Urogenital System

Early Forms of the Kidney

The common representation of mammalian kidney development includes three successive phases beginning with the appearance of the pronephros , the developmental homolog of the type of kidney found in only the lowest vertebrates. In human embryos, the first evidence of a urinary system is the appearance of a few segmentally arranged sets of epithelial cords that differentiate from the anterior intermediate mesoderm late in the fourth week. These structures are more appropriately called nephrotomes . The nephrotomes connect laterally with a pair of primary nephric ducts that grow toward the cloaca ( Figure 16.1 ). The earliest stages in the development of the urinary system depend on the action of retinoic acid, which sets the expression limits of Hox 4-11 genes that determine the craniocaudal limits of the early urinary system. Responding to a declining gradient of bone morphogenetic protein (BMP) concentration, starting in the lateral mesoderm ( Figure 16.2 ), intermediate mesoderm becomes specified. The molecular response by the intermediate mesoderm is the expression of the transcription factors Pax-2 and Pax-8 , which then induce Lhx-1 (Lim-1) and Osr-1 (odd-skipped related-1), a pair-rule gene in Drosophila , in the intermediate mesoderm. The lateral border of the intermediate mesoderm is maintained by a mutual repression by Hand-2 in the lateral plate mesoderm and Pax-8 in the intermediate mesoderm. The medial border is kept in place by antagonism between Foxc-1 and -2 expression in the paraxial mesoderm and Osr-1 in the intermediate mesoderm. Lhx-1 is required for the aggregation of the mesenchymal cells of the intermediate mesoderm into the primary nephric ducts.

The primary nephric ducts arise after a mesenchymoepithelial transformation of some cells of the intermediate mesoderm, and they soon begin to elongate as a cord of cells. Their extension continues until they reach the cloaca. Gata-3 is required for elongation of the primary nephric duct. Growth in length of the primary nephric duct is based principally on a zone of proliferating cells at the tip of the duct. In addition, retraction of the trailing edge of the cells at the tip of the duct pulls along those cells that are attached to them. The elongating duct progresses toward the cloaca along the ventral border of the somites in a manner remarkably similar to that used by outgrowing axons or dendrites. Much of the way, lead cells of the duct extend filopodia or lamellipodia-like extensions that sample the extracellular matrix that surrounds them. Both laminin and fibronectin are important substrates for the duct. Closer to the cloaca, the lead cells of the duct appear to respond to attractive signals from the cloacal region. Late in its development, cells of the terminal part of the duct (below the junction of the ureter) undergo apoptosis so that the duct and the ureter can make separate connections with their targets.

As the primary nephric ducts extend caudally, they produce Wnt-9b, which stimulates the intermediate mesoderm to form additional segmental sets of tubules. The conversion of the mesenchymal cells of the intermediate mesoderm into epithelial tubules depends on the expression of Pax-2, and in the absence of this molecule, further development of kidney tubules does not occur. These tubules are structurally equivalent to the mesonephric tubules of fishes and amphibians. A typical mesonephric unit consists of a vascular glomerulus , which is partially surrounded by an epithelial glomerular capsule. The glomerular capsule is continuous with a contorted mesonephric tubule, which is surrounded by a mesh of capillaries (see Figure 16.1B ). Each mesonephric tubule empties separately into primary nephric duct, which becomes known as the mesonephric (wolffian) duct.

The formation of pairs of mesonephric tubules occurs along a craniocaudal gradient. The first four to six pairs of mesonephric tubules (and the pronephric tubules) arise as outgrowths from the primary nephric ducts. Farther caudally, mesonephric tubules, as many as 36 to 40, take shape separately in the intermediate mesoderm slightly behind the caudal extension of the mesonephric ducts. By the end of the fourth week of gestation, the mesonephric ducts attach to the cloaca, and a continuous lumen is present throughout each. There is a difference in the developmental controls between the most cranial four to six pairs of mesonephric tubules and the remaining caudal tubules. Knockouts for the WT-1 (Wilms’ tumor suppressor) gene result in the absence of posterior mesonephric tubules, whereas the cranial tubules that bud off the primary nephric duct form normally. As is the case in the formation of the metanephros (see later), WT-1 regulates the transformation from mesenchyme to epithelium during the early formation of renal (mesonephric) tubules. Very near its attachment site to the cloaca, the mesonephric duct develops an epithelial outgrowth called the ureteric bud (see Figure 16.1A ).

