The General Layout of the Excretory System: The excretory systems of vertebrates consist of filtration units that allow small molecules from blood to pass out into the urinary space, specialised transporting epithelia that selectively recover certain solutes and return them to the blood, and also recover water in many species, and drainage ducts to take urine out of the body. The lowest vertebrates from only one pair of kidneys, the pronephroi (sing=pronephros), that consist of a few primitive nephrons connected to an unbranched nephric duct.
More advanced classes (fish, amphibia) go on to form a second pair of kidneys, the mesonephroi, each of which also consists of a linear array of nephrons attached to the unbranched nephric duct, posterior to the pronephros. In the male, there may be a connection between the mesonephros and gonad.
Amniote classes (reptiles, mammals, birds) form a third pair of kidneys, the metanephroi, which are the only ones that persist into adult life and are what people normally mean when they say 'kidney'; at the moment, almost all of this database is concerned with metanephric kidneys. The metanephric kidneys differ from their predecessors by being compact rather than elongated organs, and by being connected only indirectly to the nephric duct.
Development of the metanephric kidney: Development of the metanephric kidney begins later than that of the pro- and mesonephric kidneys. As the neprhic duct extends from these earlier kidneys to fuse with the cloaca at the posterior of the embryo, it gives out a branch, the ureteric bud, which invades a specialised region of intermediate mesoderm called the metanephrogenic mesenchyme (MM). The invasion is probably guided by signals emanating from the MM, although this has not yet been demonstrated.
When the ureteric bud makes contact with the MM, a series of inductive signals is exchanged. The MM induces the bud to grow and to branch, so that it arborises to form a tree- like collecting duct system. The ureteric bud induces the MM to gain a stem- cell phenotype and to multiply. It then induces groups of stem cells to differentiate into nephrons, a complex developmental process described in more detail below. As the ureteric bud continues to grow and branch, its tips contact fresh stem cells and induce these into the nephrogenic pathway. There is therefore a gradient of developmental age in a foetal kidney; the outermost cortex is composed of stem cells that are not yet committed to differentiation, the region just inside it contains cells undergoing the earliest phases of nephrogenic differentiation, and the region inside that contains maturing nephrons and supporting stromal cells. In the kidney, geography illuminates history.
Differentiation of nephrons: The process by which disorganised mesenchymal cells become a highly organised epithelial tubule is, perhaps unsurprisingly, complex. For convenience, it can be divided into a number of stages:
Other Events: In addition to the epithelial morphogenesis described above, developing kidneys are vascularised and innervated, and also provided with a protective capsule. Rather little is known about these events, though it is hoped that they will soon have sections of the database to themselves.
Further Information can be obtained from the following reviews;
Return to the Kidney Development Databasehome page
Date of this file 19th April 2000