My reynes or Kidneis, hath chiden me unto the night. MORE Picus Wks. 20/1, 1510.
That the watry humour may be separated from the blood, Nature hath added the Reins to the rest of the bowels. N. CULPEPER Culpeper’s Last Legacy. London, N. Brook, 1661 .
Development of the kidney, ureter, and adrenal glands
Development of the kidney nephrogenic cord and pronephros
In each somite, the intermediate cell mass of mesodermal tissue, the intermediate mesoderm, develops at the junction of the amnion and yolk sac, a region medial to the communication between the intraembryonic and extraembryonic celoms ( Fig. 12-1 A).
The intermediate mesoderm that lies caudal to the intraembryonic celom forms the nephrogenic cord (urogenital ridge), from which the embryonic kidneys, gonads, and mesonephric (wolffian) ducts arise.
Three sets of kidneys develop from the intermediate mesoderm during human embryogenesis. The pronephros is rudimentary, the mesonephros is provisional, and the metanephros becomes the permanent kidney ( Fig. 12-1 B).
The rudimentary pronephros in humans is not clearly demarcated from the mesonephros because it is made up of small aggregations of cells, the nephrotomes, which arise from stalks in the nephrogenic cord of the seven cephalad somites.
The nephrotomes are drawn into hollow tubes to form nephroceles, which in turn interconnect to join the primary excretory duct. Although the pronephros is not functional and will degenerate, the tubular portions become part of the primary excretory duct (the mesonephric or wolffian duct) that grows caudally to empty into the cloaca.
The mesonephros, like the pronephros, is formed in the intermediate mesoderm from the nephrogenic cord. As the mesonephros grows, it expands into the body cavity as part of the urogenital fold, which will later contain the müllerian duct and reproductive gland ( Fig. 12-2 ). The fold will become divided longitudinally into a genital fold and a mesonephric fold and be partially separated from the body wall by the formation of a mesentery. The genital portion subsequently acquires its own mesentery, the mesovarium or mesorchium. Mesodermal cells, beginning cranially, aggregate within the cord to form vesicles that elongate into 40 or more mesonephric tubules. One end of each tubule connects with the mesonephric duct, and the other invaginates to become the glomerular (Bowman’s) capsule. The mesonephric nephrons degenerate, starting from the cranial end, leaving only a few caudal remnants in the male.
Mesonephric duct and ureteric bud
The mesonephric (wolffian) duct develops caudally, so that by 4 weeks it joins the cloaca.
After the urogenital sinus separates from the rectum, the mesonephric duct will form the superficial part of the trigone. In the male, it contributes to the formation of the epididymis, vas deferens, ejaculatory duct, and seminal vesicle. In the female, it degenerates, leaving only vestiges. The derivations and homologies of the urogenital organs are shown in Table 12-1 .
|Precursor||Male Organ||Female Organ|
|Primordial germ cells||Spermatozoa||Ova|
|Sex cords||Seminiferous tubules||Follicular cells|
|Mesonephric tubules||Efferent ductules, paradidymis, appendix epididymis||Epoöphoron|
|Wolffian (mesonephric duct||Ductus deferens, seminal vesicles||Gartner’s canal|
|Müllerian (paramesonephric) duct||Appendix testis (hydatid), prostatic utricle||Fallopian tube, vagina (part)|
|Upper urogenital sinus||Bladder, prostatic urethra||Bladder, urethra|
|Lower urogenital sinus||Urethra||Vestibule|
|Genital folds||Penile urethra (floor)||Labia minora|
|Genital swellings||Scrotum||Labia majora|
About the middle of the fifth week of gestation, the mesonephric duct develops a single branch, the ureteric bud, where the duct bends at a right angle at the termination of the common excretory duct proximal to its junction with the cloaca ( Fig. 12-3 ). At first, the bud grows from the dorsolateral surface toward the spine and then turns cranially until it meets the mesenchyme of the caudal portion of the nephrogenic ridge, the metanephric blastema. This region of the nephrogenic cord had separated earlier. At the level of the second lumbar vertebra, the mesenchymal mass blocks further ascent of the bud. As the body lengthens and the kidney ascends, the bud (now the ureter) keeps pace. As it branches, it will eventually form the pelvis, calyces, and collecting tubules of the mature kidney. The steps of development are outlined in Table 12-2 .
