3.2 Introduction

A description of each urological organ will be described individually; hence, overlap and repetition will inevitably exist to allow a detailed understanding of the development of each organ. Three basic cell layers comprise the embryonic disc which becomes the embryo: the ectoderm (originates from the amniotic surface), the mesoderm (originates form inpouring cells from the ectoderm), and the endoderm (yolk sac). The majority of the urogenital tract is derived from the mesoderm. Organ development in general occurs between the 3rd and 10th week of gestation (Table 3.1).

3.3 Embryology of the Kidneys and Ureters

The basic unit of the kidney is the renal pyramid, which is arranged like a bunch of flowers in a vase (Figure 3.1). The flowers are the glomeruli; the stalks are the collecting tubules; and the vase is the calix. The design of the normal pyramid is important in preventing reflux of urine up into the renal parenchyma.

There are three paired kidney systems during foetal development (Figure 3.2), with only the third system being of functional importance. First, the pronephros forms and rapidly regresses in the cervical region of the intermediate mesoderm during the fourth week. The pronephros in humans is both rudimentary and segmented.

Later in the fourth week, the unsegmented mesonephros forms from the intermediate mesoderm in the upper thoracic to upper lumbar segments. These appear as a pair of sausage‐shaped swellings on the posterior abdominal wall on either side of the mesentery – the genitourinary ridges. A faint groove demarcates each ridge into a medial gonadal and lateral nephrogenic part (Figures 3.3 and 3.4). These swellings lengthen and acquire primitive nephron‐like structures, which is a collection of capillaries that form a glomerulus at the medial extremity, and Bowman’s capsule, which forms around the glomerulus, forming the renal corpuscle. These simple excretory units may function briefly before regressing in the eighth week. The mesonephros functions for a short time during early foetal life by producing urine from the sixth through to the eighth weeks of development.

On the lateral aspect and adjacent to the mesonephros, the mesonephric ducts advance distally to drain into the cloaca (this is the primitive hindgut which goes on to form the bladder and rectum) at the caudal end of the embryo. Whilst the caudal aspects of the tubules are differentiating, the cranial tubules and the glomeruli degenerate, with the majority of the mesonephros absent by the end of the second month of gestation. The mesonephric system disappears completely in the female around the eighth week. In the male, the mesonephric ducts (also known as the wolffian ducts) persist, giving rise to the efferent ductules of the testes, the epididymis, vasa, seminal vesicles, and appendix epididymis.

Third, the metanephros (the permanent kidneys) develops from the fifth week from metanephric mesoderm in the most caudal region of the nephrogenic ridge, the lateral aspect of the genitourinary ridge. As the tail end of the foetus curls up, the hindgut is curled with it and so are the nephrogenic ridges with their wolffian ducts, which twist upwards and inwards. Branches from the most caudal part of the wolffian (mesonephric) ducts enter the metanephros. These branches, or outgrowths, are called the ‘ureteric buds’ (Figure 3.5). In contrast to the first two systems, excretory units only form by a process called ‘reciprocal induction’ between the ureteric bud and metanephric tissue caps.

The collecting ducts develop from the ureteric bud (fifth week, Figure 3.6). The ureteric bud subdivides and induces formation of the glomeruli in the mesenchyme of the metanephros. The branches of the bud then grow peripherally into the cortex, dilating and splitting repeatedly until about 15 generations of ducts have formed (Figure 3.7). The first four or five generations of the dividing ureteric branches become dilated and incorporated in the eventual renal pelvis (Figure 3.8). The next four or five generations form the major calices and collecting tubules (Figure 3.8) [3]. The successive generations elongate and converge on the minor calyx (seventh week), thereby forming the renal pyramid in the flower–vase configuration described previously. Subsequent generations elongate and converge to form renal pyramids, and ultimately, they form around one million collecting ducts per kidney (until the fifth month).

