12. Development of the kidney and the urinary tract. Malformations

Learning objectives

  • Which germ layers does the urinary system develop from?
  • What is the pronephros?
  • What is the mesonephros?
  • What is the mesonephric duct and what does it give rise to?
  • What is the metanephros, when does it arise, and when does it become functional?
  • Which embryonic structure gives rise to the collecting system of the metanephros?
  • Describe the formation of the collecting system of the permanent kidney
  • Which embryonic structure gives rise to the excretory system of the metanephros?
  • Describe the formation of the excretory system of the permanent kidney
  • Which embryonic structure gives rise to the renal pelvis and ureter?
  • Describe the formation of the bladder
  • Describe the formation of the urethra
  • What is renal agenesis and dysplasia?
  • What is congenital polycystic kidney disease?
  • What is horseshoe kidney?
  • What is bladder exstrophy?
  • What is the urachus?
  • Describe the urachal abnormalities

Development of the kidney

All of the urinary system develops from intermediate mesoderm and endoderm. During embryonic life three urinary systems develop, only one of which remains as the mature urinary system. These three are the pronephros, mesonephros, and metanephros.

Pronephros

The pronephros arises in the cervical region of the embryo. It has some temporary excretory function, but it degenerates completely by week 5.

Mesonephros

The mesonephros arises in week 5 in the thoracic and lumbar regions. It has nephrons and a collecting duct called the mesonephric or Wolffian duct which drains to the cloaca. Like the pronephros, the mesonephros temporarily has excretory function.

Most of the mesonephros disappears, but in males the Wolffian duct and tubules of the mesonephros give rise to the epididymis, ductus deferens, seminal vesicle, and ejaculatory duct. In females this duct and these tubules degenerate.

A small part of the Wolffian duct remains on the caudal end, which gives rise to a bud called the ureteric bud.

Metanephros and the permanent kidney

The metanephros gives rise to the permanent kidney. Like the pronephros and mesonephros it arises from mesoderm. It also arises in week 5. The kidney becomes functional in week 12.

Collecting system

The collecting system of the permanent kidney, including the ureter, renal pelvis, collecting ducts, and everything in-between, arises from the ureteric bud. This bud is covered by so-called metanephric mesoderm (or blastema), which forms a “cap” around the it. This gives the metanephros similar appearance as the permanent kidney. The ureteric bud induces the metanephric mesoderm to differentiate into the excretory part of the kidney, the nephrons.

The ureteric bud penetrates the metanephric mesoderm. The bud then dilates and splits into portions, forming the renal pelvis and the calyces. Each calyx forms two buds, and these buds continue to subdivide until 12 generations of collecting tubules have formed.

Excretory system

Each collecting tubule is covered on its distal end by a “cap” of metanephric mesoderm. The collecting tubule induces this mesoderm to form the excretory system, i.e. the nephrons. The glomeruli, Bowman capsules, proximal convoluted tubules, loops of Henle, and distal convoluted tubules, all arise from this metanephric mesoderm.

Nephrons are continuously formed until birth, at which point there are approximately 1 million nephrons in each kidney.

The ascent of the kidney

The metanephros initially forms in the pelvis, but the final position of the kidney is more cranial, in the abdomen. The kidney ascends because of growth in the lumbar and sacral regions.

Development of the urinary tract

The renal pelvis and ureter arise from mesoderm, while the rest of the urinary tract arises from endoderm.

Renal pelvis and ureter

The renal pelvis and ureter arise from the ureteric bud, as described above.

Bladder

The primitive kidneys and gut initially empty through the same opening, the cloaca, which originates from endoderm. During weeks 4 – 7 the cloaca is divided into an anterior and a posterior portion by the urorectal septum. The anterior portion is the urogenital sinus, while the posterior portion is the anal canal. The anterior portion of the urogenital sinus differentiates into the bladder.

The ejaculatory ducts, (which originate from the Wolffian duct) and the ureters enter the bladder and gives rise to the trigone of the bladder. This means that the trigone of the bladder has mesodermal origin.

Urethra

The remaining portion of the urogenital sinus differentiates into the whole urethra in the female, and into the prostatic and membranous parts of the urethra in the male.

Malformations

Renal agenesis and dysplasia

If the ureteric bud doesn’t develop correctly or at all, the metanephric mesoderm will never be induced to form nephrons correctly or at all. This can cause agenesis or dysplasia of the kidney. In renal agenesis one or both kidneys fail to develop at all, while in renal dysplasia one or both kidneys develop abnormally.