|Gestational Days||Embryologic Event|
|22||Cloaca and pronephric duct present|
|24||Mesonephric (wolffian) ducts and mesonephric tubules develop|
|28||Wolffian duct joins cloaca; ureteral bud emerges from it|
|32||Ureteral bud enters metanephric mesenchyme; common excretory duct (wolffian duct and ureter) opens into cloaca|
|37||Pelvis and primitive calices form|
|44||Wolffian duct (caudal) and ureter (cranial) separately enter urogenital sinus after division of cloaca|
|48||Nephrons and collecting tubules are formed; urogenital membrane opens|
|52||Formation of glomeruli|
|63||Onset of renal function|
|70||Degeneration of wolffian or müllerian ducts|
|84||Urinary and genital tracts become joined in the male|
|150||Appearance of ureteropelvic junction|
At this time, the urorectal septum starts to separate the hindgut from the urogenital sinus, a process that will end when the septum arrives at the cloacal membrane.
Divisions of the ureteric bud
During the sixth week, perhaps under the inductive stimulus of the nephric cap, the tip of the ureteric bud elongates craniocaudally to form an ampulla with a central cavity, the primitive pelvis ( Fig. 12-4 A). As the ureter pushes cranially, the cap of blastema that will form the future renal parenchyma moves away from its site at the end of the nephrogenic cord. Neurons entering the cap with the bud may also play a role in renal morphogenesis.
The bud divides into paired primary branches that will form the major calyces ( Fig. 12-4 B).
Each branch progressively divides into secondary branches at the same time that the nephrogenic blastema proliferates to cap the divisions ( Fig. 12-4 C).
The bud ultimately will branch 15 times. The earliest four to six branches from the ureteric bud become incorporated into the growing renal pelvis. The next three to five branches are the primary branches: the cranial pole branch, dorsal and ventral central branches, and a caudal pole branch that become incorporated into the two or three major calyces ( Fig. 12-4 D). Because the branches are not supported by nephric tissue, with the onset of urine formation, they will dilate to assume the shape of the pelvis and calyces. Branching occurs with greater frequency at the renal poles; thus, the organ elongates and becomes reniform. The next three to five secondary and tertiary branches form ampullae and become the minor calyces. At this point, the renal parenchyma encroaches on the tip of each branch to form a papilla so that the final five to seven branchings are left to form the collecting ducts.
The fetal minor calyces, 14 in number at the most, are first arranged in pairs, one facing anteriorly and the other posteriorly. However, at the upper pole, three pairs face superiorly, and at the lower pole, two pairs face caudally. Each calyx drains a single papilla. A longitudinal groove on the surface of the kidney indicates the line between the paired pyramids of collecting tubules based on this anteroposterior calyceal division. There follows a period of calyceal fusion: The anterior and posterior calyces in the upper and in lower poles fuse across the frontal plane of the kidney, and the anterior calyces in the middle portion fuse with each other, as do posterior calyces, leaving an average number of eight or nine, with a range from 5 to 20. Papillae also fuse, especially at the poles, leaving two or more of them within one calyx. The usual result is that the upper pole has three calyces, which may be on a single major calyx with papillary fusion or on two short minor calyces with three papillae on each. The two pairs of middle calyces usually face anteriorly and posteriorly, but each of the pairs may fuse, leaving a single trunk. In the lower pole, less fusion occurs, usually leaving two pairs of minor calyces. Thus, in the adult, the pelvis has two general forms. In one, the upper major calyx is long and slender and the lower calyx is shorter and wider, which represents the double-calyceal arrangement of Sykes. In the other form, the minor calyces tend to empty directly into the pelvis without intervening infundibula.
Fused pyramids forming compound and conjoined calyces, in which the cone shape of the papilla is modified, are found most often at the renal poles, where they are more likely to be associated with intrarenal reflux.
Development of lobes and pyramids
Branching of the ureteric bud into calyces results in the development of lobes (ranunculi), each with a central calyx and peripheral tubules. In the 10-week-old fetus, only two lobes are seen, but the number increases with age. The cap over the ureteric bud segregates itself into smaller caps lying over each of the four to six first-order collecting tubules that form the individual pyramids. The lobes are separated by the interlobar septa of Bertin, which are indicated by grooves of fetal lobulation on the surface. Secondary and tertiary pyramids are similarly formed ( Fig. 12-5 ). After the sixth branching, the tip of each generation of collecting ducts joins the renal tubule with its attached glomerulus that has developed in the adjacent nephrogenic mesenchyme. The maximum number of branchings is 14, reached by the 28th week, after which some disappear. Each of these fetal lobes could be considered a separate kidney, similar to the arrangement found in marine mammals.