Early in the fifth week of gestation, the ureteric bud begins to grow into the most posterior region of the intermediate mesoderm. It then sets up a series of continuous inductive interactions leading to the formation of the definitive kidney, the metanephros .

Although there is evidence of urinary function in the mammalian mesonephric kidney, the physiology of the mesonephros has not been extensively investigated. Urine formation in the mesonephros begins with a filtrate of blood from the glomerulus into the glomerular capsule. This filtrate flows into the tubular portion of the mesonephros, where the selective resorption of ions and other substances occurs. The return of resorbed materials to the blood is facilitated by the presence of a dense plexus of capillaries around the mesonephric tubules.

The structure of the human embryonic mesonephros is very similar to that of adult fishes and aquatic amphibians, and it functions principally to filter and remove body wastes. Because these species and the amniote embryo exist in an aquatic environment, there is little need to conserve water. The mesonephros does not develop a medullary region or an elaborate system for concentrating urine as the adult human kidney must.

The mesonephros is most prominent while the definitive metanephros is beginning to take shape. Although it rapidly regresses as a urinary unit after the metanephric kidneys become functional, the mesonephric ducts and some of the mesonephric tubules persist in the male and become incorporated as integral components of the genital duct system ( Figure 16.3 ).

Metanephros

Development of the metanephros begins early in week 5 of gestation, when the ureteric bud ( metanephric diverticulum ) grows into the posterior portion of the intermediate mesoderm, where Hox-11 paralogues are expressed . Mesenchymal cells of the intermediate mesoderm condense around the metanephric diverticulum to form the metanephrogenic blastema (see Figure 16.1C ). Outgrowth of the ureteric bud from the mesonephric duct is a response to the secretion of glial cell line–derived neurotrophic factor (GDNF) by the undifferentiated mesenchyme of the metanephrogenic blastema ( Figure 16.4A ). This inductive signal is bound by c-Ret , a member of the tyrosine kinase receptor superfamily, and the coreceptor Gdnfra-1 , which are located in the plasma membranes of the epithelial cells of the early ureteric bud. The formation of GDNF in the metanephrogenic mesenchyme is regulated by influences from two major sources. One is the transcription factors Pax-2 and Eya-1 within the intermediate mesoderm. The other is Wnt-11 signals from the ureteric bud. The formation of Wnt-11 is stimulated by C-Ret. Because C-Ret is stimulated by GDNF, the Wnt-11 signaling represents an intermediary in a positive feedback loop that maintains GDNF activity throughout the period of branching of the ureteric bud.

The posterior location of the ureteric bud results from a combination of repression of GDNF expression in the more anterior regions by the actions of Slit-2/Robo-2 in the mesenchyme and Spry-1 (Sprouty) , which reduces the sensitivity of the anterior nephric duct to the action of GDNF. Branching of the stalk of the ureteric bud is inhibited by a mutually inhibitory interaction between gremlin within the ureteric bud and BMP-2 in the surrounding mesenchyme.

The outgrowing ureteric bud is associated with two types of mesenchyme: intermediate mesoderm and tailbud mesenchyme. These two types of mesenchyme create a sharp border between the forming ureter (associated with tailbud mesenchyme) and the intrarenal collecting duct system (associated with intermediate mesoderm). BMP-4 , secreted by the surrounding tailbud mesenchyme, causes the ureteric epithelium to form uroplakins , proteins that render the epithelium of the ureter impermeable to water. The renal pelvis of the adult kidney shares properties with the ureter and the collecting system, and its cellular origins are unclear.

