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PH-RN10

Developmental anomalies of the kidney and urinary tract

How the ureteric bud and metanephric blastema build the kidney, and how errors in that meeting, the ascent, the ureteric openings or the urethra, or in ciliary genes, cause agenesis, horseshoe kidney, reflux, valves and polycystic kidneys.

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  • 6 steps · 10 questions
  • 3D normal anatomy

By the end, you can

  • Describe how the ureteric bud and the metanephric blastema induce each other to form the collecting system and the nephrons, and how the kidney ascends.
  • Relate agenesis, hypoplasia, dysplasia, ectopic and horseshoe kidneys to the developmental step that failed.
  • Explain how the position of the ureteric bud decides whether a ureter refluxes or obstructs, including in a duplex system.
  • Explain how pelviureteric junction obstruction and posterior urethral valves damage the kidney, before and after birth.
  • Contrast autosomal dominant and autosomal recessive polycystic kidney disease in genes, cysts, kidneys, liver and age at presentation.
  • Recognise the classic appearances, grades and tests of these anomalies, and explain how surgery and drugs act on each mechanism.

What it is

Developmental anomalies of the kidney and urinary tract (often grouped as congenital anomalies of the kidney and urinary tract, CAKUT) are structural defects present from before birth. Most are errors of one step: the meeting of the ureteric bud and metanephric blastema (agenesis, hypoplasia, dysplasia), the kidney’s ascent (ectopic and horseshoe kidneys), the position of the ureteric opening (duplex systems, reflux), or the outflow (pelviureteric junction obstruction, posterior urethral valves, hypospadias). The polycystic kidney diseases are inherited disorders of tubular cilia rather than errors of shape.

4 types to tell apart
Too few nephrons
The kidney is absent (agenesis), small with normal structure (hypoplasia), or disorganised (dysplasia, including multicystic dysplastic kidney).
Wrong position or shape
A kidney that did not ascend (pelvic kidney), crossed the midline, or fused with its partner (horseshoe kidney).
Faulty drainage
Duplex systems, ectopic ureters and ureteroceles, vesicoureteric reflux, pelviureteric junction obstruction and posterior urethral valves.
Inherited cystic disease
Autosomal dominant (ADPKD) and autosomal recessive (ARPKD) polycystic kidney disease, from faults in proteins of the primary cilium.

Normal function

How the permanent kidney is built

The permanent kidney (metanephros) starts in the fifth week from two tissues. The grows out of the near the and pushes into a cap of mesoderm, the . Each instructs the other (reciprocal induction): signals from the blastema make the bud grow and branch, and each new bud tip makes the blastema round it form . The becomes the ureter, renal pelvis, calyces and collecting ducts; the blastema becomes the nephrons, from the glomerular capsule to the distal tubule. Nephrons keep forming until about 36 weeks; none are added later in life.

The young kidney lies in the pelvis. During the second month it rises to the upper lumbar region, turning so that its hilum faces the midline, and takes its blood supply from successively higher branches of the aorta. Meanwhile the cloaca is divided, and the lower end of the mesonephric duct is absorbed into the back of the bladder to form the trigone. This carries each ureteric opening upwards and outwards, so the ureter ends up tunnelling obliquely through the bladder wall, the valve that stops reflux.

The fetal kidneys make urine from late in the first trimester, and from mid-pregnancy fetal urine is the main source of amniotic fluid, which the lungs need in order to grow. In boys the urethra through the penis forms when the urethral folds on its underside fuse, under the influence of androgens.

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