Types of kidney and evolution of urinogenital ducts in vertebrates Excretion is a major physiological process found in all the animal groups by which nitrogenous wastes and toxins are removed from the body. Kidney is the major excretory organ of the vertebrates. Before going to discuss about the different types of kidney found across the vertebrate groups, we should have a look on the evolution of excretory system among different animal groups as discussed in the chart below:
Animal groups Main excretory structures Protozoans Plasmalemma or general body surface Porifera and Cnidaria Plasma membrane of each cell Platyhelminthes (Planaria, tapeworm etc.) Protonephridia or flame cells Nemathelminthes (Nereis, earthworm, leech Renette cells (H-shaped) etc.) Annelids Nephridia Arthropods Insects, centipedes, and millipedes Malpighian tubules Crustaceans (e.g. Prawn) Antennary or green glands Scorpions and spiders Coxal glands Molluscs Keber’s Organ and the organ of Bojanus Echinoderms No specialized excretory organ Hemichordates Proboscis gland Urochordates Neural gland Cephalochordates Solenocytes Vertebrates One pair of kidneys
Development of kidney in the vertebrates: The embryonic mesoderm differentiates on each side of the body into a segmented dorsal epimere, a small mesomere in an intermediate position and an unsegmented ventrolateral hypomere. Kidney tubules arise from the mesomere. The meomere is as long as the coelom. Gradually it pinches off from the overlying epimere. Anteriorly it remains relatively thin and becomes segmented into units called nephrotomes. Posteriorly, it does not become segmented and is called the nephrogenic cord. Nephrotomes and nephrogenic cord merge into one another at a level that varies among different animal groups.
Figure 1: Developmental origin of kidney in the vertebrates Among the vertebrates, the excretory system is composed of kidneys and urinary tubules. In vertebrates, there is a pair of kidneys situated dorsal to the abdominal cavity one on either side of the vertebral column and held in position by peritoneum. Each kidney is made up of a large number of uriniferous tubules known as nephrons. From the point of origin and evolution, the kidneys are classified as: archinephros, pronephros, mesonephros, and metanephros. Archinephros: It is believed that kidneys in all modern vertebrates evolved from a hypothetical kidney known as Archinephros or Holonephros, which is extended from anterior to the posterior end of the body, with segmentally arranged glomeruli and nephrostomes. Each tubule opens separately into the coelom by a peritoneal funnel and nephrostome. Archinephros was found in primitive vertebrate probably extended the entire length of the body cavity & had external glomeruli that drained the coelomic fluid. Each segment of the body contained one pair of Uriniferous tubules with peritoneal funnels. Malphigian bodies remained hanging in the coelom collecting the waste matter from it and pouring in the common archinephric duct. The archinephric duct eventually opens into the cloaca. Archinephric kidney is present in the larval myxine and some apodan amphibians. In the embryos of all vertebrates, archinephros appear as a transitory structure from which pronephros has evolved. In present day vertebrates, uriniferous tubules appear in two or three stages of succession. These stages are pronephros, mesonephros, and metanephros. These stages have evolved from the original archinephros.
Figure 2: Structure of Archinephros
Pronephros: Pronephros develop in the anterior most part of the nephrotome. 1-13 tubules in each, one pair to each segment. Near each tubule, a glomerulous without any Bowman’s capsule is present. This is known as external glomeruli. When Bowman’s capsule is present, it is known as internal glomeruli. The uriniferous tubules of each pronephros open into a common duct, known as pronephric duct. It runs backward to open into the embryonic cloaca. A pair of pronephros is found in all vertebrate embryos but becomes functional kidney only in some adult cyclostomes and embryos of all anamniotes. Pronephros, when functions as adult kidney, it is known as the Head Kidney found in Myxine and some teleost.
Figure 3: Nephros and development of various ducts 1. Pronephros, 2. Mesonephros, 3. Metanephros forming, 4. Metanephros
Mesonephros: Mesonephros develops from the nephrotome behind the pronephros and the pronephros later degenarates. Each mesonephric uriniferous tubule with a funnel and Malpighian body, joins the existing pronephric duct. This duct is now known as mesonephric duct or Wolffian duct. Later, the segmental mesonephric tubules undergo budding and their segmental arrangement is lost, and they may have no peritoneal funnel. As the mesonephros in an anamniote forms from the middle part of the nephrotome and as it does not extend through the length of the coelom, this adult anamniote mesonephros is called the Opisthonephros. Mesonephros are the functional kidneys of adult cyclostomes, fishes, and amphibians. They also form the kidney of amniote embryos in which they degenerate in the adult.
Figure 4: Mesonephric tubules.
Metanephros: It is the functional kidney of adult amniotes. It is formed from the posterior part of nephrotome behind the mesonephros. Metanephros has a double origin. A tubular outgrowth arises from the mesonephric duct near the cloaca. It reaches the nephrotome where it branches to form connecting tubule and calyces. The metanephric kidney begins as a small outgrowth from the more posterior end of the mesonephric duct. This is initially called a metanephric bud. Later, as the nephric tissue develops around this new duct a new kidney structure forms. The proximal part of the tubular outgrowth becomes the ureter or metanephric duct. The nephrotome behind the mesonephros gives rise to metanephric uriniferous tubules of which there are thousands with no segmental arrangements. The tubule becomes long and much coiled and have developed Malpighian body or renal corpuscles without peritoneal funnel. Thus, all