastema and the formation of the nephron Functional development



Ascent of the kidneys

Ureter anlage and metanephric blastema: reciprocal induction The interaction between the ureter anlage (epithelial tissue) and the metanephric blastema (mesenchyma) is of decisive importance for renal development. The development of the kidneys represents a classical model of a sequential and reciprocal induction between epithelium and mesenchyma. For this reason it is frequently used for investigating the molecular cell mechanisms that play a role in the entire organogenesis. Renal development comprises a whole series of developmental processes such as forming an epithelial tree structure, interactive tissue induction, differentiation, polarization, migration, cell adhesion and finally the epitheliomesenchymaltransformation. During the genesis of the metanephros the metanephric blastema first induces the

Quiz Quiz 21

branching of the ureter anlage, which, for its part, then lets the metanephric vesicleform into a predetermined blastema. Through the transformation into epithelial tissue, the renal tubules form and finally the nephrons emerge. The most recent molecular-biologic research, mainly on transgenic mice, has shown that several factors are involved in this process. These various factors can be gone into only briefly in this chapter; those wishing to dig deeper can have a look at the cited references. Meer info Genes and their products that are important in the formation of the upper urinary tract.

Morphologic differentiation of the urinary tract The ureter anlage and the formation of the evacuating urinary tract The ureter anlage 14 is an epithelial diverticulum which arises from the caudal portion of the mesonephric duct (Wolffian duct) at the level of the first sacral vertebra. Its enlarged end grows in the dorso-cranial direction and projects into themetanephric blastema. It is the origin of the intra- and extrarenal excretory passages:  Ureter  Renal pelvis  Major and minor calices



Quiz Quiz 04 Quiz Quiz 08 Quiz Quiz 09

Collecting ducts

Fig. 10 - Schematic cut through the kidney (sagittal section)

Fig. 11 - Enlargement of the inset in Fig. 10

Legenda Fig. 10, 11 The branching of the collecting ducts and calices from the renal pelvis is shown. The enlargement in B shows the collecting duct and the various elements that form the nephron. Meer info Histological picture of the vessel architecture of the renal cortex

1 2 3 4 5 6 7

Ureter Major calix Minor calix Renal pelvis Collecting duct Metanephric vesicle Kidney lobe

8 9 10 11 13

Distal tubule Proximal tubule Glomerulus Connecting tubule Intermediate tubule

Meer info Meer information on fig. 11.

The cranial end of the ureter anlage subdivides dichotomically many times due to the inductive effect of the metanephric blastema.

Fig. 12 - Development of the metanephric outflow

Legenda Fig. 12 The renal excretory passages develop from the ureter anlage. This development is described in greater detail in an interactive diagram of region C (see below).

1 2 3 2+3 4 5 4+5

Cloaca Ureter anlage Metanephric blastema Metanephros Mesonephric duct (Wolffian duct) Nephrogenic cord Mesonephros

The renal pelvis arises from the swollen end of the ureteric bud 16 that subdivides 4 times over the course of the 4th to 6th weeks. 16 branches arise from this that partially then coalesce again in order to finally form 2 to 4 major calices. At around the 7th week the minor calices start to develop. They discharge into themajor calices. Further dichotomic branching follow - up to the 15th generation (roughly 32 weeks). Thereby the caliber of the tubules is reduced more and more so that finally 1-3 millions collecting ducts are formed in the periphery of the metanephric blastema 18 .

The metanephric blastema and the formation of the nephron The process of nephron formation is complicated and thus subdivided into various steps:



The metanephric blastema surrounds each newly formed collecting duct. It condenses in order to form peritubular cell aggregates.



Through induction signals (see: beginning of the chapter), derived from the ureter anlage, the mesenchymal cells transform themselves and form vesicles.