The morphological foundations for the development of the metanephric kidney are the elongation and branching (as much as 14 or 15 times) of the ureteric bud, which becomes the collecting (metanephric) duct system of the metanephros, and the formation of renal tubules from mesenchymal condensations (metanephrogenic blastema) located around the tips of the branches. The mechanism underlying these events is a series of reciprocal inductive interactions between the tips of the branches of the metanephric ducts and the surrounding metanephrogenic blastemal cells. Without the metanephric duct system, tubules do not form; conversely, the metanephrogenic mesoderm acts on the metanephric duct system and induces its characteristic branching. The pattern of branching is largely determined by the surrounding mesenchyme. If lung bud mesenchyme is substituted for metanephric mesenchyme, the pattern of branching of the ureteric bud closely resembles that of the lung.

Before branching begins, the tip of the ureteric bud expands to form an ampulla. Through the action of Wnts secreted from the bud, mesenchymal cells from the metanephrogenic mesenchyme condense around the tip. Fibroblast growth factor (FGF)-2 and BMP-7 secreted from the tip prevent apoptosis of the surrounding mesenchymal cells and maintain these cells in the proliferative state throughout the many branching events that are involved in the formation of the million nephrons per kidney. In response, these cells continue to secrete GDNF, which works to promote branching in the same manner as initial outgrowth of the ureteric bud (see Figure 16.4 ).

The tips of the ureteric buds contain aggregations of proliferating cells that are stimulated to divide through the action of Wnt-7b signaling. Although many of the progeny of these divisions result directly in extension of the bud, other daughter cells break free and are intercalated into the ureteral epithelium away from the tip. Through this mechanism and other oriented cell divisions, the stalk lengthens. Mesenchymal BMP-4 is involved in the promotion of stalk elongation; it also suppresses the formation of new branches from the stalk.

The formation of individual functional tubules ( nephrons ) in the developing metanephros involves three mesodermal cell lineages: epithelial cells derived from the ureteric bud, mesenchymal cells of the metanephrogenic blastema, and ingrowing vascular endothelial cells. The earliest stage is the condensation of mesenchymal blastemal cells around the terminal bud of the ureteric bud (later to become the metanephric duct).

The concentrated mesenchymal cells form a mesenchymal cap around the tip of a branching ureteric bud ( Figure 16.5A ). These cells, which constitute the nephron progenitor pool, are characterized by expression of the transcription factor Six-2 . Another transcription factor, Pbx-1, places a check on their differentiation and maintains the cells in a progenitor state. At the same time, signals from Wnt-9b promote proliferation of the progenitor cells (see Figure 16.4B ). Notch suppresses the proliferation program and stimulates the differentiation of these cells into nephric epithelium.

The cells surrounding the mesenchymal cap are destined to become stromal cells and are characterized by the expression of the transcription factor Foxd-1 . Both the Foxd-1 and Six-2 cells arise from an earlier common precursor that expresses Osr-1 , the earliest marker of intermediate mesoderm. Another population of stromal cells, characterized by the expression of Tbx-18, contributes to the walls of the ureters.

Responding to FGF-8 and Wnt-4 signals, cells of the mesenchymal cap undergo a mesenchymoepithelial transformation to become a spherical renal (nephrogenic) vesicle ( Figure 16.6A ; see Figure 16.5A ). From its early formation, a renal vesicle is divided into distal and proximal portions, with Notch operating in the proximal part and Wnts in the distal part (see Figure 16.6A ). Cells of the proximal part will go on to form Bowman’s capsule and the proximal tubules, down to the descending loop of Henle, and the distal part forms the ascending loop of Henle and the distal tubule.

While still in the mesenchymal state, cells of the mesenchymal cap are associated with several interstitial proteins such as types I and III collagen and fibronectin. During their transformation to an epithelial cell type, these proteins are lost and are replaced with epithelial-type proteins (type IV collagen, syndecan, laminin, and heparin sulfate proteoglycan), which are ultimately localized to the basement membranes ( Figure 16.7 ).

Cells of the renal vesicle go through a series of defined stages to form a renal tubule. After a growth phase, mitotic activity decreases, and the primordium of the tubule assumes a comma shape. Within the comma, a group of cells farthest from the end of the metanephric duct becomes polarized and forms a central lumen and a basal lamina on the outer surface. This marks their transformation into an epithelium—the specialized podocytes , which ultimately surround the vascular endothelium of the glomerulus.

