In the course of vertebrate evolution, the kidney, as the primary osmoregulatory organ, has allowed for the exploration and colonization of new environments ranging from seawater to dry land (Smith 1953). The universal demand for tissue fluid homeostasis and osmoregulation in varied habitats has led to the development of a variety of kidney forms. With the exception of aglomerular marine kidneys, all vertebrate kidneys are composed of three functional units: the glomerulus, the tubules (proximal and distal), and the collecting system derived from the nephric duct (Fig. 1A). The glomerulus is the blood filter; capillary tufts covered in specialized basket- or sieve-like epithelial cells (podocytes) are suspended in an epithelial capsule (the nephrocoele or Bowman’s capsule), which receives the cell- and protein-free filtrate of the blood. Kidney tubules connect to the capsule and allow passage of filtered fluids to the exterior and, at the same time, perform the delicate operation of recovering needed metabolites and ions so as to maintain blood ion concentrations within the narrow limits required for the proper functioning of all other body cells. Collecting ducts continue the job of metabolite recovery begun in the tubules and provide the physical link to the exterior. Each of these three structures appears to develop as a result of a distinct series of morphogenetic events controlled by tissue patterning mechanisms during organogenesis.
KeywordsGlomerular Basement Membrane Dorsal Aorta Intermediate Mesoderm Pronephric Duct Germ Ring
Unable to display preview. Download preview PDF.