Horseshoe kidney

Horseshoe kidney, or sometimes renal fusion, is a congenital disorder where the two kidneys fuse together into one during fetal development, resulting in one large horseshoe-shaped kidney.

Normally during fetal development, the future kidneys develop in the pelvis as a structure called the metanephrose before finally migrating upward into the abdomen and becoming everyone’s favorite organ duo.

For people with horseshoe kidney, their kidneys fused together at some point during development, and there are two main working theories on how this might happen.

The first is mechanical fusion, which happens during the metanephros stage, which is around about the 5th week of gestation.

At this point, the two kidneys are still in the pelvis, and are therefore pretty close together, so close that it’s thought that some flexion or growth of the developing spine and pelvic organs essentially pushes them together, causing the lower or inferior poles of the kidneys to touch and fuse together, forming what’s called a fibrous isthmus, fibrous because it’s composed of connective tissue.

The other theory involves a teratogenic event. Teratogenic meaning something that disrupts fetal development in some way.

In this case it’s thought that the posterior nephrogenic cells, which are the cells that help to form part of the kidney, migrate and rendezvous in the wrong spot, and therefore again form an isthmus connecting the two kidneys, but this time since the isthmus is composed of kidney cells as opposed to connective tissue, it’s called a parenchymal isthmus.

Whichever one happens, now you’ve got this single, horseshoe-shaped kidney in the pelvis.