A double layer of connective tissue and a layer of less differentiated cortex lie between the pyramids of each lobe to form an interlobar septum or renal column of Bertin. Traditionally, these have been called columns even though they do not have a columnar shape; septa has been proposed as a better term.
The surface lobulation seen in fetal kidneys persisting after the age of 4 years is caused by bulging of the several pyramids as the cortex grows between the relatively fixed interlobar septa. It is often associated with extrarenal branching of the arteries and an abnormal renal pelvis. Usually, as the cortex fills out, the lobulation almost completely disappears, although in half of adult kidneys, some residual lobulation is found, usually on the anterior surface. The basic lobar arrangement within the kidney persists, however, except in the arrangement of the vessels.
The number of papillae present in the developing kidney is probably fixed at that number at the stage at which urine formation begins and causes differentiation between calyx and collecting duct. The actual number of papillae found in the adult kidney depends on the degree of subsequent fusion between the pyramids.
Ascent of the kidney
At 6 weeks, the lower margin of the nephrogenic ridge lies opposite the second sacral segment, well caudal to the level of the lower lumbar segments, when it is reached by the ureteric bud. The metanephros , which now can be called a kidney, capped by a large adrenal gland , grows cephalad behind the mesonephros to reach and pass ventral to the umbilical artery ( Fig. 12-6 A). It is at this stage that the upper and lower poles can be identified.
As the caudal end of the vertebral column straightens and that portion of the body grows during the sixth week, the kidney increases rapidly in size and rounds up to become shorter, enabling it to move away from the angle of the umbilical artery ( Fig. 12-6 B). It thus appears to ascend, so that at 6 weeks it lies opposite the 3rd lumbar vertebra.
By 8 weeks, the kidney assumes its adult level at the 2nd lumbar vertebra (Figs. 12-6C and D). Thus, ascent is the result of both renal and skeletal growth.
The renal pelvis at first lies anteriorly because the initial direction of the ureteric bud was posterior, but during ascent, the pelvis is moved to a medial orientation as the kidney rotates. The upper pole moves laterally and the lower pole moves medially so that the kidney assumes a more upright position.
Development of the renal vessels
Because the kidney ascends mainly by differential growth of the body, it acquires an arterial supply successively from the segmental mesonephric vessels as it moves cephalad relative to the major vessels. The development of the renal vasculature is closely related to that of the mesonephros and adjacent structures. Arteries enter the kidney from successive sources, and veins drain the kidney into nearby as well as distant visceral and parietal pathways. Many variations are found, as would be expected from vessels that develop from evolving embryologic systems.
At 4½ weeks, approximately 30 lateral branches develop as segmental arteries from the dorsal aorta, extending from the 6th cervical to the 3rd lumbar segments. The more cranial of these roots gradually degenerate as more caudal ones develop to supply the urogenital (mesonephric) arterial rete . At this stage the mesonephros as well as the gonad, adrenal gland, and ureter obtain their blood supply from this source. As the metanephros differentiates into a kidney, under the influence of the ureter branching from the mesonephric (wolffian) duct, and rises with differential body growth, it successively acquires the segmental arteries that connect the rete to the aorta ( Fig. 12-7 ). Most of the root vessels to the rete degenerate, leaving the kidney vascularized by a single enlarged branch, the renal artery. Accessory arteries are not rare. Because arterial degeneration begins at the cephalad end of the metanephros, the segmental branch to the lower pole is the one most likely to persist as an accessory vessel.
Although within the kidney the renal segmental arteries have a constant relationship with the renal segments (see Fig. 12-71 ), infinite variations occur in their origin and site of division in relation to the hilum ( Figs. 12-8 and 12-9 ). The apical and lower segmental arteries may originate independently directly from the aorta, in which case the renal segmental artery may supply a larger segment of the kidney than it would if it were a branch of the main renal artery ( Fig. 12-10 ).
