Developmental Stages Of Vertebrate Kidney
The major excretory organs in vertebrates are kidneys. They have adapted different modifications during their evolutionary development.
Developmental Stages Of Kidney
Archinephros: The first form of kidneys is known as archinephros or holonephros type and the collecting duct as archinephric duct. A pair of kidneys are seen in the dorsal side of the coelom and are made of large number of uriniferous tubules also called nephrons. In embryonic state a part of mesoderm called as mesomere or nephrostome extend through out the trunk. The uriniferous tubules arise from this as one in each segment of the trunk from anterior to posterior end. The uriniferous tubules are the fundamental units of the kidneys and has a basic structure made of a ciliated peritoneal funnel, a convoluted ciliated tubule, a malphigian body or renal capsule. The peritoneal funnel opens into the splanchnocoel by coelomostome. Its tubular end is connected to the convoluted tubule of the malphigian body which further opens into the longitudinal collecting duct joining the cloaca. The malphigian body is made of network of interarterial blood capillaries called glomerulus enclosed in a double layered Bowman’s capsule. Blood filtration takes place in the glomerulus. In some fishes the kidneys lack malphigian bodies and so are called aglomerulus. These kidneys are seen in the adult stages of ancestral vertebrates, some fishes, and apodan amphibians, and in the embryos of most vertebrates they appear as transitory structure.
During the developmental stages of later vertebrates, the uriniferous tubules follow three stages of succession namely pronephros, mesonephros and metanephros.
Pronephros: In this stage the uriniferous tubules have no peritoneal funnel. They are present as one in each segment and they unite with the pronephric duct which joins the embryonic cloaca. The glomeurlus sometimes lack Bowman’s capsule and is called external glomeruli otherwise internel glomeruli. A pronephric chamber may be present into which all the glomeruli project and seem to be single compound called glomus. These types are present in cyclostomes and in embryos of all anamniotes which degenerate during development.
Mesonephros: They also do not have peritoneal funnel. They undergo budding to form hundreds of tubules, thus are deviated from segmental arrangement and are still present in glomus. The earlier pronephric duct is now called as mesopnephric or Wolfian duct. This stage is seen in cyclostmes, fishes and amphibians and in embryos of all amniotes which degenerate during development. In adult anamniotes the mesonephros run through the length of the coelom, while that in amniotic embryos, is present only from the middle part so are called as opisthonephros type.
Metanephros: This stage develops only in the amniotes, and they are the functional kidneys. There is no peritoneal funnel and the metanephric tubules are much long and coiled and have glomeruli enclosed in the Bowman’s capsule. They are developed from the posterior part of the nephrostome and thousands of metanephric uriniferous tubules with no segmental arrangement are seen in the glomus. The tubular out growth from the nephrostome also give rise to ureter or metanephric duct.