Development of the Kidneys
In the embryo, the kidneys develop from three overlapping sequential systems; the pronephros, the mesonephros, and the metanephros. They are all derived from the urogenital ridge.
Pronephros
The pronephros appears in the 4th week of development.
Its development begins in the cervical region of the embryo. Segmented divisions of intermediate mesoderm form tubules, known as nephrotomes. In total, 6-10 pairs of nephrotomes are formed.
These tubules join into the pronephric duct, which is a duct that extends from the cervical region to the cloaca (distal end) of the embryo. This early system is non-functional and regresses completely by the end of week 4.
Mesonephros
The mesonephros develops caudally (inferiorly) to the pronephros. First, the presence of the pronephric duct induces nearby intermediate mesoderm in the thoracolumbar region to form mesonephric tubules.
These tubules receive a tuft of capillaries from the dorsal aorta – allowing for the filtration of blood – and they drain into the mesonephric duct (a continuation of the pronephric duct). They act as a primitive excretory system in the embryo, with most tubules regressing by the end of the 2nd month.
Additionally, the mesonephric duct sprouts the ureteric bud caudally, which induces the development of the definitive kidney.
Metanephros
The metanephros forms the definitive kidney. It appears in the 5th week of development and becomes functional around the 12th week.
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.
Adapted from work by Ashley Sawle [CC BY-SA 3.0], via Wikimedia Commons