The metanephric tissue caps covering each collecting tubule form renal vesicles which develop into nephrons. Capillaries grow into the opposite end of the nephron giving rise to glomeruli (Figure 3.9). From about the 10th week, urine is produced by the metanephros; however, nephrons continue to form until birth. After birth, no further nephrons will form (approximately 700 000 per kidney), but existing ones will continue to grow. This growth is responsible for the change from lobulated kidneys at birth to kidneys with a smooth outline.

The metanephros ascends during weeks 6–10 as a result of the elongation of the sacral and lumbar regions of the embryo as well as loss of the initial curvature of the embryo. The arterial supply originates from the aorta, and serial arteries are formed and regress during renal ascent. Some vessels may remain as accessory renal arteries.

During the fourth to sixth weeks of gestation, while the caudal end of the foetus curls up and bends the hindgut into a U configuration, the mesoderm grows down into the gap between the future rectum and bladder, thus forming the urorectal septum (Figure 3.10). This septum separates the cloaca into a primary urogenital sinus (ventrally) and rectum (dorsally). The wolffian ducts lie in this wedge‐shaped septum, and grow down with it. They also become bent into a loop and take the ureteric buds with them (Figure 3.10) [4]. Part of this septum is incorporated into the bladder to form the trigone, and because of the incorporated loop of the wolffian duct, the ureteric duct comes to open into the bladder cephalad to the duct. In males, the wolffian duct becomes the vas deferens and seminal vesicles, whilst in the females these ducts regress in the absence of testosterone [4]. At this stage, the tail end of the foetus is roughly 1‐cm long and the space between the tail and the umbilical cord is filled by the cloacal membrane. On either side of this membrane are the two small genital tubercles (Figure 3.11). This membrane is formed by tightly packed ectoderm and endoderm cells with no intervening mesoderm. As this membrane dissolves, the two genital tubercles fuse to form the united phallic tubercle (Figure 3.11).

While all this is taking place, the mesonephros is withering away, but as it regresses, the wolffian duct generates a second duct, parallel and lateral to it, the müllerian duct (Figure 3.12). As the urogenital ridge twists round, so the müllerian ducts approach each other, meet in the midline in front of the wolffian ducts, and burrow down in the urorectal septum (Figure 3.12). The urorectal septum is partially absorbed into the trigone, and the müllerian ducts open into the urethra medial to, and in front of, the wolffian ducts.

The subsequent fate of the wolffian and müllerian ducts is determined by the X and Y chromosomes. Until the fourth week, the urogenital ridge is neuter. At four weeks it is invaded by gonadal cells, which migrate by amoeboid movements from the yolk sac across the coelom and burrow into the gonadal ridge (Figure 3.13). By the sixth week, it is estimated that there are about 60 000 gonadal cells in each gonadal ridge. The male ones are active at once; the female gonadocytes stay dormant for another two weeks.

3.3.1 Relevant Congenital Malformations

Partial or complete ureteric duplication results from early splitting of the ureteric bud. In a complete duplex system, the Weigert‐Meyer rule states that the ureter draining the lower moiety tends to reflux as a result of a shorter submucosal tunnel positioned laterally and superior; the upper pole tends to obstruct, be ectopic, or form ureteroceles and is positioned medially and inferiorly (Figure 3.14). An ectopic ureter may drain into the bladder, bladder neck, or prostatic urethra in males, or into the vagina, uterus, or ovary in females (Figure 3.15). The pathophysiology of duplex kidney is explained by the insertion of the ureter into the bladder, whilst the lower pole tends to reflux due to a shorter submucosal tunnel, and the upper pole tends to obstruct, be ectopic, or form ureteroceles.

Failure of renal ascent gives rise to a pelvic kidney (Figure 3.16). Midline fusion of both kidneys during their ascent gives rise to a horseshoe kidney, with further ascent limited by the root of the inferior mesenteric artery. The midpoint joining both kidneys is known as the isthmus (Figure 3.17). Crossed fused renal ectopia results from both kidneys ascending on the same side of the body and fusing in the process (Figure 3.18).

