Development of the renal system

The renal system starts developing during week 4 of intrauterine life.

At this point, the embryo is made up of three primitive germ layers: the ectoderm, the mesoderm and the endoderm.

The mesoderm also has three parts: the paraxial mesoderm, which flanks the embryo’s future vertebral column; the intermediate mesoderm, which is just lateral to the paraxial mesoderm; and the lateral plate mesoderm, which is the most lateral of all.

The intermediate mesoderm on either side of the embryo condenses to form a cylindrical structure called the urogenital ridge.

This ridge runs parallel to the embryo’s future vertebral column, and it gives rise to both the urinary and genital systems.

The portion of the urogenital ridge called the nephrogenic cord develops into the urinary structures.

Now, during the development of the urinary system, there are three sets of structures that emerge from the nephrogenic cord, and they form in a craniocaudal fashion—from head to tail-end.

The first structure to emerge from the nephrogenic cord is the pronephros, which appears in the neck region of the embryo at the beginning of week 4.

The pronephros consists of the pronephric duct and the nephrotomes in front of it.

The pronephric duct is basically a pipe that runs down the length of the nephrogenic cord, and the nephrotomes are small chunks of tissue that break off from the nephrogenic cord.

However, the pronephros doesn’t produce urine, and regresses by the end of week 4.

Before the pronephros completely disappears, a second set of structures called the mesonephros appears in the thoracic and upper lumbar region of the nephrogenic cord.

The mesonephros has a mesonephric duct and mesonephric tubules in front of it.

The mesonephric duct develops off of the pronephric duct, making it longer so that it reaches all the way to the cloaca, which is the last part of the primitive digestive tract.

So for a short while, the urinary and digestive system share a common exit.

Just like the nephrotomes, the mesonephric tubules break off as chunks of tissue from the nephrogenic cord.

The mesonephric tubules are hollow, S-shaped tubes.

On one end, they connect to the mesonephric duct, and at the other end, the tubule forms a cup shape called a Bowman’s capsule around a clump of capillaries called a glomerulus.

This primitive structure extracts fluid from the capillaries and the fluid flows down the duct to form urine, which drains through the mesonephric duct into the cloaca.

This system is in place until week 10, when the permanent kidneys take over and the mesonephros regresses.

Around week 5, the metanephros develops, and it forms the permanent kidneys—so for a couple of weeks, the metanephros and mesonephros coexist.

The metanephros forms in the pelvic region.

First, intermediate mesoderm near the mesonephric duct differentiates into metanephric mesoderm, sometimes called the metanephric blastema.

The metanephric mesoderm produces growth factors that travel to the mesonephric duct; in response, the duct sprouts a small bud called the ureteric bud, which is connected to the mesonephric duct through the ureteric stalk.

Over time, the ureteric bud lengthens and it secretes growth factors that causes the metanephric mesoderm to grow.

This is called reciprocal induction, because the metanephric mesoderm and the ureteric bud promote each other’s growth.

Eventually, the ureteric bud reaches the metanephric mesoderm and grows into it, like two lovers running (albeit very slowly) towards one another.

The metanephric mesoderm surrounds the end of the ureteric bud, leaving just the ureteric stalk uncovered.

The ureteric stalk lengthens and forms the ureter around week 6.