Archinephros

It is believed that the ancient primitive form of the kidney was the archinephros, which had series of segmental tubules through the entire length of the trunk part of the body,^[13] and each body segment had a pair of tubules.^[31] All tubules were opened medially (closer to the midline of the body) into the body cavity known as coelom and united laterally into the two common archinephric ducts which were located in opposite sides of the body.^[13][31] And the archinephric ducts were opened into the cloaca.^[13] As an organ, the archnephros is still preserved in the larvae of hagfishes and some caecilians, and is also found in the embryos of some more developed vertebrates.^[32]

Pronephros

In lower vertebrates, the pronephros is sometimes called the head kidney due to its anterior position behind the head.^[33] In embryogenesis it is usually a transitional structure and is subsequently replaced by the mesonephros in most vertebrates.^[7] In mammalian embryogenesis, the pronephros is usually considered to be rudimentary and non-functional. A functional pronephros develops in vertebrates that have a free-swimming larval stage in their development.^[7]

Pronephros functions in amphibians in the larval stage, in the adults of some bony fishes, and in the adults of some other fish species.^[7] The pronephros is a vital organ in animals that go through the aquatic larval stage. If in larvae the pronephros becomes non-functional, then they rapidly die from edema.^[34]

The pronephros is a relatively large organ that has a primitive structure and usually consists of a single pair of bilateral nephrons with an external glomerulus or glomus.^[34][15] The typical pronephric nephron is non-integrated, and the wastes are filtered through the glomerulus or glomus directly to the coelom, in the more advanced pronephros they are filtered into the nephrocoel, which is a cavity adjacent to the coelom. The coelom is connected to the pronephric duct through the ciliated nephrostomes, which drain coelom fluid into the cloaca.^[7]

Because of its small size and simple structure, the pronephros of fish and amphibian larvae has become an important experimental model for studying kidney development.^[35]

Mesonephros and opisthonephros

Mesonephros develops after the pronephros, replacing it. The mesonephros is the final kidney in amphibians and most fish. In more advanced vertebrates (amniotes), mesonephros develops during embryogenesis and is then replaced by the metanephros.^[36] In reptiles and marsupials, it remains functional for some time after birth along with the metanephros.^[37][38] When mesonephros degenerates in male mammals, its remains are involved in the formation of the reproductive system.^[39] Sometimes the anamniote mesonephros is called opistonephros to distinguish it from the stage of development in amniotes.^[40] In anamniotes, opisthonephros develops from a region of the nephric ridge, which is derived from intermediate mesoderm, from which both the mesonephros and metanephros are developed in the embryo of amniotes.^[41][42]

Unlike the pronephros, the mesonephros consists of a set of nephrons, the glomeruli of which are enclosed in Bowman's capsules, but in some marine fish glomeruli may be absent.^[36] In fish, mesonephric kidneys has no division into cortex and medulla.^[43] Usually the mesonephros consists of 10–50 nephrons. The mesonephric tubules may have a connection to the coelom, however, the glomeruli of mesonephric nephrons still remain integrated. Nephrostomes are typically absent in the embryonic mesonephros of birds and mammals.^[44] Mesonephros in fish has the ability to add new nephrons while body mass increases.^[45]

Metanephros

In amniotes, which include reptiles, birds, and mammals, the pronephros and mesonephros are usually intermediate stages in the formation of metanephros during embryonic development, and metanephros is the final kidney.^[28] Genes that are involved in the formation of one form of kidney are reused in the formation of the next one.^[28] Metanephros differs from pronephros and mesonephros in development, position in the body, shape, number of nephrons, organization and drainage.^[46][44] Unlike mesonephros, after the end of its development process, metanephros has no longer the ability to add new nephrons through nephrogenesis,^[5] although many reptiles show ongoing nephron formation in adults.^[47]

Metanephros is the most complex form of kidney.^[44] Each metanephric kidney is characterized by a large number of nephrons and a highly branched system of collecting tubules and ducts,^[28] that open into the ureter.^[48] Such branching in the metanephros is unique in relation to the pronephros and mesonephros.^[44] Depending on classes and species urine from the ureters can be excreted directly into the cloaca, or collected in the urinary bladder and then excreted into the cloaca, or collected in the urinary bladder and then excreted outside through the urethra.^[46]

Reptile kidney

Reptiles were the first class of animals that had no larval stage and that were fully terrestrial animals.^[21] The mesonephros in reptiles functions for some time after birth simultaneously with the metanephros, while later the metanephric kidneys become permanent and the mesonephros degenerates.^[38]

The kidneys in reptiles are located mainly in the caudal part (away from the head) of the abdominal cavity^[49][50] or retroperitoneally (behind the peritoneum) in the pelvic cavity in the case of lizards.^[49] Reptile kidneys are commonly elongated^[51] with color ranging from light to dark brown.^[52] The shape of the kidneys varies between reptiles due to variations of their body form.^[8] The kidneys of snakes are elongated, cylindrical^[53][50] and lobulated.^[52] Turtles and some lizards have urinary bladder^[50] that opens into the cloaca^[54] but snakes and crocodiles do not have it.^[50]

Compared with the metanephros of birds and mammals, the metanephros of reptiles is simpler in structure.^[21] Unlike mammals, the kidneys of reptiles do not have a clear distinction between cortex and medulla.^[43] The kidneys lack the loop of Henle, have fewer nephrons (from about 3,000 to 30,000), and cannot produce hypertonic urine.^[3][21] Nitrogenous waste products excreted by the kidneys may include uric acid, urea and ammonia.^[55] Aquatic reptiles excrete predominantly urea, while terrestrial reptiles excrete uric acid, which allows them to conserve water.^[21]