A consequence of this epithelial transformation is the formation of a slit just beneath the transforming podocyte precursors in the tubular primordium (see Figures 16.5B and 16.6B ). Stimulated by VEGF (vascular endothelial growth factor) secreted by the immature podocytes (see Figure 16.5E ), precursors of vascular endothelial cells grow into this slit, which ultimately forms the glomerulus. The endothelial cells are connected with branches from the dorsal aorta, and they form a complex looping structure that ultimately becomes the renal glomerulus. These endothelial cells arise from precursor cells intrinsic to the intermediate mesoderm, rather than as outgrowths from the aorta. The other cells of the glomerular complex (renin-producing cells, vascular smooth muscle cells, mesangial cells, and pericytes) all arise from Foxd-1-expressing stroma cells. Cells of the glomerular endothelium and the adjoining podocyte epithelium form a thick basement membrane between them. This basement membrane later serves as an important component of the renal filtration apparatus.

As the glomerular apparatus of the nephron takes shape, another slit forms in the comma-shaped tubular primordium, thus transforming it into an S-shaped structure ( Figure 16.5C ). Cells in the rest of the tubule primordium also undergo an epithelial transformation to form the remainder of the renal tubule. This transformation involves the acquisition of polarity by the differentiating epithelial cells. It is correlated with the deposition of laminin in the extracellular matrix along the basal surface of the cells and the concentration of the integral membrane glycoprotein E-cadherin , which seals the lateral borders of the cells ( Figure 16.8 ). While the differentiating tubule assumes an S shape, differing patterns of gene expression are seen along its length. Lhx-1 expression and the downstream Delta/Notch system are now known to play a prominent role in generating the proximal convoluted tubule (see Figure 16.6A ). At the other end of the tubule (future distal convoluted tubule), Wnt-4 and E-cadherin remain prominent, whereas in the middle (future proximal convoluted tubule), K-cadherin is a prominent cellular marker. Many of the uninduced mesenchymal cells between tubules undergo apoptosis.

Differentiation of the renal tubule progresses from the glomerulus to the proximal and then distal convoluted tubule. During differentiation of the nephron, a portion of the tubule develops into an elongated hairpin loop that extends into the medulla of the kidney as the loop of Henle . As they differentiate, the tubular epithelial cells develop molecular features characteristic of the mature kidney (e.g., brush border antigens or the Tamm-Horsfall glycoprotein [see Figure 16.7 ]). Early in their differentiation, the ends of the distal tubules connect to the terminal ends ( collecting ducts ) of the ureteric bud system.

Growth of the kidney involves the formation of approximately 15 successive generations of nephrons in its peripheral zone, with the outermost nephrons less mature than the nephrons farther inward. Development of the internal architecture of the kidney is complex, involving the formation of highly ordered arcades of nephrons ( Figure 16.9 ). Details are beyond the scope of this text.

Later Changes in Kidney Development

While the many sets of nephrons are differentiating, the kidney becomes progressively larger. The branched system of ducts also becomes much larger and more complex, and it forms the pelvis and system of calyces of the kidney ( Figure 16.10 ). These structures collect the urine and funnel it into the ureters. During much of the fetal period, the kidneys are divided into grossly visible lobes. By birth, the lobation is already much less evident, and it disappears during the neonatal period.

When they first take shape, the metanephric kidneys are located deep in the pelvic region. During the late embryonic and early fetal period, they undergo a pronounced shift in position that moves them into the abdominal region. This shift results partly from actual migration and partly from a marked expansion of the caudal region of the embryo. Two concurrent components to the migration occur. One is a caudocranial shift from the level of L4 to L1 or even the T12 vertebra ( Figure 16.11 ). The other is a lateral displacement. These changes bring the kidneys into contact with the adrenal glands, which form a cap of glandular tissue on the cranial pole of each kidney. During their migration, the kidneys also undergo a 90-degree rotation, with the pelvis ultimately facing the midline. As they are migrating out of the pelvic cavity, the kidneys slide over the large umbilical arteries, which branch from the caudal end of the aorta. All these changes occur behind the peritoneum because the kidneys are retroperitoneal organs. During the early phases of migration of the metanephric kidneys, the mesonephric kidneys regress. The mesonephric ducts are retained, however, while they become closely associated with the developing gonads.

