The main structural and functional element of the kidney is the
primitive fish.[19] The nephrons of the mesonephros, the functional organ in most anamniotes called opisthonephros,[20] are slightly more complex than those of the pronephros.[19] The main difference between the pronephros and the mesonephros is that the pronephros consists of non-integrated nephrons with external glomeruli.[7] The most complex nephrons are found in the metanephros of birds and mammals.[19][21][22] The kidneys of birds and mammals have nephrons with loop of Henle.[23]
All three types of kidneys are developed from the
rudimentary and not functional.[18] In some lungfish and bony fishes, the pronephros can remain functional in adults, including often simultaneously with the mesonephros.[7] The mesonephros is the final kidney in amphibians and most fish.[25]
Evolution
embryogenesis, replacing each other and reflecting the evolution of the kidneys in vertebrates.[28]
At the very beginning of vertebrates, when they evolved from marine
marine fish received their kidneys after a previous adaptation of the kidneys to fresh water. As a result, early vertebrates developed renal glomeruli capable of filtering blood and perhaps tubules that reabsorbed ions.[29] Excretion of excess water from the body is the main characteristic of the pronephros in the case of species in which it develops into a functional excretory organ. In some species, the pronephros is functional during the embryonic stage of development, representing the first stage of kidney development, after which the mesonephros develops. The mesonephros probably appeared in the course of evolution in response to the increase in body mass of vertebrates, which also led to an increase in blood pressure.[28]
The evolution of the kidneys, along with the evolution of the lungs, allowed vertebrates called amniotes to live and reproduce in terrestrial environment.
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]
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 develops after the pronephros, replacing it. The mesonephros is the final kidney in
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
nephrogenesis,[5] although many reptiles show ongoing nephron formation in adults.[47]
Metanephros is the most complex form of kidney.
urinary bladder and then excreted into the cloaca, or collected in the urinary bladder and then excreted outside through the urethra.[46]
Metanephric kidneys
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
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,
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]
The structure of the avian kidneys differs from the structure of the mammalian kidneys.
bladder; urine is excreted from the kidneys through the ureters to the cloaca.[83]
Avian kidneys combine so called reptilian-type nephrons, without the
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]
