The long and winding path to understanding kidney structure in amphioxus - a review.

The history of studies on amphioxus kidney morphology is reviewed with special attention to four zoologists who made important early contributions. In 1884, Hatschek described a single anterior nephridial tubule in larval and adult amphioxus. Subsequently, in 1890, Weiss and Boveri independently found multiple branchial nephridia (morphologically similar to Hatschek's nephridium) associated with the pharyngeal gill slits. These initial discoveries set the stage for Goodrich to criticize Boveri repeatedly for the latter's contention that amphioxus nephridia develop from mesoderm and are connected to neighboring coeloms throughout the life history. In the end, Boveri was almost certainly correct about amphioxus nephridia developing from mesoderm and at least partly right about the lumen of the nephridial tubules being connected to nearby coeloms-the openings are present during larval stages but are closed off later in development. The more detailed structure of amphioxus nephridial tubules was ultimately revealed by electron microscopy. The tubule epithelium includes specialized excretory cells (cyrtopodocytes), each characterized by a basal region similar to that of a vertebrate renal podocyte and an apical region bearing a flagellar/microvillar process reminiscent of an invertebrate protonephridium. At present, in spite of considerable progress toward understanding the development and structure of amphioxus nephridia, virtually nothing is yet known about how they function, and no consensus has been reached about their phylogenetic significance.