Mesonephros

The mesonephros (Greek: middle kidney) is one of three excretory organs that develop in vertebrates. 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 the Wolffian body after Caspar Friedrich Wolff who described it in 1759. (The Wolffian body is composed of: mesonephros + paramesonephrotic blastema)

Mesonephros
Reconstruction of a human embryo of 17 mm. (Label for Mesonephros is at center right.)
Details
Carnegie stage14
Days22
PrecursorIntermediate mesoderm
Identifiers
LatinMesonephros
MeSHD008650
TEE5.6.2.0.0.0.1
FMA72171
Anatomical terminology

Structure

The mesonephros acts as a structure similar to the kidney 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 to nephrons of the adult kidney. Each of these consists of a glomerulus, a tuft of capillaries which arises from lateral branches of dorsal aorta and drains into the inferior cardinal vein; a Bowman's capsule, a funnel like structure which surrounds the glomerulus; and a mesonephric 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 called renal corpuscle, and a unit consisting of single renal corpuscle with its associated mesonephric tubule is called a "nephron"[1] or "excretory mesonephric unit".

Development

Mesonephric vesicle

Formation of each mesonephric nephron begins when a bit of the intermediate mesoderm adjacent to the mesonephric duct differentiates to form a mesonephric vesicle.[1]

Mesonephric tubules
Enlarged view from the front of the left Wolffian body before the establishment of the distinction of sex.

a, b, c, d. Tubular structure of the Wolffian body.
e. mesonephric duct.
f. Its upper extremity.
g. Its termination in x, the urogenital sinus.
h. The duct of Müller.
i. Its upper, funnel-shaped extremity.
k. Its lower end, terminating in the urogenital sinus.
l. The genital gland.

This vesicle then elongates to form the mesonephric 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 the Wolffian tubules (or Kobelt'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.

Formation

The tubules collectively constitute the mesonephros.

Function

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 as Gartner's ducts, the epoophoron, and paroophoron are common. In males, a few of the more caudal tubules will survive and give rise to the efferent ductules of the testis,[1] the epididymis, vas deferens, seminal vesicle, as well as vestigial structures such as the appendix testis, appendix epididymis, and paradidymis.

Other animals
Section of the fold in the mesonephros of a chick embryo of the fourth day. Wolffian tubules are labeled to the right.

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.

Additional images
  • Broad ligament of adult, showing epoöphoron.
gollark: Oh, and it's not a special case as much as just annoying, but it's a compile error to not use a variable or import. Which I would find reasonable as a linter rule, but it makes quickly editing and testing bits of code more annoying.
gollark: As well as having special casing for stuff, it often is just pointlessly hostile to abstracting anything:- lol no generics- you literally cannot define a well-typed `min`/`max` function (like Lua has). Unless you do something weird like... implement an interface for that on all the builtin number types, and I don't know if it would let you do that.- no map/filter/reduce stuff- `if err != nil { return err }`- the recommended way to map over an array in parallel, if I remember right, is to run a goroutine for every element which does whatever task you want then adds the result to a shared "output" array, and use a WaitGroup thingy to wait for all the goroutines. This is a lot of boilerplate.
gollark: It also does have the whole "anything which implements the right functions implements an interface" thing, which seems very horrible to me as a random change somewhere could cause compile errors with no good explanation.
gollark: - `make`/`new` are basically magic- `range` is magic too - what it does depends on the number of return values you use, or something. Also, IIRC user-defined types can't implement it- Generics are available for all of, what, three builtin types? Maps, slices and channels, if I remember right.- `select` also only works with the built-in channels- Constants: they can only be something like four types, and what even is `iota` doing- The multiple return values can't be used as tuples or anything. You can, as far as I'm aware, only return two (or, well, more than one) things at once, or bind two returns to two variables, nothing else.- no operator overloading- it *kind of* has exceptions (panic/recover), presumably because they realized not having any would be very annoying, but they're not very usable- whether reading from a channel is blocking also depends how many return values you use because of course
gollark: What, you mean no it doesn't have weird special cases everywhere?

See also

References
  1. "20.2 Development of the renal anlage". Human Embryology. Retrieved 21 May 2012.
  2. Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 371–374. ISBN 0-03-910284-X.
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