Development of Murine Urinary System

The development of the murine kidneys

As noted above, the metanephros develops from the most caudal part of the nephrogenic cord that is itself derived from the intermediate plate mesoderm (see above). The initial renal anlage that develops from the most rostral part of the nephrogenic cord is termed the pronephros. The latter is not believed to function in the mouse, or in any other mammal. However, within the pronephros, a relatively small number of pronephric (or nephrotomal) vesicles form in a cranio-caudal direction, and these “drain” into the pronephric duct. As the pronephros is a relatively transient structure in mammals, the absence of differentiated glomeruli within it strongly suggests that it probably does not act as even a primitive excretory organ. Despite the complete degeneration of the pronephros, the pronephric duct is retained. This structure is then taken over by the mesonephros (also termed the “Wolffian” body), and is only then termed the mesonephric portion of the nephric duct.

By contrast to the pronephros, the histological features of the mesonephros, with its primitive glomeruli, suggest that it probably functions as a primitive kidney, and is involved in the production of much of the amniotic fluid. Within the two mesonephroi, one located on either side of the dorsal mesentery of the hindgut, a substantial number (in the region of about 40 or more) of cranio-caudally segmented mesonephric tubules are formed. It has, however, been suggested that only the most rostrally located 4-6 pairs of mesonephric tubules drain into the mesonephric portion of the nephric duct. This is now seen to extend along the length of the mesonephroi, being located towards their lateral sides. The mesonephros is also retained over a considerably longer period than the pronephros, but gradually undergoes regression in a cranio-caudal direction. While the rostral part displays clear evidence of regression its more caudal part appears to display evidence of functional activity. Within the medial part of the mesonephros, vesicles are formed, although no glomeruli are formed there in this species. It is, however, difficult to believe that the relatively enormous mesonephroi do not have an excretory role in the mouse, only serving as a base for gonadal differentiation. In the human embryo, the medial part of the mesonephric tubules enlarges, become invaginated by capillaries, and form glomeruli. These then take on an excretory role. In the mouse, the mesonephric ducts appear to be patent throughout their length.

During the period that the majority of the mesonephroi are present, they form two quite voluminous structures. These are located one on either side of the hindgut, and occupy a substantial part of the caudal half of the peritoneal cavity. It is largely due to their presence (and that of the liver and stomach with its associated gastric dilatation in the upper part of the peritoneal cavity, and the urogenital sinus in its lower part) that the potential space within the peritoneal cavity is so limited. This thus stimulates the extrusion of the embryonic midgut region into the so-called physiological umbilical hernia where the midgut can increase in length, and undergo the initial stages of its rotation, before it returns into the peritoneal cavity in order to complete its rotation and differentiation.

Similarly, during the period when the majority of the mesonephros is still present, the caudal part of the mesonephric duct extends caudally until it makes contact with the urogenital sinus in the region that eventually corresponds with the supero-lateral part of the trigone of the future bladder. It is also at about this stage that the urogenital sinus is separated from the caudal part of the hindgut due to the down growth of the urorectal septum. Once contact is made between the mesonephric portion of the nephric duct and the urogenital sinus, it appears likely that any excretory products produced by the mesonephros will be transferred into the urogenital sinus. Shortly after contact is established between the caudal part of the mesonephric duct and the urogenital sinus, this stimulates the formation of a cranio-laterally directed diverticulum, termed the ureteric bud. It is at this stage that the mesonephric tissue shows more rapid evidence of regression, and much of its rostral part has by this time completely regressed.

The two ureteric buds grow in the direction of the most caudal part of the nephrogenic cord tissue, and this is then stimulated (possibly by inductive interaction) to form the primitive metanephros. This will in due course develop into the definitive kidney, or metanephros.

Unlike the mesonephros, the metanephros shows no obvious evidence of cranio-caudal segmentation. The metanephros, in fact, has a rounded appearance, and the future glomeruli (one of the critical components of the excretory system) tend to be located towards its periphery. When the ureteric bud makes contact with the metanephric mesenchyme, the terminal parts of the ureteric buds start to bifurcate. Mesenchyme closely apposed to the tips of the buds is called cap mesenchyme; it will later go on to form nephrons.