Congenital polycystic kidney disease

Congenital polycystic kidney disease is a congenital disease in which numerous cysts form in the kidneys. It exists in two variants, autosomal recessive polycystic kidney disease and autosomal dominant polycystic kidney disease.

Horseshoe kidney

In 1/600 people the inferior poles of the kidneys fuse, forming one horseshoe-shaped kidney. This kidney is usually unable to ascend to its normal position, as the inferior mesenteric artery prevents it. This causes the kidney to remain in the pelvic area. Horseshoe kidney is usually asymptomatic.

Professor Judith Horváth has stated that she has horseshoe kidney.

Bladder exstrophy

Bladder exstrophy is a condition in which the bladder herniates through a defect in the ventral abdominal wall. Epispadias always occurs with it.

Urachal abnormalities

The urachus is a structure which connects the urinary bladder to the yolk sac through the umbilical cord. It arises from the allantois. The urachus normally closes and degenerates to form the median umbilical ligament, but in some cases this doesn’t occur.

If the urachus doesn’t close at all, a urachal fistula remains, which allows urine to drain from the bladder and out of the umbilicus. If the urachus mostly closes but not completely a urachal cyst forms. If the urachus remains open at the part closest to the urachus a urachal sinus forms.

Summary

  • Which germ layers does the urinary system develop from?
    • The nephrons, renal pelvis, and ureters arise from intermediate mesoderm
    • The bladder and urethra arise from endoderm
  • What is the pronephros?
    • It’s a temporary embryonical structure with some excretory function
    • It degenerates completely by week 5
  • What is the mesonephros?
    • It’s a temporary embryonic structure with some excretory function
  • What is the mesonephric duct and what does it give rise to?
    • The mesonephric duct, also called the Wolffian duct, drains urine from the mesonephros to the cloaca
    • It gives rise to the epididymis, ductus deferens, seminal vesicle, and ejaculatory duct in males
    • The caudal end of the duct forms the ureteric bud
  • What is the metanephros, when does it arise, and when does it become functional?
    • The metanephros is the embryonic structure which will develop into the permanent kidney
    • It arises in week 5 and becomes functional in week 12
  • Which embryonic structure gives rise to the collecting system of the metanephros?
    • The ureteric bud
  • Describe the formation of the collecting system of the permanent kidney
    • The ureteric bud penetrates metanephric mesoderm, after which it will dilate and split into smaller and smaller portions, forming the renal pelvis, major and minor calyces, and the collecting tubules
  • Which embryonic structure gives rise to the excretory system of the metanephros?
    • The metanephric mesoderm
  • Describe the formation of the excretory system of the permanent kidney
    • Each collecting tubule induces an overlying “cap” of metanephric mesoderm to differentiate into the glomeruli, Bowman capsules, proximal convoluted tubules, loops of Henle, and distal convoluted tubules
  • Which embryonic structure gives rise to the renal pelvis and ureter?
    • The ureteric bud
  • Describe the formation of the bladder
    • The cloaca is initially divided into the urogenital sinus and the anal canal by the urorectal septum
    • The anterior part of the urogenital sinus becomes the bladder
    • The trigone of the bladder arises from the Wolffian duct and the ureters, making it originate from mesoderm rather than endoderm
  • Describe the formation of the urethra
    • The urethra arises from the part of the urogenital sinus which didn’t give rise to the bladder
  • What is renal agenesis and dysplasia?
    • Agenesis refers to failure of development of one or both kidneys
    • Dysplasia refers to abnormal development of one or both kidneys
  • What is congenital polycystic kidney disease?
    • It’s a congenital disease in which numerous cysts form in the kidneys
  • What is horseshoe kidney?
    • In some cases, the lower poles of the kidneys fuse, forming a horseshoe-shaped kidney
  • What is bladder exstrophy?
    • It’s a condition in which the bladder herniates through a defect in the ventral abdominal wall
    • Epispadias always occurs with it
  • What is the urachus?
    • An embryonic structure which connects the urinary bladder to the yolk sac through the umbilical cord
    • It closes and forms the median umbilical ligament
  • Describe the urachal abnormalities
    • If the urachus remains open a urachal fistula forms
    • If the urachus doesn’t close completely a urachal cyst forms
    • If the urachus remains open only at the upper end a urachal sinus forms

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13. Development of the female genital organs. Malformations

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