These vesicles grow longer and form an "S"-shaped tubule with three sections. o Development of the distal section into the distal part (distal convoluted limb, thick ascending limb and thin ascending limb) o Development of the middle section into the proximal part (thin descending limb, proximal straight and convoluted tubule)

o

Development of the proximal section in the glomerular capsule

Quiz Quiz 10

Meer info Histology of the renal corpuscle Scanning electron microscopy (SEM) of a vascular cast

21 . The epithelial vesicle secretes angiogenic factors. Thereby, over the course of the further development, endothelial cells are brought into the glomerular capsule. As soon as the afferent vessels come into close contact with the vesicular epithelium, it flattens and forms a cup with a bilaminar structure, Bowman's capsule (formation of the renal corpuscle: interactive diagram, overview over all the pictures).  At the same time as the formation of the renal corpuscle, the distal end of the epithelial vesicle fuses with the neighboring collecting duct. The metanephros thus becomes able to function and can filter the plasma from the glomeruli. Through the proximal tubule the glomerular filtrate (primary urine) gets into the intermediate tubule, the distal tubule, connecting tubules and collecting duct. In these tubules the secondary urine arises through resorption and secretion processes. It then reaches the renal pelvis and, via the ureter, the bladder. During the pregnancy, the fetal urine is excreted into the amniotic cavity. Physiology of the kidneys The fetal kidneys do not have to maintain the water and electrolyte households in the organism nor do they have to clean the blood from substances (mainly end products of cellular metabolism in the form of urea, uric acid and creatinine ) that are usually eliminated with the urine. These functions are performed by the placenta. The kidneys are, however, involved in the production of amniotic fluid. This is why an agenesis of the kidneys on both sides leads to an oligohydramnios (deficiency of amniotic fluid), which is described in greater detail in the chapter on pathology. Naturally, besides their excretory function, the kidneys also have an endocrine one in that they secrete hormones (renin, erythropoetin, prostaglandin and kallikrein). Even though some of the endocrine functions only begin at birth, the embryonic kidneys nevertheless have an important task to fulfill, namely the production of erythropoetin. In addition, renin-positive cells can already be found in the mesonephros of 5-6 week old embryos (19).

(following corrosion of adherent tissue).

Meer info Renal diuresis in fetuses and newborns.

Quiz Quiz 11

Summary:  The urine-forming system (filtration, resorption and secretion): distal, intermediate, and proximal tubules and the renal corpuscles (Bowman's capsule with capillaries) arise from the metanephric blastema of themetanephros.



The urine excreting system - the ureter, the renal pelvis, the major and minor calices as well as the collecting ducts - arise from the ureteric bud anlage from the Wollfian duct.

Functional development

Quiz Quiz 05 Quiz Quiz 24

At birth the kidneys have a multilobular appearance, due to the development of

Fig. 13 - Development of the smooth kidney surface

Legenda

the ureter anlage in the metanephric blastema. Normally, towards the end of the fetal period, the lobes are considerably smoothed, but they still exist until after birth. Completion of the smoothing follows during childhood by the increase in volume of the connective tissue and theincrease in size of the nephrons without any change in their number. With only few exceptions, adult kidneys no longer exhibit any lobulation. Since the renal architecture is finalized between the 5th and 15th weeks of intrauterine development; organogenesis of the kidneys lasts well beyond the embryonic phase until far into the fetal period.

Fig. 13 In humans renal lobulation is easily seen only in the embryonic and fetal periods, but can extend into childhood. At birth, however, it is strongly attenuated. In adults, the cortical zones of the individual lobes, "A to E", fuse and the smoothing of their depressions leads to a smooth and uniform renal surface structure.

1 2 3

Renal medulla Calix minor Renal cortex

Fig. 14 - Anatomy of the kidney at the end of its development

Legenda Fig. 14 The renal parenchyma is divided into an outer cortical zone and an inner medullar zone. In its interior the kidney has a hollow space, the renal sinus, into which the papillas intrude and where the trunks of the renal vessels are found. Meer info Histology Quiz Quiz 12

1 2 3 4 5 6

Ureter Renal pelvis Vena renalis Arteria renalis Major calix Minor calix

7 8 9 10 11 12 13

Cortex Capsula renis Medullary rays Papilla renalis Sinus renalis Columna renalis Medullary pyramid

Fig. 15 - Vascular supply of the kidney at the end of the development

Legenda

Fig. 15 Detail of the vascular supply of a renal lobe. One sees the renal pyramid surrounded by the interlobar vessels as well as their continuations, the arcuate vessels. From them originate the interlobular vessels that form the afferent arterioles, followed by a capillary network (glomerulus; not shown here).