Renal agenesis results from failed reciprocal induction, intrinsic defects within the mesenchyme, or involution of a multicystic dysplastic kidney. Multicystic dysplastic kidney may be the result of faulty ureteric bud development. Renal dysplasia results from defects in reciprocal induction or from obstruction during the foetal period.

3.4 Embryology of the Bladder

The foetal hindgut is curled, following the outline of the tail end of the embryo, in the shape of a hook (Figure 3.19). The caudal most part of the hindgut remains in communication with the allantois and will form the bladder. As stated, the urorectal septum descends at four to six weeks to separate the cloaca into the urogenital sinus anteriorly and the anal canal posteriorly. The cloacal membrane dissolves to open both canals (Figures 3.20 and 3.21).

As the allantois shrinks it becomes a solid cord – the urachus – linking the apex of the bladder to the umbilicus. In clinical terms, the urachus becomes the median umbilical ligament upon closing. If it remains patent, the patient will have a congenital umbilical fistula. The urogenital sinus itself will give rise to the bladder, the posterior urethra (male) or entire urethra (female), and the anterior urethra (male) up to the edge of the glans penis. These are all endodermal (originates from yolk sac) in origin.

The male anterior urethra is not initially tubular, but rather a flattened urethral plate, which is pulled forwards with the growing phallus (Figure 3.22). It then folds in sideways and closes along the midline at around 12 weeks (Figure 3.23 and 3.24). The terminal part of the male urethra (fossa navicularis and external urethral meatus) is formed by ingrowth of ectoderm (derived from amniotic sac) from the tip of the glans penis (15 weeks).

The lower ends of the mesonephric ducts (due to become the vasa efferentia) and the lower ends of the ureters become incorporated into the posterior wall of the bladder, and thus form the trigone (Figures 3.25 and 3.26). Descent of the testes then causes the vas deferens on either side to swing anteriorly over the ureter (‘water under the bridge’).

3.5 Embryology of the Indifferent Genital System

The gonads begin their development from the urogenital ridge, located behind the coelom. They divide longitudinally, in the third week, to form the gonadal or genital ridge on the medial side and the embryonic mesonephros on the lateral side, which later becomes the urinary tract. Paramesonephric ducts (müllerian ducts) develop from these genital ridges.

Between weeks five and six of embryonic development, the germ cells arising from the yolk sac integrate into the gonadal ridge after travelling by amoeboid movement through the umbilical cord and the coelom (Figures 3.27–3.28). Upon the arrival of germ cells, primitive sex cords form in the still indifferent gonad. Failure of gonadal development occurs if the germ cells do not reach the gonadal ridges because they trigger the development of the gonad into ovary or testis [5].

In the human embryo, the gonads remain undifferentiated until about week seven of development. Depending on the XY genetic constitution they then differentiate into the testes or the ovaries [6].

3.6 Embryology of the Male Genital System

Once in the gonadal ridge, the presence of the sex‐determining region Y (SRY) gene located on the short arm of the Y chromosome leads to testicular development and male phenotyping. Down streaming from SRY produces steroidogenesis factor (SF1) and SOX9 that stimulate testicular cellular differentiation. This induces the first step of the organogenesis of the testes, the formation of gonadocytes, and proliferation of Leydig cells and sustentacular cells of Sertoli (Figures 3.29 and 3.30) [7]. This development not only relies on the presence of the SRY gene, but also the lack of the DAX1 gene (which can down regulate the SRY gene action) and WNT4 gene, which is responsible for female gonadal development.

These cells then proliferate with the aid of SRY gene proteins and form the primary sex cords. These further proliferate and extend into the medulla of the gonad to form the testis or medullary cords. Once here, the cords branch with their deep ends anastomosing to form the tubules of the rete testis. The gonadal cords further develop to give rise to the testicular cords which differentiate to become the seminiferous tubules. The tubuli recti are formed from the narrowing of the deeper portion of the semiferous tubules, and it converges into the tubuli recti. The tubules’ connections with the germinal epithelium are discontinued with the formation of a dense network of fibrous connective tissue known as the ‘tunica albuginea’ [5].