Although normally supplied by one large renal artery branching directly from the aorta, the adult kidney consists of five vascular lobes. The arteries feeding each of these lobes were originally segmental vessels that supplied the mesonephros and were taken over by the migrating metanephros. Their aortic origins are typically reduced to the single pair of renal arteries, but anatomical variations are common.

Formation of the Urinary Bladder

The division of the cloaca into the rectum and urogenital sinus region was introduced in Chapter 15 (see Figure 15.13 ). The urogenital sinus is continuous with the allantois, which has an expanded base continuous with the urogenital sinus and an attenuated tubular process that extends into the body stalk on the other end. Along with part of the urogenital sinus, the dilated base of the allantois continues to expand to form the urinary bladder , and its attenuated distal end solidifies into the cordlike urachus, which ultimately forms the median umbilical ligament that leads from the bladder to the umbilical region (see Figure 16.21 ).

While the bladder grows, its expanding wall, which is derived from tailbud mesenchyme, incorporates the mesonephric ducts and the ureteric buds ( Figure 16.12 ). The result is that these structures open separately into the posterior wall of the bladder. Through a poorly defined mechanism possibly involving mechanical tension exerted by the migrating kidneys, the ends of the ureters open into the bladder laterally and cephalically to the mesonephric ducts. The region bounded by these structures is called the trigone of the bladder, but much of the substance of the trigone itself is composed of musculature from the bladder. Only small strips of smooth muscle along the edges of the trigone may arise from ureteral smooth muscle. At the entrance of the mesonephric ducts, the bladder becomes sharply attenuated. This region, originally part of the urogenital sinus, forms the urethra , which serves as the outlet of the bladder.

Clinical Correlation 16.1 presents congenital anomalies of the urinary system.

Genetic Determination of Gender

Since 1923, scientists have recognized that the XX and XY chromosomal pairings represent the genetic basis for human femaleness and maleness. For many decades, scientists believed that the presence of two X chromosomes was the sex-determining factor, but in 1959, it was established that the differentiation between maleness and femaleness in humans depends on the presence of a Y chromosome. Nevertheless, the link between the Y chromosome and the determination of the testis remained obscure. During more recent decades, three candidates for the testis-determining factor were proposed.

The first was the H-Y antigen , a minor histocompatibility antigen present on the cells of males but not females. The H-Y antigen was mapped to the long arm of the human Y chromosome. It had been considered to be the product of the mammalian testis-determining gene. Then a strain of mice (Sxr) was found to produce males in the absence of the H-Y antigen. Sxr mice were found to have a transposition of a region of the Y chromosome onto the X chromosome, but the locus coding for the H-Y antigen was not included. In addition, certain phenotypic human males with an XX genotype were shown to be missing the genetic material for the H-Y antigen.

The next candidate was a locus on the short arm of the Y chromosome called the zinc finger Y (ZFY) gene. With DNA hybridization techniques, this gene has been found in XX male humans and in mice in which small pieces of the X and Y chromosomes were swapped during crossing-over in meiosis. Conversely, this gene was missing in some rare XY human females. Certain XX males were found to lack the gene, however, and other rare cases of anomalies of sexual differentiation did not show a correspondence between sexual phenotype and the expected presence or absence of the ZFY gene.

CLINICAL CORRELATION 16.1

Congenital Anomalies of the Urinary System

Anomalies of the urinary system are common (3% to 20% of live births). Many are asymptomatic, and others are manifest only later in life. Some urinary tract anomalies can be attributed to environmental factors, such as the high glucose concentrations seen in diabetic mothers. Many others are genetically based. More than 500 syndromes with a known genetic basis involve malformations of the urinary tract. In mice, knockouts of essentially all genes involved in kidney formation result in serious urinary tract anomalies. Other cases of urinary malformations involve interactions between environmental factors and genetic susceptibility. Figure 16.13 summarizes the locations of many frequently encountered malformations of the urinary system.

