| Mesonephros | |
|---|---|
 Reconstruction of a humanembryo of 17 mm. (Label for Mesonephros is at center right.) | |
| Details | |
| Carnegie stage | 14 |
| Days | 22 |
| Precursor | Pronephros |
| Gives rise to | Kidney |
| Identifiers | |
| Latin | mesonephros |
| MeSH | D008650 |
| TE | E5.6.2.0.0.0.1 |
| FMA | 72171 |
| Anatomical terminology | |
Themesonephros (Greek:middlekidney) is one of threeexcretory organs that develop invertebrates. It serves as the main excretory organ of aquatic vertebrates and as a temporary kidney in reptiles, birds, and mammals. The mesonephros is included in theWolffian body afterCaspar Friedrich Wolff who described it in 1759. (The Wolffian body is composed of: mesonephros + paramesonephrotic blastema)
The mesonephros acts as a structure similar to thekidney that, in humans, functions between the sixth and tenth weeks of embryological life. Despite the similarity in structure, function, and terminology, however, the mesonephric nephrons do not form any part of the mature kidney or nephrons.[1]
In humans, the mesonephros consists of units which are similar in structure and function tonephrons of the adult kidney. Each of these consists of aglomerulus, a tuft ofcapillaries which arises from lateral branches of dorsal aorta and drains into theinferior cardinal vein; aBowman's capsule, a funnel like structure which surrounds the glomerulus; and amesonephric tubule, a tube which connects the Bowman's capsule to the mesonephric duct.[1] A unit consisting of a single glomerulus and the Bowman's capsule surrounding it is calledrenal corpuscle, and a unit consisting of single renal corpuscle with its associated mesonephric tubule is called a "nephron"[1] or "excretory mesonephric unit".[This quote needs a citation]
Formation of each mesonephric nephron begins when a bit of theintermediate mesoderm adjacent to themesonephric duct differentiates to form amesonephric vesicle.[1]
This vesicle then elongates to form themesonephric tubule, attaching to the mesonephric duct on one side. Meanwhile, an artery from the dorsal aorta begins extending towards the mesonephric tubule. When these two structures contact each other, they form the glomerulus and the Bowman's capsule surrounding it.[1] The mesonephric tubule is also known as theWolffian tubules (orKobelt's tubules).
On the medial side of the mesonephric duct, from the sixth cervical to the third lumbar segments, a series of tubules, the Wolffian tubules, develops. They increase in number by outgrowths from the original tubules. The change from solid masses of cells to instead become hollowed in the center. One end grows toward and finally opens into the mesonephric duct, the other dilates and is invaginated by a tuft of capillary bloodvessels to form a glomerulus.[citation needed]
The tubules collectively constitute the mesonephros.[citation needed]
The mesonephros as a whole produces urine from the 6th through the 10th week of development.[1] Despite the similarity in structure, function, and terminology, however, the mesonephric nephrons do not form any part of the mature kidney or nephrons.[1] As the more caudal nephrons form, the more cranial nephrons are already degenerating. In females, the mesonephros degenerates entirely, though vestigial structures such asGartner's ducts, theepoophoron, andparoophoron are common. In males, a few of the more caudal tubules will survive and give rise to theefferent ductules of thetestis,[1] theepididymis,vas deferens,seminal vesicle, as well as vestigial structures such as theappendix testis,appendix epididymis, andparadidymis.
The mesonephros persists and forms the anterior portion of the permanent kidneys in fish and amphibians, but in reptiles, birds, and mammals, it atrophies and for the most part disappears rapidly as the permanent kidney (metanephros) begins to develop[2] during the sixth or seventh week. By the beginning of the fifth month of human development, only the ducts and a few of the tubules of the mesonephros remain.
