Embryology of the urinary tract

Embryology of the urinary tractKIDNEYS AND HIGH URINARY APPARATUS:


The kidneys and the upper urinary tract develop from the intermediate mesoderm.

Nephrogenic territory is in fact situated between the para-axial mesoblast forming the somites and the lateral blade which, by cleavage, is at the origin of the somato- and splanchnopleure. From the 18th day of embryonic life, the nephrogenic cords gradually differentiate in the craniocaudal direction. The appearance of the metanephros at the origin of the definitive kidney is preceded by two transient sketches: the pronéphros and the mesonephros.

The pronéphros, vestigial structure, is individualized during the 3rd week in the form of metamerized cellular clusters. It does not acquire any functional character and disappears completely at the end of the fourth week. The mesonephros begins to differentiate at the beginning of the 4th week.Like the pronéphros, it appears progressively in the form of metamerized cellular clusters, the nephrotomes, which hollow out into vesicles and then elongate in tubules. The outer ends of these tubular structures form the beginning of a collecting channel, the Wolff channel. Their internal extremities swell and then are arranged in cups opposite arterial handles issuing from the aorta, thus drawing the prefiguration of the glomerular chambers. From the fifth week, cranial tubules begin their involution, while the caudal tubules are still in the process of formation. They will disappear completely between the 8th and the 10th week, without having acquired a real excretory function although it is possible that processes of excretion and reabsorption occur very temporarily. For some authors, the Wolff Canal would result not from the confluence of the mesonephrogenic tubules but from a direct dorsolateral delamination of the nephrogenic cord between the 9th and 13th pairs of somites. Anyway, it will form secondarily with the mesonephric tubes the entire male genital tract. It also participates in the induction of the Müller canals at the origin of the female genital tract.

This mesonephrotic duct or Wolff’s canal grows by growing from its caudal end and progresses rapidly towards the cloaca which it joins at the 5th week. In this process, it is guided by a process of recognition and interaction between the cells. In its juxta- cloacal portion about the 30th day, it emits a diverticular bud, the ureteral bud, which first elongates towards the back and then in the cranial direction towards the nephrogenic cord.

The development of the definitive kidney begins during the 5th week, when metanephros appears in the lower pelvic portion of the nephrogenic cord. Its differentiation takes place under the inductive action of the ureteral bud which penetrates it by dividing according to the dichotomous mode. Consequently, the differentiation of the collecting and secreting structures occurs by reciprocal interaction between the metanephrogenic blastemus and the ureteric bud.The blastema cells determine the urethra’s roar. In turn, the terminations of ureteric ramifications induce the differentiation of blastematous cells into secretory tubes.

The inducing power of blastema on the division of the ureteral bud is specific. On the other hand, some experimental data show that the ureteral bud shares with other tissues (heterologous inducers) its ability to induce the differentiation of nephrons within the blastema. Moreover, when they have received the determining impulse, the cells of the blastema are able to continue their differentiation in an autonomous way. It seems therefore that the area of ​​the metanephros is predetermined within the nephrogenic territory before it is in contact with the ureteral inducer whose presence simply allows the cells of the blastema to express their nephrogenic potentialities.

Renal agenesis results from a non-formation of the metanephrogenic blastema or a defect of induction due to poor development of the ureteral bud. The ureter is absent in approximately 60% of cases of renal agenesis, and some cases, labeled unilateral renal agenesis, are in fact multi-cystic dysplasia that has been totally involuted and whose residues have disappeared in the first years of life.

Bilateral renal agenesis is obviously incompatible with life. The absence of renal function in utero determines an oligoamnios responsible for the characteristic facies described by Potter (nose in parrot beak, low ears implanted with soft cartilages, retrognathism, epicanthus). There is also pulmonary hypoplasia and other skeletal or visceral malformations can be observed.

Unilateral renal agenesis is more frequent (1/1000) and more commonly occurs in boys than in girls. It can be isolated or associated with other skeletal or visceral malformations, especially digestive. Moreover, the development of the internal genital tract may be disturbed by an abnormal development of the Wolffian system and consequently of the Müllerian system. For example, the boy may experience testicular agenesis or epididymodeferential aplasia and in the daughter of tubal or uterine malformations. Additionally, the ipsilateral adrenal may be absent.