Multiple renal arteries
Although they are anomalous, accessory renal arteries are evidence of the persistence of one or more of the segmental mesonephric roots extending from the 6th cervical to the 3rd lumbar segments, the more caudal of which once supplied the renal arterial rete. The five segments of the normal kidney are each provided with an artery; thus, accessory arteries may be considered to be normal arteries that have a more cranial or caudal origin. Such vessels may supply either pole of the kidney ( Figs. 12-11 and 12-12 ).
On the right side, persistent arteries may lie anterior or posterior to the vena cava, and on the left, they may actually enclose the renal vein. Some arise from the base of the renal artery or from the aorta. Extra vessels are found in 10 to 40% of autopsy cases, and vessels to the lower pole are twice as common as those to the upper pole.
A persistent segmental artery to the lower pole may provide the origin of the gonadal artery, and one to the upper pole may also provide an adrenal artery. In addition to segmental arteries, smaller accessory arteries, usually multiple, may come from the inferior phrenic or from an adrenal artery.
The renal veins develop from venous plexuses and pass through a complicated evolution by formation and absorption of the postcardinal, supracardinal, and subcardinal veins, which are involved in the formation of the inferior vena cava. Venous maldevelopment provides a continuum from almost normal to frankly abnormal configurations. Should the renal vein or vena cava become occluded, these persistent embryonic transitional pathways can provide alternative routes of drainage.
The veins do not follow the arterial pattern. In fact, those vessels that make up the venous complex develop at a deeper level than that occupied by the arteries, although near the vena cava, the renal veins come to lie anterior to the arteries.
The right renal vein rarely has tributaries, except for the gonadal vein in a fifth of cases, but the left renal vein (a vessel that embryologically could be considered a segmental left vena cava) always receives the adrenal vein and gonadal vein on that side, and most often has lumbar, ascending lumbar, or hemiazygos communications. The reasons for this difference may be found in the development of the inferior vena cava.
To understand renal and ureteral venous anomalies, it is necessary to review the development part of Chapter 2 . The left renal vein from the hilum of the kidney to the ends of the adrenal and gonadal veins is formed from the subcardinal vein, in addition to connection with the residua of the intersubcardinal anastomosis. Also, as the renal vein crosses the midline it picks up the veins draining adjacent organs and the lumbar veins. This complex origin explains the greater length and larger number of veins draining into the left renal vein compared with the veins on the right, where these tributaries drain directly into the vena cava.
The anomalies of the venous supply to the kidney that result from retention of embryologic pathways are described in Figures 2-9 and 2-10 . Most apparent clinically are the persistence of the left caval vein, the circumaortic venous ring, and the formation of retroaortic renal veins.
Absence of the metanephros may be due to defects in the development of the nephrogenic ridge, but the usual finding in clinical practice is its absence from failure of formation of the mesonephric duct and the ureteric bud. Thus, renal agenesis in the male is often associated with defects of the other derivatives of the duct. In 12% of males with a single kidney, a genital abnormality is found, including absence, hypoplasia, or cyst formation of the seminal vesicle, vas deferens, and ejaculatory ducts. Important surgically is that the remaining kidney may be abnormal in formation or position.
In the female, genital anomalies are frequently associated with renal agenesis (44%) because of the close developmental association of the müllerian duct with the wolffian duct at the urogenital sinus. The uterus may be unicornuate, bicornuate, or hypoplastic. The vagina may fail to form, or it may be septate or even obstructed, resulting in unilateral hematocolpos. A syndrome is recognized secondary to interruption of growth of the wolffian duct consisting of unilateral renal agenesis, absence of the fallopian tube, and absence of half of the uterus. With bilateral absence of the ducts, and thus absent kidneys, multiple anomalies associated with oligohydramnios are the rule, including pulmonary hypoplasia, and the infant will show Potter’s facies ( Fig. 12-13 ).
Duplex, ectopic, and horseshoe kidneys
Renal duplication , the most common anomaly, occurs when the ureteric bud divides and two ureters enter the blastema ( Figs. 12-14 and 12-15 ). These kidneys have normal vasculature because the arterial distribution is not influenced significantly by abnormalities of the pelviocaliceal system. In contrast, the rare supernumerary kidney that results from a split of the nephrogenic blastema often has abnormal vessels.
The anomalies of renal fusion and ectopia may be placed in five categories: crossed with and without fusion, not crossed with and without fusion, and fused caudally, the horseshoe kidney.
