Everything about Pronephros totally explained
Pronephros the most primitive of the three
excretory organs that develop in
vertebrate, corresponding to the first stage of
kidney development.
The pronephros develops in the anterior
nephrotomes of all vertebrates. It is a paired organ, consisting of a series of
nephrons filtering
urine from both the
pericardium fluids via openings called
nephrostomes and blood from the
glomerulus.
The organ is active in adult forms of some primitive
fish, like
lampreys or
hagfish. It is present at the embryo of more advanced fish and at the
larval stage of
amphibians. In
human beings, it's 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 (for example 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 (for example
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's 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, doesn't 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's essential to the development of the more complex later kidneys.
Further Information
Get more info on 'Pronephros'.
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