The ureteral bud formation is made by symmetric or asymmetric dichotomous division, varying considerably from one metanephros to another and, within the same blastema, from one region to another. It progresses more rapidly at the poles than in the interpolar region. The first divisions determine the disposition of the future pyellocal system and the corresponding renal lobules. The next 3 to 5 generations will constitute the small chalices and the papillae. The 6 to 9 subsequent generations will be the origin of the papillary tubes.


The differentiation of the nephrons begins on contact with the first divisions of the ureteral bud and the first secreting tubes in the process of formation are identifiable with certainty as early as the 8th week. However, they degenerate during development and will hardly be found in the mature kidney, where only the nephrons left by the ramescences at the origin of the collecting tubes will remain.

The metanephrogenic blastema discharged by the twigs from the ureteral bud is the site of intense mitotic activity, which results in the individualization of cellular condensations that are arranged in a cap around the terminal ampule of the ramifications.

The formation of these condensations represents a critical stage during development. Laterally dislodged by the progression of the ureteral divisions, these condensations form spherules which rapidly burrow into vesicles remaining attached to the excretory pathway. Very quickly, these vesicles elongate by bending, forming the S-shaped body that will give rise to the epithelial structures of the nephron.

The near end of the excretory path soon opens into it; it is at the origin of the segment of connection of the distal tube and the handle of Henle. The middle zone will constitute the proximal secreting tube. At the other extremity there appears a swelling which depresses into a cupule with two cellular layers in the concavity of which a capillary system develops. This capillary network, enclosed by the development of the cup forming the capsule of Bowman, constitutes the corpuscle of Malpighi. The growth of the various tubular segments then continues asymmetrically. The differentiation of the cells of the proximal bypass tube precedes that of the cells of the distal tube, whose growth in length will, on the other hand, be faster. For each nephron, the process of differentiation is spread over 4 to 5 weeks.

The first nephrons differ in the medullary zone. Although their differentiation occurs, they are progressively transported to the future cortical zone, driven by the development of arborization excretory and then, when this arises, by the formation of arches spreading from the arborization terminal. In fact, the induction of nephrons continues beyond the 14th week, when the division of the collecting system ends. The connecting segments of the oldest nephrons, deportees, are gradually attached to the connecting segments of the youngest nephrons forming arches constituted by 4 to 7 nephrons. The growth of the collecting tubes beyond the points of attachment of these arches still allows the induction of 5 to 7 additional nephrons. The differentiation of the nephrons continues until about the 32nd week; they are now 800 000 to 1 000 000. The growth of the kidney will now be a function of the increase in length of the various segments of the secretory tubes and in particular of their proximal segment which will continue well beyond the birth.The Malpighi corpuscles also undergo a very long phase of maturation, which only ends at the age of 12 years.

Paleo-functional system:

At the same time, the pyélocaliciel system has acquired its final disposition. The first divisions are superior and inferior.One or the other of these axes will generally give a predominant interpolar branch. It is therefore by its asymmetry that the dichotomous process leads to the usual formation of three calicial groups: superior, middle and inferior. This process explains the variations in disposition of the pelvis and the calicious groups that can be observed from one subject to another and from one kidney to another.

It is by a complicated process involving a variable number of divisions that the pelvis and calyxes are modeled during phases of confluence and dilatation. This dilation appears progressively from the pelvis between the 10th and the 13th week. From the 14th to the 16th week, small chalices and papillae develop prefiguring the disposition encountered in the mature kidney. The initiation of renal function plays an essential role in the modeling of the excretory pathways. It begins at the 9th week.

Defects in the differentiation of the metanephrogenic blastema determine hypoplasia or renal dysplasia and the association of the two disorders (hypodysplasia) can be observed.

It is a simple reduction in the size of the kidney that characterizes the simple hypoplasia (doll kidney); there is hardly any parenchymatous lesion. It is mostly bilateral.