The detailed events associated with the differentiation of the nephrogenic mesenchyme are somewhat complex. It has been suggested that each terminal branch of the ureteric bud, the ureteric tips, stimulate the associated cap mesenchyme tissue to form a renal vesicle (the most primitive stage of nephron development: a stage I nephron). This then elongates, becomes a comma-shaped and then an S-shaped body (stage II nephron), and makes contact with and fuses with the distal component of the ureteric bud, the ureteric tips. The ureteric tips and trunks (ureteric tree) then form the collecting duct. The proximal end of the S-shaped body gives rise to Bowman’s capsule (also termed the glomerular capsule) and glomerulus, whilst the medial and distal ends will become the nephron tubules. Endothelial cells invade the cleft in the S-shaped body to make a capillary knot-like outgrowth, the glomerular tuft, which goes on to form the glomerulus. The visceral epithelium, or podocyte layer because it consists of podocytes, of the renal corpuscle is closely apposed to the endothelial cells of the glomerulus. Together, the Bowman’s capsule and the glomerulus comprise the definitive renal corpuscle. The rest of the nephron elongates to form components of the proximal tubule, the loop of Henle, the distal tubule and connecting tubule segments. The distal pole of the developing nephron connects to the ureteric tip that induced it at an early stage of nephron/ collecting duct development, by S-shaped body stage, before differentiation of the proximal tubule, the loop of Henle and the distal tubule are complete. This connection allows the excretory products produced by the kidney to be removed and subsequently transferred, via the ureter, into the bladder where they are stored until it is appropriate to empty the bladder.

Thus, while the ureteric bud tissue differentiates into the drainage system of the definitive kidney, the metanephric mesenchymal tissue differentiates into its excretory component. By this means, large numbers of nephrons are induced to form from the metanephric tissue, and each of these units, as indicated above, drains into the definitive bladder through the derivatives of the ureteric bud. As indicated above, further differentiation of the excretory component of the system subsequently occurs, with the formation in the nephrons of the proximal and distal tubules, sometimes called 'convoluted tubules' because of their shape when fully mature, and the loop of Henle. The most caudal part of the ureteric bud gives rise to the ureter. For details of the ascent of the kidneys, and their associated blood supply during this period, see Kaufman & Bard (1999).

The development of the murine bladder

Before discussion of the development of the external genitalia, it is appropriate to discuss the development of the bladder here. As will be seen, this is a complex organ. It initially has a common origin with the caudal part of the hindgut, and is first evident at about TS15 (9.5 dpc). By about TS17 (10.5 dpc), this whole region then dilates to form the cloaca, and accordingly initially possesses an endodermal lining. The cloacal membrane develops where the cloaca makes contact with the overlying surface ectoderm and, characteristically, no intervening mesoderm develops in this location. On either side of the latter, the surface ectoderm is elevated to form the two genital folds (see below). A shallow depression forms in the region between the two genital folds. Shortly afterwards, at about TS17 (10.5 dpc), the first evidence of the substantial down growth, termed the urorectal septum is seen. This will, by about TS19-20 (11.5-12 dpc), completely separate the dorsally located future hindgut region from the ventrally located urogenital sinus. Most of the hindgut region at this stage is suspended by an elongated dorsal mesentery. With the division of the cloaca into these two distinct regions, the cloacal membrane becomes subdivided into a dorsally located anal membrane, and a ventrally located urogenital membrane. In the case of humans the future fibrous perineal “body” (also termed the central tendon of the perineum) then develops in the midline between the latter two membranes. A similar structure develops in the mouse.

Because the caudal one-third of the future anal canal is formed by the indentation of the surface ectoderm, to form the anal pit (or proctodaeum), this region constitutes the most caudal part of the future gastrointestinal tract. It subsequently develops into the lower one-third of the future anal canal, and consequently has an ectodermal lining. By contrast, the most caudal part of the embryonic hindgut forms the upper two-thirds of the anal canal, and is therefore lined by endoderm. These two regions fuse, and the region of fusion soon canalises. The relatively bloodless site of fusion is sometimes termed the pectinate line (or Hilton’s white line). The cloacal sphincter, being the derivative of the cloacal membrane, gives rise to the external anal sphincter, dorsally, and to the urogenital diaphragm, ventrally. Branches of the pudendal nerves subsequently supply both of these sphincters. By about TS20-21 (12-13 dpc), the endodermally derived urogenital sinus initially forms all of the lining of the future bladder, with the urachus at its apex. The latter structure is directed ventrally and slightly rostrally towards the umbilical region, and indeed runs in the umbilical cord. Initially, it contains a lumen that remains canalised until shortly before birth. The most rostral part of the urogenital sinus gives rise to the majority of the lining of the bladder, and the most caudal part is believed to give rise to its urethral derivatives, although exactly which parts has yet to be determined (see above).

Initially, the ureteric bud emerges from the caudal portion of the nephric duct following the physical connection between the caudal end of the nephric duct and the wall of the urogenital sinus. Technically, therefore, the ureter is thus not directly connected with the bladder, the final insertion site. In order to establish direct connections with the bladder, the caudal ureteric bud (the future ureter orifice) undergoes transposition. During this process, the base of the nephric duct including the ureteric bud stalk (the common nephric duct) becomes absorbed into the urogenital sinus, establishing a direct connection between the future ureter orifice and the primitive bladder. At the same time, degeneration of the common nephric duct occurs via an apoptotic mechanism, enabling the ureter to separate from the nephric ducts. Once separated from the nephric duct, the position of the ureter orifice shifts further anterior as the surrounding urogenital sinus tissue undergoes growth and expansion forming the bladder.

