Pronephros
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215760512
2008-05-29T16:14:20Z
Emeraude
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System = |
Days = 20 |
CarnegieStage = 10 |
Precursor = [[nephrotome]] |
GivesRiseTo = [[mesonephros]] |
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DorlandsPre = p_36 |
DorlandsSuf = 12670025 |
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'''Pronephros''' the most primitive of the three [[excretory system|excretory organ]]s that develop in [[vertebrate]]s, corresponding to the first stage of [[kidney]] development.
The pronephros develops in the anterior [[nephrotome]]s of all vertebrates. It is a paired organ, consisting of a series of [[nephron]]s filtering [[urine]] from both the [[pericardium]] fluids via openings called [[nephrostome]]s and blood from the [[glomerulus]].
The organ is active in adult forms of some primitive [[fish]], like [[lamprey]]s or [[hagfish]]. It is present at the embryo of more advanced fish and at the [[larva]]l stage of [[amphibian]]s. In [[human being]]s, it is rudimentary, appears at the end of the third week (day 20) and replaced by [[mesonephros]] after 3.5 weeks.
==Development==
The pronephros is the first in a sequence of kidneys that form in vertebrate embryos. The pronephric primordium develops from the [[intermediate mesoderm]], lying between the [[paraxial]] (somitic) mesoderm and the [[lateral plate]]. In many organisms (e.g. amphibians) this primodium forms anteriorly then migrates posteriorly to fuse with the cloaca, while in others it forms along the length of the intermediate mesoderm (e.g. [[zebrafish]]).
==Drainage==
In both amphibians and zebrafish the pronephros has a single nephron attached to a nephric duct, which in turn is linked to the cloaca. Although these kidneys have a simple anatomical organization with only a single nephron, the nephrons have a segmental and functional complexity that is very similar to that in more complex kidneys such as [[mesonephroi]] and [[metanephroi]].
==Filtration==
One unique feature of pronephroi is the arrangement by which the glomerular filtrate is generated and collected by the nephron. In pronephroi the [[glomerulus]] (or [[glomus]] if it extends over multiple body segments) projects into the coelom rather than into the proximal tip of the nephron. The glomerular filtrate flows directly into the coelom, or a dorsal compartment of the coelom known as the nephrocoel. In jawless fishes, the pronephric glomus projects into the pericardial cavity. Fluids are swept from the filtration cavity into the nephron through cilated funnels known as nephrostomes. These thin epithelial tubes are densely packed with cilia and have a distinct morpology to the other tubular epithelia of the kidney.
==Relationship to nephrotomes==
Older anatomical texts describe the pronephros as condensing from nephrotomes, but modern visualization techniques have shown that this represents a histological artifact.
==In amphibians, fishes, and mammals==
Once the more complex mesonephros forms the pronephros undergoes apoptosis in amphibians. In fishes the nephron degenerates but the organ remains and becomes a component of the immune system.
In mammals a functional pronephros, in the context of an organ performing waste excretion or osmoregulation, does not develop. However, a kidney primodia that runs along the intermediate mesoderm does form and links up to the cloaca. This duct is known as the pronephric duct, mesonephric duct or Wolffian duct. While this transient primordium never forms functional nephrons, the duct derived from it is essential to the development of the more complex later kidneys.
==See also==
*[[Mesonephros]]
*[[Metanephros]]
==External links==
* [http://www.uoguelph.ca/zoology/devobio/210labs/meso3.html#pro Overview at uoguelph.ca]
* [http://www.xenbase.org/WWW/Marker_pages/pronephros/balinsky.gif Diagram at xenbase.org]
* [http://receptor.mgh.harvard.edu/Research/labs/Drummond/devel.html Development of the pronephros in zebrafish]
* {{MUNAnatomy|embryo/kid5}}
{{Development of urinary and reproductive systems}}
[[Category:Embryology]]
[[Category:Kidney]]