Male genital development depends on the SRY gene, anti‐müllerian hormone (AMH; also called müllerian inhibitory substance [MIS]), insulin‐like hormone 3, testosterone, and dihydrotestosterone (DHT).

The gene does two things consecutively:

1. It makes the gonadocytes differentiate into Sertoli cells which secrete another simple polypeptide, the müllerian duct‐inhibiting factor. This has a dramatic effect: the entire müllerian duct system disappears within a single day, leaving behind only the tiny vestige of the utriculus masculinis in the verumontanum (Figure 3.31). MIS also inhibits the formation of the uterus and fallopian tubes (the müllerian structures) [7–9].
   
2. Approximately one week later (approximately week eight), the SRY gene enables the differentiation of the germ cells located between the testicular cords into Leydig cells derived from the mesenchyme of the gonadal ridge. These contain 17‐ketosteroid reductase which is involved in the synthesis of testosterone, which is activated by the enzyme 5‐alpha reductase to 5‐alpha DHT. This active substance reacts with a cytosol receptor in the phallic tubercles and wolffian ducts to secrete growth factor. This results in the two changes necessary to convert the neuter foetus into the male. The cytosol receptor factor is a product of one of the genes in the X chromosome.

In the presence of testis determining factor (SRY protein), medullary cords of the testis, and the rete testis form (Figure 3.32). Formation of the tunica albuginea follows. Sertoli cells (from epithelium) and Leydig cells (from mesenchyme) form dependent on the SRY protein. SRY protein stimulates AMH production by Sertoli cells (seventh week), which in turn causes Leydig cells to produce testosterone and insulin‐like hormone 3 (ninth week). The testis cord continues to remain solid until the onset of puberty, when it acquires a lumen to form the seminiferous tubules, which connects with the rete testis and enters the ductuli efferentes, which are remnants of the mesonephric system. The ductus deferens is formed when rete testis is joined to the wolffian duct with the aid of the ductuli efferentes [5].

AMH causes involution of the paramesonephric ducts. Only small remnants from the paramesonephric ducts persist (i.e. appendix testes, utricle). Testosterone influences development of the mesonephric ducts and male external genitalia. Mesonephric ducts differentiate into the central zone of the prostate, ejaculatory ducts, vas deferens, seminal vesicles, epididymis, and efferent ductules (Figures 3.33 and 3.34)

3.6.1 The Descent of the Testis

Testicular descent happens in two phases, guided by the action on the gubernaculum (Figure 3.35).

1. Passive phase: Dependent on AMH/MIS and insulin‐like hormone 3 guiding abdominal descent to the inguinal ring (8–15th week)
   
2. Active phase: Dependent on testosterone through the inguinal canal (24–28th week), and to the base of the scrotum (28th‐33rd week).

During their descent, the testes acquire a layer of peritoneum, which becomes the tunica vaginalis.

During development the testis begins its journey in the lumbar area of the retroperitoneum. It transfers from here to the scrotum near the end of the third month of pregnancy. This is mediated by testosterone and the gubernaculum (Figure 3.36), a lump of jelly, which forms an expanding track, which leads to and inserts into the genital swelling, the future scrotum [10]. Ectopic testes are formed when the gubernaculum leads them in the wrong direction, such as towards the penis or the thigh, and incomplete descent occurs if the gubernaculum fails to form a path to the scrotum (Figure 3.37) [11–13].

3.6.2 Relevant Congenital Malformations

As the testis descends, it is accompanied by the processus vaginalis. The lumen of the processus vaginalis is normally obliterated within a few weeks of birth; however, if it persists, it gives rise to defects such as a congenital hernia, hydrocele, an encysted hydrocele of the cord, or an abdominoscrotal hydrocele (Figures 3.38 and 3.39) [14].