In oligomegranophore hypoplasia, the kidneys are small, harmonious; the reduction of the number of nephrons is accompanied by hypertrophy of the existing nephrons. In these two cases, they are fundamental parenchymal disorders, the causes of which are due to the fact that the segmental renal hypoplasia has a probably ischemic origin and therefore localized in peripheral-based triangular areas, both with cortical depressions and the medulla; the tubes are sometimes dilated, pseudothyroid, sometimes atrophic, and the glomeruli may be invisible.

Renal dysplasia results from abnormal differentiation of the metanephrogeneous blastema. Primitive tubes bordered by a cylindrical epithelium are then observed, surrounded by a concentric mesenchymal sleeve, sometimes with marked fibromuscular differentiation; there are also metaplastic cartilage, small cortical or medullary small islets, hyaline cysts. These abnormalities may be cortical and / or medullary, total or partial, focal and segmental. Depending on the case, the kidney is normal or totally deformed.

Multi-cystic renal dysplasia is a very special entity: the urethra is atresic, the vessels are slender, and the kidney is reduced to a cluster of cysts, containing a citrinous liquid, isolated from each other and appended around a more or less fibrous tissue less dense in which the essential elements of dysplasia are found.

The anomaly of induction is obvious.

A displacement of the meeting point of the ureteral bud and the metanephrogenic blastema may be the cause of dysplasia. This is what happens when the bud emerges higher or lower than normal from the Wolff Canal, or when the latter gives rise to two, exceptionally three, buds. The area of ​​the metanephrogenic territory being, as we have seen, predetermined at the level of the nephrogenic cord, if the bud (or in the case of duplication one of the two buds) comes into contact too high or too low with the preform it will induce an area which does not have the same potential for differentiation either at the boundary between mesonephros and metanephros or at the completely terminal portion of the nephrogenic cord. The result will be dysplastic states such as can be seen in ectopic implantations of the ureter, in one of the pyelons of a duplication, or in certain vesico-ureteral refluxes which indicate abnormal abortion of the ureter in the ureter. bladder.

During development, the condition of the kidney changes, but gradually acquires its definitive form. Initially pelvic, it becomes lumbar due mainly to very rapid growth of the lumbosacral region of the embryo. This “migration” spans about 6 weeks. A rotation of 90 ° brings outside the original dorsal renal convexity. Due to the division of the ureteral bud and the resulting segmentation of the blastema, the kidney retains a polylobed appearance that fades from the end of pregnancy through progressive filling of the grooves but may persist in early childhood and sometimes in -of the.

Any disturbance of the process of “migration” of the kidney leaves it in ectopic position: pelvic, iliac or lumbar. It then keeps a vascularization of stage coming from the hypogastric artery, the primitive iliac or the aorta. The pathogenesis of very rare renal ectopias remains mysterious. One of the two kidneys is then situated beneath the other in the iliac fossa, whereas the ureter which drains it normally ends in the bladder on the opposite side.

Parenchymal fusions carrying various forms of L, J or S (sigmoid kidney) can also be observed in these cases.

The fusion of the two parenchymal masses is indeed possible leading to various aspects of renal symphysis. They can be explained by the fact that during their “ascent” the two kidneys are brought together and sometimes temporarily compressed in the arterial fork of the umbilical arteries. The symphyses most frequently observed are those which unite the two kidneys by their homologous poles, exceptionally superior, generally inferior, then realizing the horseshoe-shaped kidney; the fusion zone constitutes a fibrous or parenchymatous isthmus sometimes receiving a clean vascularization directly from the aorta or even from a primary iliac. A complete symphysis leads to the formation of discoidal kidneys or cakes; the common parenchymatous mass then often remains low, pelvic.

The structure of the excretory route remains conjunctive until the 15th week. The appearance of muscle cells is gradually and slowly from the lumbar segment. The differentiation of pericual muscle cells, which are probably the starting point of ureteral peristalsis, only begins around the 21st week and continues until birth. The epithelium is gradually differentiated by successive ranges.

The muscle bundles become truly important only after the 30th week. The differentiation of the cells of the connective tissue is slower still and will hardly be completed until birth. For some, there would be a short phase of obstruction followed by a secondary rewinding of the light but this notion remains very uncertain. Little is known about the initiation of ureteral peristalsis.