Quiz Quiz 06 10 14 15 16 17 18

Papilla renalis Interlobar artery Interlobar vein Arcuate artery Arcuate vein Interlobular arteries and veins

Ascent of the kidneys The metanephros is formed in the sacral region at the level of the first sacral vertebra (S1) and the bifurcation of the aorta. In the adult, the kidney lies at the upper lumbar level. The "migration" (ascent) of the kidney occurs between the 6th and 9th week and the kidneys finally come to lie at the level of the 12th thoracal vertebra (Th12) under the suprarenal glands. The mechanism that leads to this ascent of the kidneys is not an active migration but rather much more the result of the differing growths of the sacral and lumbar regions, which lead to an unfolding of the lower pole of the embryonic body. During its ascent, the kidney is supplied by a number of transitory vessels that all originate from the aorta. The definitive renal arteries stem from the lumbar region of the aorta, while the transitory vessels normally disappear. It must also be mentioned here that during their development the kidneysturn

Fig. 16 - Migration of the kidneys Stage 15 (ca. 36 days)

Quiz Quiz 13 Quiz Quiz 19

Legenda

Fig. 16 Ascent of the kidney at the end of the 5th week. The kidneys still find themselves in the sacro-lumbar region. The mesonephros, which in this stage is still to be found under the undifferentiated gonads, is in the process of atrophying.

90° towards the vertebral cords, so that the hili are medially oriented at the end, while they originally face ventrally. (Animation). Various anomalies are connected with this ascent and will be described in detail in the chapter on pathology.

1 2 3 4 5 6 7 8 9

Gonad Mesonephros Allantois Tuberculum genitale Cloacal membrane Cloaca Posterior intestine Metanephros Mesonephric duct (Wolffian duct) 10 Ureter anlage

Fig. 17 - Migration of the kidneys Stage 18 (ca. 44 days)

1 3 4 5 6 7 8 9

Gonad Bladder being formed Genital tubercle Cloacal membrane Rectum Posterior intestine Metanephros Metanephros Mesonephric duct (Wolffian duct)

Fig. 18 - Migration of the kidneys Stage 23 (ca. 56 days)

1 3 4 6a 7 8 9

Gonad Bladder being formed Genital tubercle Rectum Posterior intestine Metanephros Mesonephric duct (Wolffian duct) 10a Ureter 11 Urogenital orifice 12 Anal orifice

20.4 Lower urinary system 

Subdividing the cloaca o Separating the cloaca o The perineum and the urorectal septum

Legenda Fig. 17 Ascent of the kidneys at the end of the 7th week. At this point the ureter - independent of the mesonephric duct (Wolffian duct) inserts into the bladder. The mesonephros has disappeared. Fig. 18 Ascent of the kidneys at the end of the 8th week. The kidneys find themselves at the upper lumbar level in their definitive position and a



Development of the bladder



Development of the male and female urethra

Subdividing the cloaca

Quiz Quiz 22

We have seen that the upper urinary system - consisting of the collecting ducts, the calices, the renal pelvis, and the ureters - arises from the ureter anlage. The lower urinary system - composed of the bladder and the urethra - is formed from the endoderm of the posterior intestine.

Separating the cloaca In stage 13 13 the cloaca is the common end of the rectal tube and the urogenital tract. Towards the outside it is closed by the cloacal membrane. Between the 4th and 6th weeks the urorectal septum separates the cloaca into aprimary urogenital sinus (ventrally) and the rectum (dorsally).  The bladder and the pelvic limb of the urethra arise from the primary urogenital sinus and the caudal portion of the urethra comes from the definitive urogenital sinus (see Fig. 26)



The urorectal septum divides the cloacal membrane into two membranes: theurogenital membrane (ventrally) and the anal membrane (dorsally). These two membranes atrophy 19 , like the bucco-pharyngeal membrane 11 , in order to form the intestinal and urogenital openings.

Fig. 19 - Development of the cloaca, Stage 13, roughly 32 days

1 2 5 8

Urorectal septum Cloacal membrane Cloaca Allantoïs

Fig. 20 - Migration of the kidneys, Stage 23, roughly 56 days

3 4 6 7 8a

Urogenital orifice Anal orifice Urogenital sinus Rectum Future bladder

Legenda Fig. 19 The white arrow indicates the direction of growth of the urorectal septum. This separates the cloaca into a urogenital sinus (ventrally) and the rectum (dorsally). Fig. 20 In this diagram the kidneys are found in their definitive position at the level of the upper lumbar region. The urorectal septum (white arrow) has divided the cloaca. The urogenital sinus, bounded on the outside by the urogenital orifice, lies ventrally. The rectum, which opens to the outside through the anal orifice, is found behind it. The cloacal membrane disappears in stage 19.