Maldescent of a testis results in an undescended testis. Malposition of the gubernaculum or an abnormally long gubernaculum result in an ectopic testis. Failure of, incomplete, or inappropriate development of the genital system leads to disorders of sexual differentiation.

In males, the müllerian duct lingers as two tiny vestiges, the utriculus masculinis in the verumontanum and the appendix testis, neighbouring the appendix epididymis which is a vestige of the wolffian duct (Figures 3.31 and 3.34 ) [5]. If there is a congenital deficiency of MIS, phenotypical males are born with fallopian tubes and a uterus, usually found by chance at laparoscopic orchidopexy or hernia repair, and occasionally, associated with testicular tumours [7].

3.7 Embryology of the Prostate

As described previously, the male foetus develops in the presence of a Y chromosome, which encodes the SRY protein, thus enabling testicular differentiation and the production of androgens such as testosterone. In addition to the actions of SRY protein and androgens, a third factor is required for male development, AMH, also known as MIS [15]. This causes the müllerian (paramesonephric) duct to degenerate, forming the prostatic utricle (or utriculus masculinis). Blandy originally described this as, ‘a volcanic crater on the summit of the verumontanum’.

The prostate forms in the mesenchyme of the urogenital septum. The cloacal membrane, a thin film which covers the convexity of the hindgut, regresses to leave gaps in front of and behind the urogenital septum, and in so doing, forming the urethra and anus. Running down in this septum are the müllerian and wolffian ducts and the ureteric buds (Figure 3.40).

At approximately week eight, the Leydig cells of the foetal testis secrete testosterone [16, 17], and as a consequence, the human prostate begins its development at about the 10th week of gestation. The initial outgrowths of the epithelial ‘prostatic’ buds from the urethra into the urogenital sinus (UGS) occur in response to the binding of 5α‐dihydrotestosterone to androgen receptors localised in the surrounding mesenchymal tissue [18–21]. There are five pairs of buds. The top pairs are derived from mesoderm (i.e. wolffian structures) and form the transitional, periurethral, and central zones of the prostate, whereas, the bottom pairs are derived from endoderm and form the peripheral zone.

These prostatic buds begin as solid cords of epithelial cells that elongate and undergo extensive branching morphogenesis during the latter stages of foetal growth to develop primitive lumens [22]. During weeks 13–15, serum testosterone elevates and remains high until week 25, which in turn induces epithelial differentiation. At this point, the three important epithelial populations, other than the stem cells, are distinct: luminal, basal, and neuroendocrine cells [22]. The stromal component compromises of fibroblasts, smooth muscles, and myofibroblasts. At week 25, the testosterone level diminishes, and the gland remains in a quiescent state until puberty. The central zone is primarily sensitive to testosterone, whereas the peripheral and transitional/periurethral are sensitive to DHT.

Summary: DHT prompts development of the prostate from urethral endoderm and surrounding mesoderm (13–16th weeks), giving rise to the transitional zone and peripheral zone. The central zone is formed from the mesonephric duct.

3.8 Embryology of the Penis and Urethra

Two crucial events occur in the male embryo, between the fifth and seventh week:

• the disappearance of the müllerian ducts
  
• the transformation of the phallic tubercles

Both events are orchestrated by the two sex chromosomes. On the Y chromosome, the SRY gene controls germ cell differentiation into Sertoli cells (whose müllerian inhibiting factor causes the müllerian ducts to disappear). By the eighth week of gestation, the mesenchymal cells of the genital ridge differentiate into Leydig cells which secrete testosterone, which enters the wolffian ducts and the phallic and genital tubercles. These tissues contain 5‐alpha reductase, an enzyme which activates testosterone to a more potent form, DHT. DHT binds to a cytosol receptor protein and sets off the changes in growth, which allow the wolffian ducts to develop into the vasa efferentia, seminal vesicles, and epididymis. The phallic and genital tubercles become the penis, scrotum, and urethra (Figure 3.41). A gene on the X chromosome codes the cytosol receptor protein. Each step on this ladder of events is carried out by a certain enzyme coded by a single gene. In the absence of this organised testicular development, the differentiated gonads will develop into ovaries by the 13th and 14th weeks of gestation [23]. Each step may go wrong and result in one of the variations of intersex.

