Abstract
BackgroundThree kidney systems appear during vertebrate development: the pronephroi, mesonephroi and metanephroi. The pronephric duct is the first or primary ureter of these kidney systems. Its role as a key player in the induction of nephrogenic mesenchyme is well established. Here we investigate whether the duct is involved in urine modification using larvae of the freshwater amphibian Ambystoma mexicanum (axolotl) as model.ResultsWe investigated structural as well as physiological properties of the pronephric duct. The key elements of our methodology were: using histology, light and transmission electron microscopy as well as confocal laser scanning microscopy on fixed tissue and applying the microperfusion technique on isolated pronephric ducts in combination with single cell microelectrode impalements. Our data show that the fully differentiated pronephric duct is composed of a single layered epithelium consisting of one cell type comparable to the principal cell of the renal collecting duct system. The cells are characterized by a prominent basolateral labyrinth and a relatively smooth apical surface with one central cilium. Cellular impalements demonstrate the presence of apical Na+ and K+ conductances, as well as a large K+ conductance in the basolateral cell membrane. Immunolabeling experiments indicate heavy expression of Na+/K+-ATPase in the basolateral labyrinth.ConclusionsWe propose that the pronephric duct is important for the subsequent modification of urine produced by the pronephros. Our results indicate that it reabsorbs sodium and secretes potassium via channels present in the apical cell membrane with the driving force for ion movement provided by the Na+/K+ pump. This is to our knowledge the first characterization of the pronephric duct, the precursor of the collecting duct system, which provides a model of cell structure and basic mechanisms for ion transport. Such information may be important in understanding the evolution of vertebrate kidney systems and human diseases associated with congenital malformations.
Highlights
Three kidney systems appear during vertebrate development: the pronephroi, mesonephroi and metanephroi
Identification of functional pronephroi and pronephric ducts We determined the interval in which the axolotl pronephros and pronephric duct is functional by investigating kidney structure in freshly dissected larvae and in larvae prepared for histology (Figure 1 and 2)
We show that the pronephric duct, which is the first or primary ureter in all vertebrates, participates in urine adjustment in the axolotl
Summary
Three kidney systems appear during vertebrate development: the pronephroi, mesonephroi and metanephroi. A characteristic of vertebrate kidney organogenesis is the development of a pronephric duct in association with each pronephros [3] These ducts are the first or primary ureters of vertebrate kidney systems. They form the collecting duct system of the mesoand metanephroi, and they, and their derivates, are the key players in the induction of the nephrogenic mesenchyme, which forms these latter kidney generations. Gene expression assays have indicated high expression of transporters known to be involved in ion transport, such as the Na+/ K+-ATPase and the ROMK channel [17,18,25,26,27] These data suggest that the pronephric duct may play an important role in regulation of extracellular fluid homeostasis. We ask the question: Is the duct involved in urine modification?
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