Ectopia occurs when ascent is prevented at the time that the kidney lies at a level between the 3rd sacral and the 2nd lumbar vertebra. Thus, there may be pelvic, iliopelvic, iliac, or lumbar ectopia ( Fig. 12-16 ). Because arrest occurs at a relatively early stage of embryologic development, ectopia is usually associated with incomplete rotation, a short ureter, and a blood supply that arises from local lateral segmental vessels, connections that account for the fixation of the kidney found at operation. In addition, anomalies of the external and internal genitalia and of structures associated with the cloaca are common. The typical pelvic kidney is usually smaller, lobulated, and of an abnormal (pancake-like) shape. The adrenal gland, however, is usually in a normal position. At any level of arrest, the kidney, ureters, and associated vessels will reside inside the envelope of the renal (Gerota’s) fascia.
The kidney may be malpositioned from any one of the factors responsible for its arrest; including malformation of the ureteric bud or of the metanephric tissue, or persistence of the primitive segmental structure of the arterial system, although this condition is usually secondary. In addition, vertebral anomalies have been shown experimentally to result in renal ectopia, similar to abnormalities of the urinary tract found clinically with congenital scoliosis.
The ectopic kidney may be found low on the ipsilateral side or, as in crossed ectopia, on the opposite side, or it may be fused with the other kidney as crossed ectopia with fusion. It may remain wholly within the pelvis as a pelvic kidney. In half of the cases the opposite, normally situated kidney is abnormal, and in a tenth of the cases, it is absent.
Should upward movement be arrested, as with an ectopic or horseshoe kidney, the regional blood supply is maintained, arising from the iliac, inferior mesenteric, or the middle sacral arteries, or even from segmental vessels from the aorta below the inferior mesenteric artery.
In fewer than 5% of cases of renal ectopia, the affected kidney undergoes excessive cranial migration; this results in a superior ectopic kidney . Most superior ectopic kidneys lie below the diaphragm, but rarely part or all of the kidney may lie above the diaphragm, and in this circumstance the kidney is designated as an intrathoracic kidney ( Figs. 12-17 , 12-18 , and 12-19 ).
By definition, fused kidneys are a single conglomerate mass of renal tissue having two ureters that empty into each side of the bladder ( Fig. 12-20 ). They include two major groups: (1) crossed ectopia with fusion, and (2) horseshoe kidneys, although there are many variations.
In crossed ectopia with fusion, the ectopic renal mass lies on one side of the vertebra and its ureter reaches the bladder on the opposite side ( Figs. 12-21 to 12-24 ). The anomaly may result from lateral flexion of the lumbosacral spine in the tail portion of the embryo that displaces the distal portion of the nephrogenic cord across the midline, thus requiring one of the ureteric buds to cross to join the single asymmetric nephrogenic mass. Vertebral and high anorectal anomalies may be anticipated with fusion disorders. The location of the ureteric orifice in the bladder is variable, sometimes being in an ectopic position.
The horseshoe kidney is the result of fusion in the midline of a portion of one metanephric blastema with the opposite blastema before the sixth week, when the two normally lie close together. At that time, the definitive kidney moves out of the pelvis and its blood supply shifts to segmental aortic branches. At the same time, the kidney normally rotates medially on its long axis. Should the caudal portions of the metanephric blastemas come in contact with each other and fuse, normal rotation and ascent is prevented, resulting in the persistence of an anteriorly oriented pelvis, with ureters passing anterior to the fused poles, and in persistence of some of the pelvic arterial supply ( Figs. 12-25 to 12-28 ). It has been speculated that the fusion might result from crowding in the pelvis by the large umbilical arteries at the time that the blastemas would normally diverge after passing them.
Horseshoe kidney occurs once in about 500 births, occurs twice as often in males as in females, and is the most common of the fusion anomalies. It probably outnumbers crossed ectopia by a ratio of 6 to 1. A wide variety of associated anomalies are often seen, some of which may be incompatible with life.
The pattern of the blood supply within each half of a horseshoe kidney is usually the same as that of a normal kidney: Each kidney has single or double arteries angled caudally. Each segment of the kidney is supplied by a segmental branch of the renal artery, without collateral connections between. Thus, within the renal substance the distribution of blood is little different from that in normal kidneys.
