The mesonephros, often called the “middle kidney,” is a temporary excretory organ that forms during embryonic development. This structure plays a transient yet important role, particularly in higher vertebrates like reptiles, birds, and mammals, where it functions as an early kidney before being replaced by a more advanced organ. Its presence highlights the sequential development of the excretory system, serving as an initial filtering mechanism in the developing embryo. The mesonephros is one of three kidney structures that appear and, in most cases, largely regress during embryonic growth.
Anatomy and Early Function
The mesonephros develops from the intermediate mesoderm between the fourth and eighth weeks of human embryonic development. It consists of several distinct parts that work together to filter waste. These components include mesonephric tubules, which are small tubes that form from the nephrogenic cord, and the mesonephric duct, also known as the Wolffian duct, that serves as a collecting channel.
Each mesonephric tubule is associated with a glomerulus, a small tuft of capillaries that arise from branches of the dorsal aorta. Blood is filtered through these glomeruli into Bowman’s capsule, which surrounds the glomerulus. The filtered fluid, containing waste, then enters the mesonephric tubule and flows towards the mesonephric duct.
The mesonephros functions as a primitive excretory system, producing urine from the sixth through the tenth week of human development. While its structure and function are similar to the nephrons of a mature kidney, the mesonephric nephrons themselves do not become part of the permanent adult kidney. In lower vertebrates like fish and amphibians, the mesonephros persists as the functional adult kidney.
Transition and Transformation
In higher vertebrates, the mesonephros undergoes changes. In reptiles, birds, and mammals, most of the mesonephros degenerates and disappears as the metanephros, the permanent kidney, forms and takes over excretory functions. This regression occurs around the tenth week of human development, with only ducts and a few tubules remaining by the fifth month.
While much of the mesonephros regresses, portions are retained and transform into structures, particularly in the reproductive systems of males and females. In male embryos, the mesonephric ducts and some mesonephric tubules persist and differentiate into components of the male reproductive tract. These include the epididymis, which is involved in sperm maturation and storage, and the vas deferens, a tube that transports sperm.
In female embryos, most mesonephric structures regress. Remnants can persist as vestigial structures. These may include the epoophoron, paroophoron, and Gartner’s ducts, which are non-functional and located near the ovaries or uterus. The distinct fates in males and females underscore its close developmental relationship with the urogenital system.
Importance in Development and Beyond
The mesonephros is important as a developmental precursor. Its formation and transformation are steps in the development of the permanent kidney, the metanephros. The mesonephric duct, for instance, induces the formation of the ureteric bud, an outgrowth that forms the definitive kidney. It also contributes to the reproductive system, providing structures that become functional components in males and, in some cases, vestigial remnants in females.
Evolutionarily, the mesonephros provides insights into vertebrate kidney development. Its role as a functional adult kidney in aquatic vertebrates like fish and amphibians, contrasted with its temporary role in higher vertebrates, reflects an evolutionary progression towards more complex excretory systems. The sequential appearance of the pronephros, mesonephros, and metanephros during embryonic development mirrors the evolutionary history of kidney development.
Incomplete regression or abnormal transformation of mesonephric remnants can have clinical implications. While benign, these remnants can lead to cysts or other anomalies in adults. Such occurrences, though uncommon, highlight the relevance of understanding embryonic development and the impact of developmental variations on adult anatomy.