The perineum and the urorectal septum Today, the urorectal septum is no longer regarded as an isolated cellular layer of mesoderm cells that slowly grow towards the cloacal membrane. It consists of two mesodermal structures that are fused together. An upper fold (Tourneux), located frontally, grows caudally. Near the cloacal membrane two lateral folds (Rathke) form that fuse at the median level. They subdivide the cloaca and also grow in the direction of the upper fold (Tourneux) that is located frontally. A disorder in the formation of these two structures leads to recto-urethral or rectovesical fistulas. Connective tissue and the perineal musculature, which keep the pelvic organs in place, arise from the mesoderm, which surrounds the rectal tube. The central fibrous part of the perineum corresponds anatomically to the region between the anal and urogenital orifices.

Fig. 21 - Development of the urorectal septum

1 2

Fig. 22 - Separation of the cloaca and the formation of the perineum

1 2 3 4

Peritoneal cavity Upper fold (Tourneux; pink arrow) ; Lower folds (Rathke; blue arrows) Primary urogenital sinus Anal canal

Peritoneal cavity Upper fold (Tourneux) (pink arrow) Lower fold (Rathke) (blue arrow)

Fig. 23 - Separation of the cloaca and the formation of the perineum

5 6 7

Urogenital orifice Anal orifice Perineum

Development of the bladder The bladder develops from the upper part of the urogenital sinus (UGS) and is connected with the allantois. The allantois is obliterated during the development and forms a fibrous cord, the urachus, which following birth becomes the median umbilical ligament. While the cloaca is being divided, the caudal, originally common part of the mesonephric duct (Wolffian duct) and the ureter anlage is taken up into the upper, postero-lateral wall of the urogenital sinus (future bladder).

Legenda Fig. 21 The urorectal septum consists of two different mesoderm structures. Over the course of the 4th week an upper fold (Tourneux) divides the cloaca in the craniocaudal direction. The two lower, lateral folds (Rathke) are responsible for the division in the lower section.

Legenda

The rapid growth of the back wall of the urogenital sinus has the result that the common lowest part of the ureter and the mesonephric duct (Wolffian duct) are both taken up into the bladder wall. Further complicated growth processes have the result that the ureteral orifices and the orifice locations of the mesonephric duct (Wolffian duct) go through a craniocaudal positionexchange during the course of the further development. The ureteral openings appear to migrate thereby in a cranio-lateral direction and the mesonephros orifices appear to be shifted caudo-medially. The triangular zone that is thus created is termed the vesical trigonum. In males, the wolffian duct forms the future deferent duct on both sides.

Fig. 24 - Development of the bladder

Legenda Fig. 24 Dorsal view of the entry level of the mesonephric canal (common extremities of the mesonephric duct (Wolffian duct) and the ureter) while it is incorporated into the dorsal wall of the bladder.

Animation 1 2 3 4 5

Primary urogenital sinus Mesonephric duct (Wolffian duct) Ureter anlage Common drainage urinary passages Pelvic limb of the urogenital sinus

A B

Part of the genital tract Part of the urinary tract

Reversal of the orifice locations of the ureter and mesonephric duct (Wolffian duct) in the bladder wall

The trigonum thus originates from the mesoderm while the ventral bladder wall has an endodermal origin. Later, though, the trigonum will be completely covered by endodermal epithelial cells. The smooth musculature of the bladder develops during the 12th week from the splanchnopleural mesoderm, which coats the endoderm on the outside.

Development of the male and female urethra The urethra forms itself from the lower part of the urogenital sinus (UGS).





In a man the prostate and membranous part of the urethra arise from the pelvic part of the UGS while the spongy urethra comes from the phallic part (urethral plate). In a woman the whole urethra and part of the vagina arise from the pelvic part of the UGS while the phallic part (urethral plate) forms the vestibule and the labia minora. Fig. 26 - Development of the urethra out of the cloaca

Meer info In the genital tract module you will learn more about the development and differentiation of the urogenital sinus inmales and females.

SUR

Septum urorectal