The exception to a fully normal vascular pattern is the presence of an artery to the lower segment . This vessel typically has an abnormal origin, most often arising from the aorta at a level lower than the normal renal artery or from the common iliac artery or even from the internal iliac artery. Thus, an accessory artery may enter the kidney above or below the isthmus. If a substantial isthmus forms, it may be partially supplied by an additional vessel arising from the caudal part of the aorta or from the common iliac artery .
Malrotation is actually the result of arrest of rotation. The renal pelvis remains in an anterior position, its orientation before ascent. However, the kidney may occasionally be found overrotated, so that the pelvis lies posteriorly.
No consistent embryologic explanation for these several renal anomalies is available. However, associated anomalies of the genitalia and vertebrae are not unusual, suggesting a common embryologic disturbance.
The development of cystic disease of the cortex depends on deviation from the exact process with which the collecting duct must join the renal tubule. Disturbances in the connection result in various forms of cystic disease depending on the time of interference ( Figs. 12-29 to 12-32 ).
Development of the ureter
Musculature and luminal development
Longitudinally oriented elastic fibers form in the adventitia of the ureter at about 10 weeks, coincident with the onset of the formation of urine. These are followed by the appearance of randomly oriented muscle fibers in the layer that will become the muscularis. The number of elastic fibers increases linearly with the thickness of the ureteral wall. In contrast to the muscularis, those fibers associated with the submucosa are radially disposed. Fewer fibers develop in the lower ureter than in its middle portion, a finding that might help explain the occurrence of adynamic segments distally.
By 16 weeks, muscle extends the length of the extramural ureter and, by the 36th week, involves the entire ureter, including the orifice. Muscle cells and, to a lesser extent, elastic fibers increase progressively to double their number by the age of 12 years, either by multiplication of the existing cells or by mesenchymal differentiation. The size of the cells also increases but to a lesser degree.
Into the early postnatal period, the muscle fibers are arranged more circularly but become more oblique with time. By adulthood, the fibers have assumed the characteristic helical arrangement (see Fig. 12-83 ).
At first, the ureter has a lumen. Beginning at around 5½ weeks, the lumen becomes occluded, first in the midportion and, then by 7 weeks, is blocked proximally and distally as well, events coincident with cessation of function of the mesonephros. By 8 weeks, recanalization begins, extending in both directions from the middle of the ureter, so that a week later, the channel is open. Metanephric function is not a factor because it will not begin until the 10th week, but ureteral lengthening may be a factor in clearance of the obstruction. These observations may explain the greater frequency of ureteric valves in the proximal and distal segments, where delayed developmental arrest would be more likely.
Most anomalies of the ureter occur either at the ureteropelvic or the ureterovesical junctions. A rare exception to this is the blind-ending ureteric bud, a blind-ending hollow structure that joins the normal ureter at an acute angle, and that is by definition at least twice as long as it is wide. Its wall structures are identical to those of the normal ureter ( Fig. 12-33 ). Ureteral anomalies that occur in the ureterovesical junctions are described in Chapter 13 .
Congenital obstruction at the ureteropelvic junction
Obstruction that occurs where the ureter joins the kidney may be due to extrinsic factors, but more often, the cause is faulty development.
Because the helical arrangement of muscle fibers needed for urine transport develops progressively with time, arrest of this process would leave only circularly oriented fibers at the junction, an arrangement more likely to be obstructive than conductive. Alternatively, elongation of the ureter leaving predominantly longitudinal fibers would likewise be a poorly conducting arrangement. Other factors such as agenesis or reduction in fiber numbers would also leave a nonconducting segment. It is clear that several factors are involved. Primary defects in programming the development of the ureteric bud may lead to intrinsic, focal lesions such as ureteropelvic junction obstruction or ureteral valves ( Figs. 12-34 to 12-36 ).
Extrinsic causes of ureteropelvic junction obstruction are usually an aberrant vessel or a band passing anterior to the junction, although this may only contribute to intrinsic factors ( Fig. 12-37 ). Another cause may be reflux that overloads a quasi-normal junction as urine returning to the bladder supplements newly secreted urine.
Retrocaval ureter (see fig. 2-9 )
The postcardinal vein may remain dominant rather than giving way to the supracardinal vein, or the periureteric ring may persist. As the permanent kidney ascends, the postcardinal vein runs at first lateral and then medial to the kidney. As a result of the displacement and consolidation of the veins, the ureter lies dorsal to the vena cava.
