Urinary System Development

• A longitudinal elevation of the mesenchyme referred to as the urogenital ridge forms on each side of the dorsal aorta
• The nephrogenic cord is the part of the urogenital ridge that gives rise to the urinary system

Development of the Urinary System

• Three excretory organs develop, these include the pronephros, mesonephros and the metanephros

Pronephros

• The first set of kidneys
• Rudimentary and non-functional
• It appears in the fourth week of development
• A cluster of cells in the neck region
• It degenerates and will contribute to the next set of kidneys (mesonephros)

Mesonephros

• Mesonephric ducts make an appearance late in the fourth week
• They consist of rods that are caudal to the pronephros
• Ducts induce the formation of mesonephric buds which form the mesonephroi
• The mesonephroi consist of approximately 40 glomeruli with mesonephric tubules
• Tubules open into the mesonephric ducts
• Ducts open into the cloaca
• It creates urine between weeks 6 and 10
• It degenerates at the end of the first trimester and regresses in females

Mesonephros in Males

• In males, the mesonephric tubules become efferent ductules of the testis
• The mesonephric ducts become the appendix of the epididymis
• The mesonephric ducts also turn into the duct of the epididymis, the vas deferens, and the ejaculatory duct and seminal gland

Metanephros

• Serves as the primordium of the permanent kidney
• Appears in the fifth week
• Will function about 4 weeks later
• Urine is excreted into the amniotic cavity, where it mixes with amniotic fluid
• It develops from two sources; the ureteric bud (metanephric diverticulum) which forms the collecting portion and the metanephrogenic blastema which forms the excretory portion

Ureteric Bud (Metanephric Diverticulum)

• It elongates and penetrates the metanephrogenic blastema
• It forms the ureter, renal pelvis, major & minor calyces and collecting tubules
• The stalk of the ureteric bud becomes the ureter
• The collecting tubules undergo repeated branching
• The first four generations of tubules enlarge and coalesce to form the major calices
• The other four generations coalesce and form the minor calices

Metanephrogenic Blastema

• It consists of mesenchymal cells
• Each newly formed collecting tubule induces mesenchymal cells of metanephrogenic blastema to form metanephric vesicles
• Vesicles elongate and become metanephric tubules
• Tubules with their glomeruli form nephrons

Nephrons

• The proximal ends of the tubules become invaginated by glomeruli
• The distal end becomes confluent with a collecting tubule to form a uriniferous tubule

Positional Changes of the Kidneys

• Kidneys lie close to each other in the pelvis and gradually move to lie in the abdomen further apart
• They also rotate medially almost 90 degrees
• Ascent of the kidney results from straightening of the vertebral curvature and growth of the thorax and abdomen
• Attain adult position around week 9

Blood Supply

• The renal arteries are branches of the common iliac arteries
• As they ascend, the kidneys receive blood supply from the distal aorta
• The kidneys receive new branches from the aorta as they ascend further and the caudal branches usually disappear
• Ascent stops when the kidneys reach the level of the suprarenal glands
• The kidneys receive their most cranial branches from the abdominal aorta, these becoming the permanent renal arteries

Accessory Renal Arteries

• Accessory renal arteries are because of a persistence of embryonic vessels
• Approximately 25% of adult kidneys have 2-4 renal arteries that usually arise from the aorta and may enter the kidney directly
• If a polar renal artery (enters the inferior pole) is present, it may obstruct the ureter causing hydronephrosis
• They are end arteries so if the area they supply is damaged or ligated it may become ischemic

Renal Agenesis

• This is the absence of one or both kidneys
• The ureteric bud(s) fail to develop caused by faulty tissue-tissue interactions, the failure of the ureteric bud to penetrate the metanephric blastema, and no nephrons are induced by or connected to the collecting tubules

Unilateral Renal Agenesis

• Occurs in approximately 1/1000 births, is more common in males, and the remaining kidney undergoes hypertrophy

Bilateral Renal Agenesis

• Occurs in approximately 1/3000 births, is more common in males, causes oligohydramnios, and is incompatible with postnatal life

Abnormal Position

• This is when there is a failure to ascend, remaining usually in the pelvis, or in the inferior abdomen

Pancake Kidney

• This is also known as a discoid kidney, occurs when the kidneys fuse in the pelvis

