The elimination of water and the reclamation of salts may have been the earliest functions of vertebrate kidneys. Tufts of blood vessels - glomeruli, filtered water out of the blood stream into the body cavity and convoluted tubules with openings into the coelom collected the filtrate, retrieved any salts from it and emptied the final filtrate into a longitudinal duct that passed to the cloaca.
Archinephros
Glomeruli arise as localised modifications of blood vessels, which can be traced from segmental branches of the dorsal aorta - supplying the glomerulus is an afferent glomerular arteriole and emerging from the glomerulus is an efferent glomerular arteriole. The latter leads to capillary beds that surround the kidney tubules.
The most anterior embryonic/larval glomeruli may be suspended in the coelomic cavity. They are sometimes called "external" glomeruli to differentiate them from "internal" glomeruli, which are encapsulated by the kidney tubule.
In internal glomeruli, each tubule typically commences as a Bowmann's capsule. This is a blind end of the tubule that surrounds an internal glomerulus and receives the glomerular filtrate. The more anterior tubules may exhibit a ciliated funnel-shaped nephrostome, which is an opening into the coelom. Nephrostomes are usually confined to the embryo and larvae. If the embryonic tubules that exhibit a nephrostome are not altogether closed, the nephrostome may close at a later stage of development.
Kidney tubules arise from the intermediate mesoderm. This is a ribbon of nephrogenic tissue extending uninterrupted from the level of the heart to the cloaca. It lies just lateral to the segmental/dorsal mesoderm. Almost the entire ribbon produces kidney tubules. The anterior-most tubules are always metameric, since one tubule develops a the level of each mesodermal somite. Farther back, numerous tubules develop in each segment and the metamerism is lost.
The longitudinal ducts of the basic patter appear first at the anterior end of the nephrogenic mesoderm as posteriorly directed extensions of the first tubule. Each duct grows caudal until it achieves an opening into the cloaca. At this time, it is known as the pronephric duct. The kidneys of myxinoid cyclostomes closely resemble an archinephros.
Pronephros
The first embryonic kidney tubules in all vertebrates arise from the anterior end of the intermediate mesoderm. They are called pronephric tubules because they are the first of what will become a series of tubules extending the length of the coelom.
Each pronephric tubule arises in the intermediate mesoderm as a solid bud of cells, which later organises a lumen and in most anamniotes, a nephrostome. Associated with each tubule, typically, is a glomerulus. The number of pronephric tubulesis never large. The tubules lengthen and become coiled.
The pronephros in most vertebrates is functional only until such a time as the tubules farther back are prepared to supersede them. This is at the end of the larval stage in amphibians or at an equivalent development stage in fish. Occasionally, in larvae, several glomeruli unite to form a single glomus. The glomus and the pronephric tubules are generally enclosed in a pronephric chamber. The pronephros is retained in the adult only in cyclostomes and a few teleosts. The pronephros is often called head kidney because it lies immeadiately behind the head.
Mesonephros
Under the stimulus of the pronephric duct, acting as an inductor, additional tubules develop sequentially in the mesoderm behind the pronephric region. The new tubules establish contact with the pronephric duct. For at least several segments, these tubules too, may be segmentally disposed. They exhibit the same convolutions as the ones anterior to them and often have open nephrostomes. Later, the pronephros is obliterated. With the disappearance of the pronephric region, the erstwhile pronephric duct is now called the mesonephric duct. The mesonephros is the functional adult kidney of fish and amphibians. It is sometimes called opisthonephros. The mesonephros is also the functional kidney in the embryonic amniotes.
In sharks and apoda (amphibians), the adult kidney begins far forward and extends the lenth of the coelom, lying against the dorsal body wall. In other fish and other amphibian, the kidney is much shorter. The mesonephric duct may lie along the lateral edge of the kidney, or on the ventral surface or embedded in it. The posterior ends may enlarge to form the seminal vesicle in males.
The mesonephros of amniotic embryos has essentially the same structure as the adult kidneys of fish and amphibians, except that nephrostomes are rudimentary in most birds and seldom appears in mammals. Although the mesonephros is basically an embryonic kidney in amniotes, it functions for a short time after birth in reptiles, prototherians and metatherians.
During the time that the mesonephros is functionin, a new kidney, the metanephros, is in the process of developing. When the metanephros takes over the functions of a kidney, the mesonephros involutes and only remnants remain after birth.
Metanephros
The metanephros, or adult amniotic kidney, organises from the caudal end of the nephrogenic mesoderm, which is displaced anteriorly and laterally during development. The number of tubules that form this caudal section is extremely large (several million) and the tubules are highly convoluted. This has a duct of its own, the metanephric duct/ureter.
The proximal portion of the metanephric duct becomed the ureter. The distal tip becomes the pelvis (of the kidney). Many finger-like outgrowths of the pelvis invade the nephrogenic mass to become collecting tubules. Meanwhile, the metanephric tubules get organised. They commence as 'S'-shaped tubules. The upper arm opens into a collecting tubule. The lower arm becomes invaginated by developing a glomerulus to become a Bowmann's capsule.
The mammalian metanephros exhibits a greater organisation than that of reptiles and birds. The organisation is the result of the formation of a long, thin, 'U'-shaped loop of Henle, positioned between the proximal and distal convolution of the metanephric tubule. As the loop of Henle elongates, they grow away from the surface of the kidney and towards the renal pelvis. The kidney therefore comprises of a cortex, in which the renal corpuscles are concentrated and a medulla, consisting of loops of Henle and collecting tubules.
The metanephric tubules of reptiles have no loop of Henle and those of birds have only a very short equivalent segment. Small glomeruli result in the conservation of water. Many metanephric kidneys are lobulated, each lobe consisting of clusters of tubules. In snakes and legless lizards, the kidneys are elongated to conform to the slender body. The kidneys of birds are flattened against the sacrum, iliun and ribs and fit snugly against the contours of these bones.
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