User:Linz.N.W./sandbox/Evolutionary Development of the Human Kidney

The development of the kidney, or nephrogenesis undergoes three phases in development that can be linked to the evolutionary history of humans. The first kidney form is known as the pronephros and resembles that of a jawless fish. This structure is seen in the early embryological development and appears to be nonfunctional. The second kidney structure formed in developing humans is the mesonephros which functions to excrete waste in the embryo and resembles the kidney found in most fish and amphibians. This structure is finally replaced with the metnephros which is the kidney structure found in reptiles, birds and mammals. In each of these forms the structure and function is conserved and share common units and developmental pathways.

Evolutionary Development
Evolutionary developmental biology or evodevo is a field of biology that focuses on comparing developmental processes to understand their evolutionary histories. The field studies changes in the timing or positioning of an aspect of development relative to an ancestor. For example all vertebrates have gill slits at some point in their development as a remnant of the common ancestors they share with fish. This was the cause of the famous, yet false, theory that “ontogeny recapitulates phylogeny” by Ernst Haeckel.

Human Kidneys
Human kidneys are bean shaped organs around the size of a tightly clenched fist. They are partially protected by the rib cage and extend from the level of T12 to L3. The right kidney is lower than the left due the position of the liver as superior to that kidney. Each kidney is roughly 11cm long, 5cm wide, 3 cm thick and weighs around 130g. The kidneys are behind the peritoneum and are anchored to the abdominal wall with adipose tissue by the renal fascia.

Function
The kidneys play a large role in urine formation. As the blood flows through the nephron waste products are filtered out which eventually leaves the kidneys as urine heading through the ureters to the bladder. By functioning to remove compounds that are in excess, such as water, urea or ions it helps to maintain homeostasis of the body. In this way it plays a role in the regulation of blood pressure, acid-base balance and electrolyte balance. The kidneys also function in hormone secretion of erythropoietin which stimulates red blood cell production and renin which regulates aldosterone levels and helps control blood pressure.

Nephron
The nephron is the functional unit of human kidney. At the renal corpuscle fluid is filtered out into the proximal convoluted tubule, it then travels down and up the loop of Henle before entering the distal convoluted tubule and then the collecting duct. During this process excess wastes and water are filtered out and nutrients are reabsorbed back into the blood stream. The countercurrent network of vessels ensures that there is always a gradient.

Human Kidney Development
The human kidneys are formed from the mesoderm. Differentiation begins at around 21 days after fertilization.

Pronephros
This structure is formed around 21 days after fertilization in the cervical region. The pronephros is a duct that filters waste out of the coelem. The nephrons found in the pronephroi are different from the integrated neurons found in other adult vertebrate species. These nephrons are non-integrated into the kidney tubule. The glomeruli in these nephrons appear superficially very similar to other vertebrate glomeruli.

Jawless Fish
This type of kidney is present in adult jawless fish such as hagfish and lampreys.

Other Vertebrates
The pronephros is present in the embryonic phase of other vertebrates including bony fish and amphibian larvae. In mammals the pronephros are nonfunctional and eliminated using programmed cell death roughly 4 weeks into development. In humans it consists of 6-10 pairs of tubules and extends from the forth to the 14th somites.

Amphibians and Bony Fish
The mesonephros is the kidney form found in amphibians and most fish. Even in animals where the mesonephros are the adult form the embryos of these species still form the pronephros during development.

Other Vertebrates
In humans the mesonephros develops around 4 weeks and last until about the 8th week. Unlike the pronephros, the mesonephros is functional and produces urine. None of the mesonephros becomes part of the adult kidney in humans and in females they degenerate completely, however in males parts of the mesonephros become part of the reproductive system

Evolutionary Link
Analysis of these structures has found that they share common units and develop using conserved molecular pathways. All the forms have similar functional organization and differ in special organization and nephron types and numbers. They also all derive from the same intermediate mesoderm and the nephrons, while different preform, similar functions. It has been hypothesized that similar genes are involved in the development of all the kidney forms. In most vertebrates the basic structure and function of the kidneys are conserved.