Horseshoe Kidney

• When the poles of the kidneys fuse
• This usually happens at the inferior pole
• Positioned anterior to the inferior lumbar vertebrae
• Normal ascent is prevented by the inferior mesenteric artery
• It is functional, and each kidney has a normal ureter and blood supply
• It may produce no symptoms but is prone to increased occurrence of renal stones and infection

Duplications

• A supernumerary kidney is rare
• Occurs due to early division of the ureteric bud
• Duplication of ureter (Y shaped bifid ureter)
• The caudal end of the ureter attaches normally to the bladder
• One branch may end blindly
• Infections of the ureter are more common and asynchronous contractions lead to reflux and stagnation of urine

Ectopic Ureter

• The ureter is carried caudally with the mesonephric duct and is incorporated into the caudal portion of the vesical part of the urogenital sinus
• In males, an ectopic ureter may open into the neck of the bladder, the prostatic part of the urethra, the ductus deferens, prostatic utricle, or seminal gland
• In females, an ectopic ureter may enter the neck of the bladder, urethra, vagina, or vestibule of the vagina

Vesico-ureteric Reflux

• It is when the intramural region of the ureter is too short
• It increases the risk of backflow
• It is commonly congenital (primary reflux)
• Secondary causes are high pressure voiding states and duplex collecting systems or bladder exstrophy
• It is associated with recurrent UTIs
• The imaging of choice is micturition cystourethrogram(MCUG) for girls and Fluoroscopy for boys
• Management can be conservative using antibiotic prophylaxis or surgical options

Polycystic Kidney Disease

• Numerous cysts form within the kidney
• May be inherited as autosomal dominant or autosomal recessive

Polycystic Kidney Disease: Autosomal Recessive (ARPKD)

• Mutations in genes that encode proteins important for the function of cilia
• Is a progressive disorder in which cysts form from collecting ducts
• Kidneys enlarge, renal failure occurs in childhood and are diagnosed in utero or at birth by ultrasonography

Polycystic Kidney Disease: Autosomal Dominant ADPKD

• Results in the formation of cysts from all parts of the nephron
• Usually does not cause renal failure until adulthood

Development of the Bladder

• A cloacal region of the hindgut is divided into urogenital sinus anteriorly and anorectal canal posteriorly
• The urogenital sinus is divided into 3 parts:
  • Vesicle/Bladder part – develops into the bladder
  • Pelvic part – becomes the urethra (membranous and prostatic urethra in males, entire urethra in females)
  • Phallic part – contributes to the penile urethra or vestibule of the vagina

Bladder Development

• The bladder develops mainly from the vesicular part of the urogenital sinus
• The trigone is derived from the caudal ends of the mesonephric ducts
• As the bladder enlarges, distal parts of the mesonephric ducts are incorporated into its dorsal wall andcontribute to connective tissue of the trigone of the bladder
• The ureters open separately into the bladder
• The remodeling and expansion of the bladder wall lead to the ureteric openings shifting laterally and superiorly while the mesonephric ducts in males shift inferiorly to empty into the developing prostatic urethra

Urachal Defects

• The lumen of the allantois obliterates and becomes a thick fibrous cord called the urachus
• The median umbilical ligament is the fibrous remnant of the urachus in the adult
• A remnant of the lumen may persist
• A urachal cyst occurs when a patent area of the urachus dilates
• The urachal sinus is the patent end of the urachus and opens at the umbilicus (or at the bladder)
• A urachal fistula occurs when the entire urachus is patent so urine can escape from the umbilicus

Ventral Body Wall Defects

• This includes bladder exstrophy, which is the incomplete closure of the anterior abdominal wall
• It results in exposure and protrusion of the mucosal surface of the bladder
• The epispadias is when the urethra opens on the dorsum of the penis
• Exstrophy of the cloaca leads to a severe body wall defect in which the lumens of both the bladder and the anorectal canal are exposed

Development of Genital Ducts

• Male and female embryos have two pairs of genital ducts initially; the mesonephric (Wolffian) ducts and the paramesonephric (Müllerian) ducts

Mesonephric Ducts

• The drain urine from mesonephric kidneys
• A key component of the male reproductive system by forming the epididymis, vas deferens and ejaculatory duct
• Mostly degenerate in females

Paramesonephric Ducts

• Key component of the female reproductive system, they form the uterine tubes, uterus and upper vagina
• Degenerate in males