mesonephros

The mesonephros is the middle stage of vertebrate kidney development, a transient excretory organ that forms after the pronephros and before the definitive metanephros. In humans and other amniotes, it develops from the intermediate mesoderm along the urogenital ridge, typically in the thoracic to upper lumbar region.

Structurally, the mesonephros consists of a linear series of mesonephric tubules that coil and connect to a duct called the mesonephric (Wolffian) duct. Each tubule typically receives a filtrate from a nearby glomerulus and drains into the duct. The renal blood supply arises from segmental arteries of the embryo. The entire structure is transient, forming caudally to the pronephros and cranially to the developing metanephros.

Function and fate are temporary in mammals. The mesonephros acts as a functional interim kidney during early fetal life, but its role is overtaken by the metanephros as the definitive kidney develops. In humans, the metanephros begins to form around the sacral region and becomes functional later in gestation, while the mesonephros degenerates. The ureteric bud, which gives rise to the ureter, renal pelvis, calyces, and collecting ducts of the permanent kidney, emerges from the caudal end of the mesonephric duct and interacts with metanephric tissue to induce nephron formation.

In addition to its transient excretory function, the mesonephric duct contributes to the reproductive system. In males, remnants persist as structures that differentiate into parts of the epididymis, vas deferens, and seminal vesicles; in females, most mesonephric ducts regress.

Clinical considerations include abnormalities in early renal development, which can affect the formation or regression of the mesonephros and related structures.