Kidney (vertebrates)

Evolutionary pressure and the need to regulate body fluid homeostasis have led to pre-adaptation of the vertebrate kidneys to different environment conditions and to development of three kidney forms: the pronephros, mesonephros and metanephros. The kidneys of amniotes are unique compared to other internal organs, since three different kidneys are sequentially developed during embryogenesis, replacing each other and reflecting the evolution of the kidneys in vertebrates. 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. Metanephros, the permanent kidney of amniotes, has the unique ability to efficiently retain water in the body. In addition to water conservation, terrestrial life also required maintenance of salt levels in the body along with the excretion of waste products. The first class of animals to become fully terrestrial without a larval stage were the reptiles, which were the first amniotes. The kidney takes a key role in maintenance of the constant internal environment. The relative ionic composition of the extracellular fluid is similar between marine fish and all subsequent species. Therefore, it can be said that the kidneys made it possible to preserve approximately the same composition of extracellular fluid in vertebrates as it was in the primordial ocean. == Kidney forms ==

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, 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. Pronephros In lower vertebrates, the pronephros is sometimes called the head kidney due to its anterior position behind the head. In embryogenesis it is usually a transitional structure and is subsequently replaced by the mesonephros in most vertebrates. 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. 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. In reptiles and marsupials, it remains functional for some time after birth along with the metanephros. When mesonephros degenerates in male mammals, its remains are involved in the formation of the reproductive system. Sometimes the anamniote mesonephros is called opistonephros to distinguish it from the stage of development in amniotes. 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. 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. Mesonephros in fish has the ability to add new nephrons while body mass increases. 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. Genes that are involved in the formation of one form of kidney are reused in the formation of the next one. Metanephros is the most complex form of kidney. Such branching in the metanephros is unique in relation to the pronephros and mesonephros. 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. == Metanephric kidneys ==

Reptile kidney Reptiles were the first class of animals that had no larval stage and that were fully terrestrial animals. 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. or retroperitoneally (behind the peritoneum) in the pelvic cavity in the case of lizards. with color ranging from light to dark brown. The shape of the kidneys varies between reptiles due to variations of their body form. but snakes and crocodiles do not have it. The kidneys lack the loop of Henle, have fewer nephrons (from about 3,000 to 30,000), and cannot produce hypertonic urine. Aquatic reptiles excrete predominantly urea, while terrestrial reptiles excrete uric acid, which allows them to conserve water. The glomeruli in reptiles have also decreased in size compared to amphibians. Mammalian kidney , bovine, and canine kidneys In mammals, the kidneys are usually bean-shaped and located retroperitoneally on the dorsal (posterior) wall of the body. 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. 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. 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. In monotremes, the ureters open into the urogenital sinus, which is connected to the urinary bladder and cloaca, and urine is excreted into the cloaca instead of the urethra. 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. Kidneys can also be with a single renal papilla (the unipapillary kidneys), with several, as in spider monkeys, or with a large number, as in pigs or humans. The renal crest usually appears in animals larger than rabbits. Marine mammals, bears and otters have reniculate kidneys which are made of large amount of lobes called reniculi. Each reniculus can be compared to a simple unipapillary kidney as a whole. Nitrogenous waste products are excreted by the kidneys of mammals primarily in the form of urea, which is highly soluble in water. 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. 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. The renal portal system is absent in mammals. and located dorsally in the abdominal cavity in the pelvic skeletal depressions. The structure of the avian kidneys differs from the structure of the mammalian kidneys. The avian kidney is lobulated and usually consists of three lobes. and each lobule has a separate branch to the ureter. Avian kidneys combine so called reptilian-type nephrons, without the loop of Henle, and mammalian-type nephrons, with the loop of Henle. 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. Additional water reabsorption occurs in the cloaca and distal intestine. Altogether, this allows birds to excrete their wastes without significant loss of water, allowing them to fly long distances with limited water. 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. == References ==