Cholinergic innervation precedes adrenergic innervation, which occurs late. In any case, there is a long period during which the excretory pathway has no really motor structures whereas it delivers passage to an already large urinary flow. This may result in a transient expansion.

The implantation of the muscular structures ensuring the ureteral peristalsis can be done in an abnormal or unbalanced way. These disorders are observed mainly in two areas: the pyelo-ureteral junction and the terminal portion of the ureter. The muscle bundles may be restricted in number and have connection abnormalities.

We can also observe an imbalance in the distribution of these structures with preponderance of collagen. Such anomalies, disturbing the transmission of the peristaltic wave, are the cause of the hydronéphroses by syndrome of the pyeloureteral junction and the so-called “primitive” mega-ureters.


The development of the lower urinary tract is closely related to that of the posterior intestine, the genital tract and the external genitalia.

The bladder and the lower urinary tract originate essentially from the anterior portion of the primitive, endodermal cloacus, which is extended by the allantois.

By its ventral surface, the cloaca is directly in contact with the epiblast, at the level of the cloacal membrane. This zone is not penetrated by the mesoblast and thus remains didermic. It marks, during the first stages, the caudal limit of the embryonic axis as the oral membrane, of the same structure, marks the cranial limit. When the embryo is delimited, it is transferred to the ventral surface because of the forward folding of the caudal end, which determines the formation of the cloaca and the caudal appendix. Primitively reaching the annular boundary of the future umbilical region, it is gradually separated during the development of the allantoic bud. The mesoblast gradually surrounds this cloacal membrane. During the third week, it raises two lateral thickenings, the folds

cloacaux, which soon unite to form, as of the fifth week, the genital tubercle. This tubercle then separates the membrane from the caudal portion of the circumferential fold which will progressively constitute the ventral wall of the embryo at the hypogastric level.

At the end of the 5th week, a mesenchymal projection depresses the vault of the cloaca. This perineal spur, which is laterally joined by the two mesodermal folds which accompany the mesonephrotic canals, isolates the urogenital sinus in front of the anorectal canal behind. Reaching the cloacal membrane in the course of the 8th week, it subdivides it into an elongated urogenital membrane in front and anal membrane in the back. Henceforth, the separation is complete, up to the superficial planes, between the digestive tract and the urinary axis in which the Wolff canals open up.

An abnormal development of the cloacal membrane with respect to its situation or its extent and a caudal displacement of the genital tubercle are at the origin of the malformations integrating in the complex frame of the bladder exstrophy and the epispadias.

They are accompanied by a defect and a mismatch of the mesenchymatization at the origin of the spacing of the pubis and the muscles of great right. However, many points remain obscure in this pathological organogenesis. It is possible that the process of partitioning the cloaca is initially offset forward, determining the extension of an entoblastic plate and then its vacuolation in the thickness of the genital tuber until before a didermic membrane of which the second opening can explain all the degrees of ecstrophy and epispadias.

On the posterior surface of the bladder which is isolated, the lower extremity of the ureteral bud is carried along by the Wolff canal, which has given birth to it. The portion between the emergence of the bud and the abutment in the posterior wall of the urogenital sinus expands into a large ampule at the top of which the ureteral and Wolffian ducts open into a gun barrel. The development of the posterior wall of the bladder gradually encompasses this ampulla, so that the ureteral canal and the Wolff canal soon open into the sinus through distinct orifices. It is during the 7th week that the canals thus arrive in isolation. The presence of a membrane (Chwalla’s membrane) which closes the ureteral orifice and is resorbed towards the 9th week when the secretion of urine begins is generally accepted. There may therefore be a phase of modeling replication of the excretory pathway. The modeling of the urogenital sinus, because of its growth, will gradually move these two orifices away.

The ureter, caused by the spreading of the sinus wall, assumes a high position and enters the vesical portion while the fixed Wolff canal remains in the urethral portion.

