Embryology of the kidney and the urinary system

Question 1

State the 3 structures in the kidney formation that are formed from cranial to caudal sequence.

Answer 1

Pronephros
Mesonephros
Metanephros

Question 2

Describe how the kidney forms.

Answer 2

1. Pronephros Formation (4th Week):

The earliest stage of kidney development is the formation of the pronephros. However, in humans, this structure is not functional and regresses quickly.
The pronephros forms from the intermediate mesoderm, and it consists of primitive tubules that drain into the pronephric duct.

2. Mesonephros Formation (4th-8th Week):
   The pronephros regresses, and the mesonephros takes over. This is the temporary kidney that functions during early embryonic development.
   The mesonephros consists of nephric tubules and a mesonephric duct. Some components of the mesonephros contribute to the male reproductive system.
3. Metanephros Induction (5th Week):

The metanephros is the definitive kidney and begins to form in the pelvic region.

Inductive signals from the mesoderm initiate the formation of the ureteric bud, a diverticulum of the mesonephric duct. This bud penetrates the metanephric mesenchyme.

The ureteric bud from the mesonephric duct makes contact with a caudal region of intermediate mesoderm – the metanephric blastema (Fig 2). Collectively, these blastema form the metanephric system, which has two components:

Collecting system – derived from the ureteric bud.
It dilates to create the ureter, renal pelvis, major and minor calyces and collecting tubules – terminating at the distal convoluted tubule.
If the uretic bud splits too early, two ureters, or two renal pelvices connecting to one ureter may result.

Excretory system – derived from the metanephric blastema.

Each collecting tubule from the collecting system is covered by a metanephric tissue cap which gives rise to the excretory tubules.
These excretory tubules (along with the developing glomeruli) form the kidney’s functional units – the nephron.

The proximal end of the excretory tubule forms the Bowman’s capsule around a glomerulus, while the distal end elongates to form the proximal convoluted tubule, loop of Henle and distal convoluted tubule.

The definitive kidney initially develops in the pelvic region before ascending into the abdomen. In the pelvis, the kidney receives its blood supply from a pelvic branch of the abdominal aorta and as it ascends, new arteries from the abdominal aorta supply the kidney. The pelvic vessels usually regress but can persist as accessory renal arteries.

Question 3

Describe the following anomalies in kidney embryology.
1. Horseshoe kidney.
2. Pelvic kidney
3. Double ureter
4. urachal fistula
5. Exstrophy of the bladder
6. Exstrophy of the cloaca

Answer 3

1. Horseshoe Kidney:

Normal Kidney Development: During normal embryonic development, the kidneys ascend from the pelvic region to their final position in the retroperitoneal space, usually around the level of the first or second lumbar vertebra.

Anomaly: In the case of a horseshoe kidney, there is a fusion of the lower poles of the kidneys, resulting in a U-shaped or horseshoe-shaped structure. This occurs due to the failure of the kidneys to ascend completely during embryonic development. Instead of the usual ascent, the lower poles of the kidneys become trapped below the inferior mesenteric artery, leading to fusion.

2. Pelvic Kidney:

Normal Kidney Development: In a typical embryonic development, the kidneys initially form in the pelvic region and gradually ascend to their final position in the retroperitoneal space.

Anomaly: A pelvic kidney occurs when one or both kidneys fail to ascend completely and remain located in the pelvic area instead of reaching the normal adult position. This can happen due to incomplete ascent or a failure to migrate altogether during embryonic development.

3. Normal Ureter Development: The ureter is the tube that carries urine from the kidneys to the bladder. During normal embryonic development, the ureteric bud, an outgrowth from the mesonephric duct, gives rise to the ureter and other parts of the urinary system.

Anomaly: Double ureter, also known as duplex kidney or bifid ureter, occurs when there are two separate ureters draining a single kidney. This anomaly arises due to abnormal branching of the ureteric bud during development.

4. Urachal Fistula:

Normal Urachus Development: The urachus is a tubular structure that connects the fetal bladder to the umbilicus during early development. It typically degenerates and becomes the median umbilical ligament.

Anomaly: A urachal fistula occurs when there is a persistent connection between the bladder and the umbilicus, resulting from the incomplete closure or patency of the urachus.

5. Exstrophy of the Bladder:

Normal Bladder Development: The bladder forms from the cloaca during embryonic development. The cloaca is a common chamber that initially receives the urinary and digestive tracts.

Anomaly: Exstrophy of the bladder is a congenital anomaly where the anterior wall of the bladder is open and exposed on the abdominal surface. This occurs due to a failure of the lower abdominal wall to close properly during development.

6. Exstrophy of the Cloaca:

Normal Cloaca Development: During early embryonic development, the cloaca is a common chamber that receives the urinary, reproductive, and digestive tracts. It later differentiates into separate structures.

Anomaly: Exstrophy of the cloaca is a severe malformation where the abdominal wall, bladder, and sometimes the intestines are open and exposed. It results from the failure of the cloaca to properly differentiate during development.