By week seven, the cloacal membrane dissolves, and the primitive bladder opens on the ventral aspect of the genital tubercle (Figure 3.42). This elongates to form the penis under which a groove is folded in from either side to form the urethra (Figure 3.43). Rods of mesenchyme in each fold differentiate into the corpora cavernosa and corpus spongiosum. At the tip of the penis, a groove demarcates the glans through which a solid cord extends and then becomes canalised as, the terminal urethra (Figure 3.44). Skin grows forwards from the coronal sulcus to enclose the glans in the prepuce and then becomes adherent.

The scrotum is formed by the meeting together in the midline of the two genital tubercles over the urethra (Figure 3.45).

All these processes must be complete within a critical window of time. If the genital folds and penis are not completed by the 12th week they never will be. However much androgen is given later on, all it can do is slightly enlarge the penis [24, 25].

Summary: Under the influence of DHT (formed from testosterone by the action of 5‐α‐reductase), the penis, scrotum, and prostate form. DHT causes elongation of the genital tubercle after six weeks to form the phallus with the urethral plate. The genital and labioscrotal folds on either side move caudally and fuse along the midline, which carries the urethra to the tip of the formed penis and forms the scrotum with the midline scrotal septum. DHT sets off the train of events leading to the growth and fusion of the phallus, in‐rolling of the urethral tube, formation of the scrotal sac, and the downward migration of the testes [7, 8]. The wolffian ducts are dependent on testosterone to develop and form the epididymis, vas deferens, and seminal vesicles.

3.9 Neuter State

Without a Y chromosome or its SRY genes, the foetus stays neuter. The neuter state seems at first glance to be female: There is no phallus; the müllerian ducts persist; and the wolffian ducts fail to turn into the vas deferens.

3.10 Embryology of the Female Genital System

The paramesonephric ducts form the basis of the female genital system. In the absence of the SRY gene, default development is down the female pathway. The mesonephric ducts regress and only leave a few remnants, including Gartner’s cysts, paroophoron, and epoophoron. The paramesonephric ducts develop into fallopian tubes, uterus, and the upper two‐thirds of the vagina (Figure 3.46).

With regards to external genitalia, the genital tubercle develops into the clitoris, the urogenital sinus forms the introitus and vestibule of the vagina (distal one‐third), the genital folds become the labia minora, and the labioscrotal folds become the labia majora (Figure 3.47).

3.11 Embryology of the Adrenal Gland

There are two distinct components of the adrenal gland: cortex and medulla. The cortex is derived from the mesoderm. At about the fifth to sixth weeks of life, the foetal cortex develops arising from the genitourinary ridge, subsequently surrounded by a second wave of mesothelial cells, which will eventually form the definitive cortex near the developing gonads and kidneys; tiny rests of adrenal cortical tissue are common in the renal cortex, retroperitoneum, and testis as well as the broad ligament near the ovary (Figures 3.48 and 3.49). After birth, the foetal cortex regresses except for its outermost layer, which differentiates into the reticular zone. The adrenal cortex shares many of the enzymes of gonads – notably those for the synthesis of steroids – so that some inborn errors of metabolism affect them both.

In the seventh week of foetal life, neuroblasts from the sympathetic system of the neural crest (ectodermal cells) invade the medial aspect of the developing adrenal cortex to form the medulla. After a week, they differentiate into sympathicoblasts and pheochromocytes containing the intracellular catecholamines, adrenaline, and noradrenaline.

Cells derived from the neural crest migrate on each side of the aorta to form the sympathetic chains. It is from these paraganglia cells that extra‐adrenal neuro‐ectodermal tumours (paragangliomas) arise.

In foetal life, the adrenals are larger than the kidneys and are still about one‐third of their size at birth.