The segment of the wall between the ureteral and wolffian orifices thus assumes a triangular shape. This trigon, of Wolffian origin, is therefore included in what will become the bladder wall. The mucous membrane which covers it is of entoblastic origin but the muscular plane which constitutes it is mesoblastic. Trigone and ureter have the same origin and are in continuity as shown by the disposition of the muscular fibers at the end of development. This smooth musculature is clearly individualized from the 22nd week and at the same time at the level of the ureter and the trigone.

The point of emergence of the ureteral bud from the Wolff Canal determines its future level of abortion at the level of the bladder. When the bud emerges below the usual point, the common channel is short. The ureter will soon be included in the posterior wall of the bladder, where it will emerge in a higher and more external position than normal;the transparietal path is then shorter and the flap arrangement which opposes the reflux is insufficient. When the bud emerges above the usual point, the common canal is longer and the ureter separates later from the Wolff canal. It will debouch all the lower in the urinary tract that it will be born higher. It may thus be carried into the lower part of the sinus: the urethra sur-montanal in the boy or urethra in its entirety in the daughter, or even upper portion of the vestibule. It can even keep a connection with the Wolff canal and then open in the seminal tract: vas deferens, ejaculatory canal and especially seminal vesicle. When the ureteral bud is drawn below the bladder, there is an agenesis of the trigone.

A malformative process of the same type occurs in case of duplicity (or exceptional triplicity) of the ureteral bud, one of the two buds, very generally the upper bud, being inevitably shifted.

A stenosis of the ureteral orifice by abnormal resorption of the Chwalla membrane or mesenchymal disorder may allow the development of an ureterocele in the early stages of the ureterovesical junction, a pseudocystic dilatation of the submucosal segment of the ureter- ureter. The existence of a duplication of the ureteric bud promotes this process at the level of the meatus of the ureter of the upper pyelon which will be delivered in ectopic position intra- or extravesical.

On the surface, the mesenchyme that surrounds the cloacal membrane has been differentiated into two orders of concentric structures. The ischiopubic zone at the periphery marks the boundary between the perineal blank and the neighboring blanks, that is, the hypogastric wall and the lower limbs. It will be at the origin of the ischiopubian branches (which develop like bones of membranes), the corpora cavernosa and the muscles ischiocaverneux. At the immediate periphery of the membrane, the cloacal sphincter will be at the origin of the primary perineum.

During the 7th week, on both sides of the cloacal folds, the future perineal region swells, forming symmetrical genital swellings. At this stage, the disposition is identical in both sexes. The mesenchyme of the perineal spur thickens in contact with the cloacal membrane and forms the outline of the central fibrous nucleus of the perineum. The fibers of the cloacal sphincter will now be organized and intertwined at the level of this central nucleus. The cloacal beads will be attached in front of the anal membrane and form the perineal raphe. Their posterior parts circumscribe and overhang the anal membrane.

The urogenital sinus is parallel in three stages. Its cranial portion, in continuity with the allantois, flares out and constitutes the bladder. The allantois will undergo a progressive obliteration in the course of the 16th week. It will then leave a fibrous vestige, the urachus, connecting the bladder to the umbilicus but remains finely permeable until the 32nd week.

Muscle differentiation of detrusor begins as early as the 11th week. The smooth sphincter appears around the 13th week. The connection of the various muscular elements of the vesico-urethral junction which determines the morphogenesis of the bladder neck occurs around the 26th week.

The intermediate portion of the sinus in which the Wolff channels terminate becomes ductal and will constitute the prostatic urethra in the male sex, almost the entire urethra in the female sex. At the end of the third month, the epithelium of the urethra proliferates and the resulting buds penetrate the neighboring mesenchyma, which determines the formation of the prostate and in the daughter of the urethral and paraurethral glands. The terminal portion, immediately in contact with the cloacal membrane, will undergo a very different evolution according to the sex.

The primary mesenchymation of the perineum is enhanced by a second wave from the caudal somites. The deep elements of the cloacal sphincter give origin to the rectal strap, while the caudal mesenchyme forms the external bundles of the levator muscle of the anus. The sphincteric elements situated in the plane of the cloacal membrane are responsible for the greater part of the external sphincter of the anus, the sphincter striated of the urethra and the constrictor of the vulva.

