The mesonephros, sometimes referred to as the middle kidney, is an organ that helps developing mammals, reptiles, and birds with excretion. This organ develops in all vertebrates, and in fishes, it remains an excretory organ even in adulthood. In other types of animals, it only serves a temporary role, and either develops into other useful structures, or regresses to vestigal, or unused structures, depending on species and gender. Along with another developing kidney structure, the paramesonephrotic blastema, it is considered part of the Wolffian body, named for its discoverer, Caspar Wolff.
Excretion is an important function for any animal, which involves removing from the body any compounds that it has ingested. Due to this importance, the mesonephros forms quite early, appearing at just four weeks into embryonic development in humans. Initially, it arises from the mesoderm, a type of developmental tissue, in the nephrogenic cord, near the lower section of the developing spine.
This structure consists of two main sections. The mesonephric corpuscle includes bodies called vesicles, where foreign compounds can enter, and an s-shaped tubule that guides waste to the middle kidney's other main section. This area is called the mesonephric, or Wolffian duct, and helps to excrete waste from the embryo.
Humans have different developmental fates for the mesonephros, which are contingent upon gender, and occur around five months into development. Males see the mesonephric tubules develop into efferent, or outgoing ducts, of the testicles. The mesonephric ducts turn into several components of the reproductive system, such as the seminal vesicles and vas deferens. Other portions of this structure develop into portions of the vestigal organ known as the appendix.
Females do not have any usable structures that develop from the mesonephros. It shrinks in size, instead, although it does develop into some vestigal structures in the reproductive system and urinary tract. This is similar to this structure's fate in other species, including other mammals, birds, and reptiles, where the mesonephros regresses during development. Such a process occurs even more quickly in most other species, at around six to seven weeks into development.
The presence of the mesonephros is important as a guide to development, even if it is only used for a short time in many species. Studies have shown that the tissue of this structure send chemical messages to other cells. These signals guide the cells to this structure's location, where they develop into more mature organs like the gonads.