What is also evident, is that by the time of birth the wall of the bladder is thick and muscular. The latter constitutes the detrusor muscle. What is also evident is that the mucous membrane that lines the bladder, the urotheloum of the bladder, is of the transitional variety, while the loose nature of the submucosa, the lamina propria of the bladder, allows the mucosa of the empty bladder to be thrown into numerous folds.

Observations on urinary development obtained from the analysis of staged histologically sectioned mouse embryos

The principal information about the stages of development when the initial features of the renal system are observed has been obtained from the detailed analysis of serially sectioned mouse embryos isolated at sequential stages of development. For further information, see the following sources: Theiler (1972, or 1989); Kaufman (1994, or subsequent reprints). For anatomical database, see: Kaufman & Bard (1999). For additional information, see also the anatomical ontology prepared by M. Little et al. (2007)  A high-resolution anatomical ontology of the developing murine genitourinary tract. Gene Expr Patterns. 7(6):680-99. 

TS12 (about 8 dpc): The first appearance of the intermediate plate mesoderm is seen, and this approximately corresponds with the first appearance of the somites.

TS13-15 (about 8.5-9.5 dpc): First evidence of nephrogenic cord formation and the differentiation of its most rostral part to form the pronephros. Later, the nephrogenic cord differentiates into the urogenital ridge. Mesonephric tubules are first recognised (at about TS15), and these drain into the mesonephric duct that is clearly seen to be patent along its length. The duct extends caudally towards the urogenital sinus.

TS16-17 (about 10-10.5 dpc): Mesonephric tubules are now clearly seen, as are mesonephric vesicles. The first evidence of the urorectal septum is seen, and will soon separate the hindgut from the urogenital sinus, and divide the cloacal septum into hindgut and urogenital components.

TS18-19 (about 11-11.5 dpc): The first evidence of differentiation of the metanephric region (caudal component) of the nephrogenic cord is first seen, although much of the mesonephros with its mesonephric ducts and vesicles are still present. The ureteric buds (sometimes termed the metanephric ducts) are also first seen at this stage. By TS19, the first evidence of branching of the ureteric bud derivatives within the nephrogenic interstitium (peripheral blastema) is also noted.

TS20-21 (about 12-13 dpc): The future renal cortex of the metanephros (now clearly the definitive kidney) is recognizable, and shows evidence of peripheral blastema and early nephrons. It should be noted that the primitive S-shaped bodies are more readily seen at TS21 of development than at TS20. The medullary component of the future kidney is now also delineating. As far as the “drainage” component of the future kidney is concerned, branching of the ureteric bud tissue is now clearly seen. This gives rise to the renal pelvis associated with the presence of a number of primitive renal collecting ducts within the hilar region of the kidney. As the general location of the kidneys rise rostrally within the peritoneal (or abdominal) cavity, the ureters correspondingly increase in length, and are clearly seen to be patent from an early stage of their differentiation. The mesonephros at this stage has now almost completely regressed.

TS22-23 (about 14-15 dpc): Early nephrons are now seen in the cortical region of the kidney as are numbers of maturing glomerular tufts, while medullary interstitium (stromal cells) are dispersed throughout much of the renal medulla. By TS23, the outermost region of the kidney now shows evidence of differentiation into a well-defined renal capsule, while the rest of the kidney is clearly subdivided into an outer renal cortex, subjacent to which is a zone that represents the region occupied by the cap mesenchyme tissue. There is also an inner medullary region. With regard to the ureteric bud-derived tissue, the renal pelvis is now clearly seen. The primitive collecting ducts and the ureter are also showing increased evidence of differentiation.

TS24-25 (about 16-17 dpc): In the cortical region of the kidney, proximal and distal tubules are first recognised, as well as glomerular tufts and Bowman’s capsules. In relation to the drainage system of the kidney, collecting ducts are also now clearly recognised.

TS26 (about 18 dpc): Within the renal cortex, the region subjacent to the capsule contains all components of the excretory system, including large numbers of glomerular tufts and the associated proximal and distal tubules (although these may occasionally be seen at earlier stages of development). The associated ascending and descending components of the loops of Henle, also termed the immature loops of Henle at this stage, may also be seen within the renal medulla. This region is now seen to contain considerably less undifferentiated mesenchyme tissue than formerly appeared to be the case. All of the components of the drainage system are now present and readily recognised, including the renal pelvis and the collecting tubules. They also now appear to possess a distinct endothelial lining. The ureter is also clearly seen, although readily recognised from about TS21. At all stages, the ureters are surrounded by a substantial amount of mesenchyme tissue.

 

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