Renal Agenesis

Renal agenesis is the unilateral or bilateral absence of any trace of kidney tissue ( Figure 16.14A ). Unilateral renal agenesis is seen in roughly 0.1% of adults, whereas bilateral renal agenesis occurs in 1 in 3000 to 4000 newborns. The ureter may be present. This anomaly is usually ascribed to a faulty inductive interaction between the ureteric bud and the metanephrogenic mesenchyme. As many as 50% of cases of renal agenesis in humans have been attributed to mutations of RET or GDNF, which are key players in the earliest induction of the ureteric bud. Individuals with unilateral renal agenesis are often asymptomatic, but typically the single kidney undergoes compensatory hypertrophy to maintain a normal functional mass of renal tissue.

An infant born with bilateral renal agenesis dies within a few days after birth. Because of the lack of urine output, reduction in the volume of amniotic fluid ( oligohydramnios ) during pregnancy is often an associated feature. Infants born with bilateral renal agenesis characteristically exhibit the Potter sequence , consisting of a flattened nose, wide interpupillary space, a receding chin, tapering fingers, low-set ears, hip dislocation, and pulmonary hypoplasia ( Figure 16.15 ). Respiratory failure from pulmonary hypoplasia is a common cause of neonatal death in this condition, especially when pulmonary hypoplasia is caused by disorders other than renal agenesis. A sequence is classified as a set of malformations secondary to a primary disturbance in development. In the Potter sequence, reduced urinary output secondary to renal agenesis or a urinary blockage is the factor that sets in motion the other disorders seen in this constellation ( Figure 16.16 ). The actual structural effects result from the lack of mechanical buffering by the greatly reduced amount of amniotic fluid.

Renal Hypoplasia

An intermediate condition between renal agenesis and a normal kidney is renal hypoplasia (see Figure 16.14B ), in which one kidney or, more rarely, both kidneys are substantially smaller than normal even though a certain degree of function may be retained. Although a specific cause for renal hypoplasia has not been identified, some cases may be related to deficiencies in growth factors or their receptors that are active during later critical phases of metanephrogenesis. As with renal agenesis, the normal counterpart to a hypoplastic kidney is likely to undergo compensatory hypertrophy.

Renal Duplications

Renal duplications range from a simple duplication of the renal pelvis to a completely separate supernumerary kidney . Similar to hypoplastic kidneys, renal duplications may be asymptomatic, although the incidence of renal infections may be increased. Many variants of duplications of the ureter have also been described (see Figure 16.14 ). Duplication anomalies are commonly attributed to splitting or wide separation of branches of the ureteric bud, the latter resulting from ectopic expression of GDNF more proximally along the mesonephric duct.

Anomalies of Renal Migration and Rotation

The most common disturbance of renal migration leaves a kidney in the pelvic cavity ( Figure 16.17A ). This disturbance is usually associated with malrotation of the kidney, so that the hilus of the pelvic kidney faces anteriorly instead of toward the midline. Another category of migratory malformation is crossed ectopia , in which one kidney and its associated ureter are found on the same side of the body as the other kidney ( Figure 16.17B ). In this condition, the ectopic kidney may be fused with the normal kidney.

In the condition of horseshoe kidney , which can occur in 1 in 400 individuals, the kidneys are typically fused at their inferior poles ( Figure 16.18 ). Horseshoe kidneys cannot migrate out of the pelvic cavity because the inferior mesenteric artery, coming off the aorta, blocks them. In most cases, horseshoe kidneys are asymptomatic, but occasionally pain or obstruction of the ureters may occur. This condition may be associated with anomalies of other internal organs. Pelvic kidneys are subject to an increased incidence of infections and obstructions of the ureters.

Anomalies of the Renal Arteries

Instead of a single renal artery branching off each side of the aorta, duplications or major extrarenal branches of the renal artery are common. Because of the appropriation of segmental arterial branches to the mesonephros by the metanephros, consolidation of the major external arterial supply to the kidney occasionally does not occur.