Several theories have been proposed for the origin of ureteral valves, including failure of complete canalization after a normal period of closure in the sixth week. They may occur when the axes of the lumina are eccentric so that the ureter is overlapped at the site of the obstruction, resulting in a common wall, or they may result from persistence of mucosal folds left over from the pleats formed during ureteral lengthening. Finally, they might arise if the ureter elongates faster than the kidney ascends.
Development of the adrenal glands
Fetal adrenals are very large during fetal life, especially in the second trimester, mostly resulting from expansion of the cortical cells. They are still relatively large at birth, only to regress rapidly during the first 3 weeks of life (see Fig. 12-13 ). During the eighth gestational month, the zona glomerulosa appears, followed by the zona fasciculata at term; the zona reticularis follows in the next 3 to 6 months, with the cortex completely differentiated by 2 years.
At birth, the large adrenal gland is vascular and easily injured during delivery.
Origin of the adrenal cortex and medulla
Two separate tissues contribute to the formation of the adrenal glands, providing layers that remain distinct into adult life ( Table 12-3 ).
The adrenal cortex comes from mesothelial buds on the upper third of the mesonephros that project into the primitive celom with the gonad ( Fig. 12-38 ). The buds form a cellular aggregate on either side of the aorta . Some cells may not join the aggregation, accounting for accessory adrenal cortical tissue about the adrenal gland and kidney, with the spermatic vessels or testis in the male and within the broad ligament and ovary in the female.
The adrenal medulla is derived from primitive ectodermal cells of the neural crest in the developing sympathetic nervous system (see Fig. 4-2 ). These sympathogonia normally mature into sympathoblasts and ultimately into ganglion cells within the sympathetic ganglion . Alternatively, and of importance to the formation of the adrenal medulla, they may migrate and differentiate into chromaffin endocrine cells, the pheochromoblasts, that will mature into chromaffin cells after penetrating the adrenal cortical primordium to form the adrenal medulla ( Table 12-4 ).
Distribution of fetal chromaffin bodies
In fetal life, the pheochromoblasts form chromaffin bodies that are distributed along the aorta , providing the main source of catecholamines ( Fig. 12-39 A). These cells migrate to and invade the adrenal cortical aggregation to form the adrenal medulla.
Some of these chromaffin bodies regress only partially after birth and remain as the paraganglion system distributed within and adjacent to the prevertebral sympathetic ganglia and in the several sympathetic plexuses and ganglia ( celiac, mesenteric, renal and adrenal , and hypogastric ) as well as a plexus at the aortic bifurcation, the organ of Zuckerkandl ( Fig. 12-39 B). This system is a secondary source of catecholamines throughout life and may become the tissue of origin of pheochromocytomas. This is especially possible in children in whom 30 percent of pheochromocytomas are extra-adrenal; they are not infrequently malignant.
Two larger aggregates of chromaffin tissue related to the superior hypogastric plexus, the para-aortic bodies , remain on either side of the aorta in an inverted U-shape looped over the inferior mesenteric artery . These bodies enlarge during early postnatal life, only to virtually disappear at puberty.
Adrenal blood supply
The source of blood for the adrenal is the most cranial of the segmental mesonephric roots that once supplied the urogenital (mesonephric) arterial rete. Especially on the right side, smaller accessory arteries, usually multiple, may come from the inferior phrenic or the renal arteries.
Resolution of the renal venous plexus on the right leaves a single, short vein running obliquely that connects to the posterior surface of the vena cava. On the left, the residual left subcardinal vein (see Fig. 2-7 ) is also single but often receives blood from the inferior phrenic vein in addition to that from the capsular veins. The left vein is longer and descends vertically to join the renal vein.
At 8 weeks, the adrenal gland has migrated to lie adjacent to the kidney and will subsequently rise as the kidney ascends. However, the position of the adrenal gland is independent of that of the kidney, because with an ectopic kidney, the adrenal gland is found in its normal position.
Agenesis may occur, often associated with renal agenesis secondary to failure of the entire blastema. Adrenal ectopia may be found. Of surgical importance when operating on a solitary kidney is the rare possibility that the single adrenal gland lies under the renal capsule ( Figs. 12-40 and 12-41 ). Accessory adrenal cortical rests are found intra-abdominally and retroperitoneally within abdominal and sexual organs ( Fig. 12-42 ). As noted previously, extramedullary chromaffin tissue is commonly found and may become transformed into a pheochromocytoma.