Female Sex: Male

In the female sex, the development of the lower urinary tract is parallel to that of the müllerian system at the origin of the genital tract. Müller channels or paramesonephrotic channels born from the invagination of the coelomic epithelium descend parallel to the Wolff channels and then cross them to lengthen in the mediocal direction and back behind the urogenital sinus at the 9th week of development. The effacement of their walls of backing causes the formation of a median utero-vaginal canal, while their cranial extremities outline the future fallopian tubes. At this time, two evaginations appear at the pelvic portion of the urogenital sinus and by active proliferation meet on the median line to form the vaginal plate that joins the uterovaginal canal, encloses its caudal end and creates a light from of the 11th week. Its complete pipeline towards the end of the 5th month constitutes the vagina. The vaginal cavity remains separated from the urogenital sinus by a micromembrane: the hymen. At the same time, the Wolff channels disappear, sometimes leaving vestigial vestigial residues.

Their all-caudal portions, plated against the genital axis, may be included in the form of a vestigial ductal tract: the Gartner canal. The differentiation of the lower part of the open urogenital sinus by resorption of the urogenital membrane at the 9th week, progresses in parallel. It begins in the 3rd month. The gradual diminution of its depth causes the urethral meatus and hymen to become relatively superficial in a vestibule bordered by the genital folds forming the labia minora, which unite in front of the genital tubercle to constitute the clitoral cap, while the genital rims constitute the big lips.

Total Male

In the male sex, the Wolff channels persist after regression of the mesonephros.

They constitute the pathway of genital excretion. Having become vas deferens, they form the excretory pathways of the testicles formed in contact with the mesonephros. In the terminal part, before their opening at the posterior surface of the urogenital sinus, each vas deferens bulges into an ampule and emits a hollow diverticulum, the seminal vesicle.Below this point of emergence, it becomes the ejaculatory canal. The lower portion of the urogenital sinus between the neck of the bladder and the opening of the ejaculatory ducts forms the initial portion of the urethra. In the course of the third month, epithelial buds detach from the posterior surface and, penetrating the adjacent mesenchyme, form the prostatic gland.

The Müller canals have only a very temporary existence and have practically disappeared by the end of week 8. Their terminal portions, which have initiated their fusion, persist in the form of a small diverticulum which will give the prostatic utricle, although some consider it to be formed at the expense of an urogenital sinus evagination. This diverticulum fits between the orifices of the two Wolff channels, which have become ejaculatory channels at this level.The elevation which determines this confusion draws the veru montanum.

Two membranous folds develop on the lateral surfaces of the urethra, which regress and migrate backwards to form the crest of the veru.

The absence of involution of the two urethrovaginal folds is at the origin of the congenital valves of the posterior urethra in their most frequent form.

At the same time, on the external level, the genital tubercle lengthens, sketching the future penis. It draws with it the cloacal folds which underline the median furrow of the sinus

urogenital, opened by the resorption of the membrane, and extending as far as the base of the anterior bulge of the penis, forming the sketch of the glans. This furrow depresses into a true urethral, ​​sinusal, and therefore endodermic, gutter. This gutter closes progressively from back to front as from the 12th week, while the ectodermal folds merge below the urethra thus formed. There is, however, relative independence between endodermal tubulization and ectodermal fusion. In any case, the whole of the penile urethra over its entire circumference is of endodermic origin. As the penile urethra is thus formed, the surrounding mesenchymal gangue becomes denser and forms the outline of the spongy body; a comparable process occurs in the heart of the glans but independently. The cavernous bodies’ individualize in their turn, but the differentiation of the mesenchyme into erectile tissue will only intervene secondarily between the 4th and 5th months, first in the glans, then in the corpora cavernosa and finally in the spongy body. At the same time, the genital cuts migrate backwards, around the base of the penis and their fusion on the median line forms the scrotum.

During the 3rd month, a longitudinal epithelial crest or wall appears on the underside of the glans, in the extension of the penile urethral gutter. When the urethral gutter closes at the crown of the glans, this epithelial wall is hollowed out into a urethral cleft, which closes in turn, back and forth, to form the balanic portion of the urethral canal. For some, the formation of the balanic urethra would result from a different process: canalization of a full epithelial cord developing in the glans from the top until reaching the terminal part of the penile urethra. In any case, the balanic urethra forms secondary to the penile urethra from the ectodermal tissue.