Polycystic Disease of the Kidney

Congenital polycystic disease of the kidney occurs in more than 1 in 800 live births and is manifested by the presence of hundreds to thousands of cysts of different sizes within the parenchyma of the kidney ( Figure 16.19 ). The most common form, autosomal dominant, is the result of mutations of the genes PKD1 and PKD2 , which produce the proteins polycystin-1 and polycystin-2 . These proteins, which are surface membrane receptors, affect various cellular processes, such as proliferation, polarity, and differentiation. Affected individuals exhibit persisting fetal patterns of location of these proteins, along with receptors for epidermal growth factor and sodium, potassium–adenosine triphosphatase (Na ⁺ ,K ⁺ -ATPase); the result is the budding off of spherical cysts from a variety of locations along the nephron. In some genetic mutants, the cysts are caused by disturbances in the orientation of mitoses within the developing ducts. In normal ductal development, mitoses are aligned along the long axis of the duct. In mutations that result in randomly oriented mitoses, the collecting ducts and even tubules begin to balloon out, forming cysts, instead of elongating. These cysts enlarge and can attain diameters greater than 10 cm. Cysts of other organs, especially the liver and pancreas, are frequently associated with polycystic kidneys.

Wilms Tumor

One of the most common childhood cancers is Wilms tumor , which forms on one or both kidneys. An important genetic basis for many of these cases is a mutation in the WT-1 gene, which plays an important role in the early development of the mesonephros (see p. 384). Another cause is the clonal expansion of rests of embryonic cells in the kidney that did not properly differentiate.

Ectopic Ureteral Orifices

Ureters may open into a variety of ectopic sites ( Figure 16.20 ). Because of the continuous supply of urine flowing through them, these sites are symptomatic and usually easy to diagnose. Their embryogenesis is commonly attributed to ectopic origins of the ureteric buds in the early embryo.

Cysts, Sinuses, and Fistulas of the Urachus

If parts of the lumen of the allantois fail to become obliterated, urachal cysts, sinuses , or fistulas can form ( Figure 16.21 ). In the case of urachal fistula, urine seeps from the umbilicus. Urachal sinuses or cysts may swell in later life if they are not evident in an infant.

Exstrophy of the Bladder

Exstrophy of the bladder is a major defect in which the urinary bladder opens broadly onto the abdominal wall ( Figure 16.22 ). Rather than a primary defect of the urinary system, this condition is most commonly attributed to an insufficiency of mesodermal tissue of the ventral abdominal wall. Although initially the ventral body wall may be closed with ectoderm, it breaks down in the absence of mesoderm, and degeneration of the anterior wall of the bladder typically follows. In male infants, exstrophy of the bladder commonly involves the phallus, and a condition called epispadias results (see p. 416). A reduction in the expression of sonic hedgehog (shh) signaling in the pericloacal epithelia may contribute to a deficiency of tissue in the bladder and the external genitalia. According to a different hypothesis, aneurysmic swellings of the dorsal aortae in the area may prevent tissues from fusing along the ventral midline and keep the walls of the cloaca from closing around the future bladder.

The most recent candidate for the testis-determining gene is one called Sry, a member of the Sox family of transcription factors and probably an evolutionary derivative of Sox-3 ; it is also located within a 35-kb region on the short arm of the Y chromosome ( Figure 16.24 ). The Sry gene encodes a 223-amino acid nonhistone protein belonging to a family of proteins that contain a highly conserved 79-amino acid DNA-binding region called a high mobility group box . After the gene was cloned, it was detected in many cases of gender reversal, including XX males with no ZFY genes. The SRY gene on the human Y chromosome is located near the homologous region, thus making it susceptible to translocation to the X chromosome.

The Sry gene is also absent in a strain of XY mice that are phenotypically female. Further experimental evidence consisted of producing transgenic mice with the insertion of a 14-kb fragment of the Y chromosome containing the Sry gene. Many of the transgenic XX mice developed into phenotypic males with normal testes and male behavior. In situ hybridization studies in mice have shown that expression of the Sry gene product occurs in male gonadal tissue at the time of sex determination, but it is not expressed in the gonads of female embryos.

