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.