Since the reptile kidneys are unable to produce concentrated urine due to the absence of the loop of Henle, glomerular filtration rate is decreased if water loss needs to be reduced.^[56] The glomeruli in reptiles have also decreased in size compared to amphibians.^[52] In addition to the renal artery blood supply, reptiles also have a renal portal system, which can redirect blood to the kidneys during periods of water deprivation, bypassing the glomeruli, to prevent ischemic necrosis of tubular cells.^[21][57]

Mammalian kidney

In mammals, the kidneys are usually bean-shaped^[58] and located retroperitoneally^[59] on the dorsal (posterior) wall of the body.^[60] The outer layer of each kidney is made up of a fibrous sheath called the renal capsule. The peripheral layer of the kidney is called the cortex and the inner part is called the medulla. The medulla consists of one or more pyramids, the bases of which start from corticomedullary border. Medulla pyramid with overlying cortex comprises the renal lobe.^[28] In multilobar kidneys, the pyramids are separated from each other by dipped into the kidney areas of cortical tissue known as the renal columns.^[61] Blood enters the kidney through the renal artery, which in the multilobar kidney then branches in the region of the renal pelvis into large interlobar arteries that pass through the renal columns.^[62][63] The pyramids consist mainly of tubules that transport urine from the cortex, that produces it by blood filtration, to the tips of the pyramids, that form the renal papillae. Urine is excreted through the renal papillae into the calyces and then into the pelvis, ureter, and bladder.^[62][28] Then it is excreted outside through the urethra.^[64] In monotremes, the ureters open into the urogenital sinus, which is connected to the urinary bladder and cloaca,^[65] and urine is excreted into the cloaca instead of the urethra.^[66][65]

Structurally, kidneys vary between mammals.^[67] Which structural type a particular species will have depends mainly on the body mass of the species.^[68] Small mammals have simple, unilobar kidneys with a compact structure and a single renal papilla, while large animals have more complex multilobar kidneys, such as those of bovines.^[67][69] Kidneys can also be with a single renal papilla (the unipapillary kidneys),^[69] as in mice and rats,^[70] with several, as in spider monkeys, or with a large number, as in pigs or humans.^[69] Most animals have a single renal papilla.^[69] In some animals, such as horses, the apices of the renal pyramids fuse with each other to form a common renal papilla, called the renal crest.^[71] The renal crest usually appears in animals larger than rabbits.^[68] The kidneys of bovines are multilobar with external lobation.^[72] Marine mammals, bears and otters have reniculate kidneys which are made of large amount of lobes called reniculi.^[73] Each renculus can be compared to a simple unipapillary kidney as a whole.^[74]

Nitrogenous waste products are excreted by the kidneys of mammals primarily in the form of urea,^[75] which is highly soluble in water.^[76] Each nephron is located in both the cortex and the medulla. The most proximal part of the nephron is glomerulus, which is located in the cortex.^[28] The nephrons of the mammalian kidneys have loops of Henle, which are the most efficient way to reabsorb water and produce concentrated urine to conserve water in the body.^[12] The mammalian kidneys combine both nephrons with a short and nephrons with a long loop of Henle.^[77] The medulla is divided into outer and inner regions. The outer region consists of short loops of Henle and collecting ducts, and the inner region consists of long loops of Henle and collecting ducts.^[28] After passing through the loop of Henle, the fluid becomes hypertonic relative to the blood plasma.^[78] The renal portal system is absent in mammals.^[56]

Avian kidney

In birds, the kidneys are typically elongated^[79] and located dorsally in the abdominal cavity in the pelvic skeletal depressions.^[80][81]

The structure of the avian kidneys differs from the structure of the mammalian kidneys.^[67] The avian kidney is lobulated and usually consists of three lobes.^[80] The lobes are divided into lobules, each of which has a cortex and a medulla.^[67][3] The medulla of the each lobule is shaped like a cone, and, unlike mammals, it is not subdivided into the inner and outer regions, while structurally it is similar to the outer medulla of the mammalian kidney.^[67] In the avian kidney, the renal pelvis is absent,^[82] and each lobule has a separate branch to the ureter.^[3] No birds, except for the ostrich, have a bladder; urine is excreted from the kidneys through the ureters to the cloaca.^[83]

Avian kidneys combine so called reptilian-type nephrons, without the loop of Henle, and mammalian-type nephrons, with the loop of Henle.^[23] Most nephrons are reptilian-type.^[84] The loop of Henle of birds is similar to that of mammals, the main difference is that the nephron of birds has only a short loop of Henle.^[77] Like mammals, although to a lesser extent,^[67] birds are able to produce concentrated urine, thus conserving water in the body.^[23] Nitrogenous waste products are excreted mainly in the form of uric acid, which is a white paste that is poorly soluble in water, which also helps to reduce water loss.^[85] Additional water reabsorption occurs in the cloaca and distal intestine. Altogether, this allows birds to excrete their wastes without significant loss of water.^[5]

In birds, the arterial blood is supplied to the kidneys by the cranial, middle and caudal renal arteries.^[86] Like reptiles, birds have a renal portal system, but it does not deliver blood to the loops of Henle, blood is delivered only to the proximal and distal tubules of the nephrons. When birds are in a state of dehydration, nephrons without a loop of Henle stop filtering, while nephrons with a loop continue, but due to the presence of a loop, they can produce concentrated urine.^[56]