Specification of Germ Cells, Migration Into the Gonads, and Entry Into Meiosis

The early appearance of primordial germ cells (PGCs) in the lining of the yolk sac and their migration into the gonads in human embryos is briefly described in Chapter 1 . Both descriptive and experimental studies in the mouse have shown that PGCs originate in the epiblast-derived extraembryonic mesoderm at the posterior end of the primitive streak. Recent research on monkeys suggests that in primates, PGCs may originate in the amnion. In the mouse, as few as six precursor cells become specified to become PGCs in response to BMP-2, BMP-4, and BMP-8b, which are secreted by nearby extraembryonic ectoderm. These cells maintain pluripotency by expressing Sox, Nanog, and Oct-4, much as these genes maintain the undifferentiated condition of blastomeres of cleaving embryos (see p. 65). They are protected by the transcriptional repressor Blimp-1 from entering the default transcriptional program that directs cells of the epiblast to become somatic cells.

Once specified, the PGCs begin a phase of active migration (see Figure 1.1 ), which takes them first from the base of the allantois to the future hindgut and, in a second phase, from the hindgut up the dorsal mesentery into the genital ridges (future gonads). During the migratory phase, PGCs are protected from undergoing apoptosis by the actions of Nanos-3 , an evolutionarily conserved protein involved in germ cell maintenance. The initial stages of migration of PGCs at some distance from the gonads are accomplished by active ameboid movement of the cells in response to a permissive extracellular matrix substrate. Tissue displacements through differential growth of the posterior region of the embryo may also contribute. During their migration, many PGCs are linked through long cytoplasmic processes. How these interconnections control either migration or settling down in the gonads remains to be determined. While they migrate through the dorsal mesentery, the PGCs proliferate in response to mitogenic factors such as leukemia inhibitory factor and Steel factor (Kit-ligand).

As the germ cells approach the genital ridges late in the fifth week of development, they may be influenced by chemotactic factors secreted by the newly forming gonads. Such influences have been shown by grafting embryonic tissues (e.g., hindgut, which contains dispersed germ cells) into the body cavity of a host embryo. The PGCs of the graft typically concentrate on the side of the graft nearest the genital ridges of the host or sometimes migrate into the genital ridges from the graft. Approximately 1000 to 2000 PGCs enter the genital ridges. When the PGCs have penetrated the genital ridges, their migratory behavior ceases, and a new set of genes is activated.

After entry into the genital ridges, PGCs in females enter meiosis, whereas those in males undergo mitotic arrest. Initially, male and female PGCs are equivalent, and under the influence of Dazl (deleted in azoospermia-like), they both progress to a meiosis-competent stage ( Figure 16.25 ). At this point, differing environments in male and female gonads exert a profound effect on the PGCs. In the female, retinoic acid , produced in the tubules of the adjoining mesonephros, is found in the gonad. Working through Stra-8 , which is required for premeiotic DNA replication, retinoic acid stimulates the PGCs to enter the meiotic cycle. In the male gonad, the action of the cytochrome P450 enzyme Cyp26b1 , produced by the Sertoli cells, catabolizes the mesonephrically derived retinoic acid into inactive metabolites. This action, along with the antimeiotic activity of Nanos-2 within the germ cells, prevents entrance of the PGCs into meiosis. Instead, they become arrested at the G ₀ phase of the mitotic cycle, where they remain until after birth. If Cyp26b1 is inactivated, male PGCs, like their female counterparts, are also propelled into the meiotic cycle. The female gonad suppresses the formation of both these meiosis-inhibiting factors (Cyp26b1 and Nanos-2).

Whether the PGCs, when in the gonad, develop into male or female gametes depends on the environment of the somatic cells in the gonad, not on their own genetic endowment. XY PGCs begin to develop into eggs if they are transplanted into a female gonad, and XX PGCs begin to develop into spermatogonia when they are transplanted into a male gonad.

Some PGCs follow inappropriate migratory pathways that lead the cells to settle into extragonadal sites. These cells normally start to develop as oogonia, regardless of genotype; they then degenerate. Rarely, however, the PGCs persist in ectopic sites, such as the mediastinum or the sacrococcygeal region, and ultimately may give rise to teratomas (see Chapter 1 ).

Establishment of Gonadal Gender