If we consider the different stages of organogenesis of the urethra and the penis, we can conceive that there are three basic types of hypospadias which may be called anterior, middle, and posterior. The anterior hypospadias result from a defect in the formation of the balanic urethra. If they are usually accompanied by a malformation of the foreskin, cases may be observed in which the balanic cleft remains open, while the foreskin has evolved normally in its anterior part.The mean hypospadias correspond to an incomplete development of the penile urethra.

In all cases, the portion of the spongy body corresponding to the unformed urethra remains atrophic, poorly differentiated. The foreskin is always abnormal because, without support at the crown of the glans, the annular bead that gives it birth remains incomplete on its lower face. In the posterior hypospadias, the malformation bears on the whole of the urethral gutter and the rod is very deformed. When there is malformation of the urethra, the temporary curvature of the penis can be controlled by the abnormal growth of the structures on the lower side, resulting in a bending that is maintained by fibrosis as a result of the persistence of mesenchymal blade corresponding to the undeveloped spongy body.


The urinary tract of the fetus is thus in place, as a whole, at the end of the first trimester of gestation, but the kidneys have only part of their nephrotic equipment. Around the 20th week, when the collection system is complete, they have about one third of the nephrons and represent only one tenth of the mass of the mature kidneys. Their growth is parallel to the gestational age and it seems that fetal hypertrophy does not affect their development.

Ultrasonic and anatomical measurements suggest that the mean height of the kidney is 2 cm at the 20 th week, 3 cm at the 28 th week and 4 cm at the end, while its width is respectively at these three moments of 1.4 cm, 2.2 cm and 2.7 cm and its thickness of the order of 1 cm, 1.7 cm and 2.5 cm.

The ratio between the maximum renal circumference and the abdominal circumference remains during gestation in the range of 0.27 to 0.30. If the kidney is in its normal position and has acquired its definitive shape between the 10th and the 20th week, ultrasonic recognition is hardly possible before the 15th week because of its small size and the absence of fat which impedes its individualisation. Its ultrasound definition becomes very satisfactory from the 20 thweek. It should be noted that the adrenal, relatively voluminous, can have a similar echogenicity, which can be a trap during the measurements.

As soon as the primitive glomeruli have acquired their vascularization, the secretion of urine begins. Renal function begins well before the phase of nephrogenesis is complete. The kidney has a certain function at the end of the 9th week by the appearance of a glomerular filtration but the cells of the tubular epithelium only come into operation from the 15th week. Because of this diuresis, the bladder is visible on ultrasound as early as the 12th week. It has been seen that the hydrostatic pressure generated by the elimination of urine has an influence on the development of the excretory system.

Although it does not intervene in homeostasis before birth, since this function is assured by placental exchanges, urine excretion plays an important role in regulating the volume and composition of the amniotic fluid. It certainly does not present the only mechanism of regulation. However, renal agenesis and obstructive uropathies involving the urinary tract are accompanied by an oligoamnios which is therefore a warning sign for the surveillance of the urinary tract of the fetus.

Upon initiation of renal function, the mean urine output is high in relation to the mass of the fetus. It is about 0.5 to 0.7 ml / h at the 18th week, 1.5 ml / h at the 20th week, 9.6 ml / h at the 30th week and can reach 25 to 27 ml / h at the end. The volume remains relatively constant with respect to weight, at 10 ml / kg / hr, from the 20th week at birth. The mechanisms of regulation of this diuresis are still poorly understood. Its importance leads to frequent urination. The duration of the bladder replication cycle varies between 50 and 150 minutes. Urination occurs for an average bladder replication of 40 ml.

The ureter is never distended in the normal state; its echographic evidence is therefore hardly possible. On the other hand, the pelvis and the chalices are individualizable and analyzable especially from the 30th week. The diameter of the pelvis is in any case less than 15 mm. The calyces have a variable thickness. They are never globular or spherical.