The ultrastructure of the protonephridial tubules of the freshwater planarian Bdellocephala brunnea

The maintenance and regeneration of the planarian excretory system are regulated by EGFR signaling

“…In both ciliated and non-ciliated tubule sections we observed morphological subtypes of the bounding cells ( Fig. 2E-H), consistent with the functional differentiation of constituent cell types inferred previously [ciliated 'ductule' and ciliated 'collecting duct' (McKanna, 1968b); non-ciliated 'trunk' and 'distal tubule' (Ishii, 1980b); 'transitional region' or 'osmoregulatory duct' (McKanna, 1968b)]. Jointly, these experiments provide strong evidence to support the existence of multiple protonephridial cell types.…”

“…The flame cells were often attached to a muscle fiber (not shown) and were always surrounded by a comparatively large volume of extracellular space (Fig. 2C), which in past studies using chemically fixed material appears to have occasionally been misinterpreted as 'fixed parenchymal cells' (Pedersen, 1961;Ishii, 1980b). Cross-sections through ciliated tubules were much more frequent than through flame cells and tended to occur in clusters, consistent with the tortuous course of ciliated trunk and side branches (Fig.…”

“…The tightly coiled CAVII-1-positive tubules are no longer ciliated, and thus are very likely to correspond to the non-ciliated tubule profiles seen by electron microscopy (Fig. 2B,G,H) (McKanna, 1968b;Ishii, 1980b). CAVII-1-positive segments often connected two neighboring proximal units, continuing the convergence trend of many proximal elements into fewer and fewer distal structures.…”

“…The suggested loose organization is consistent with the almost instantaneous dissociation of the planarian mesenchyme upon removal of the epithelium (not shown). Readily identifiable ciliation and other criteria previously established in chemically fixed material (McKanna, 1968b;McKanna, 1968a;Ishii, 1980b;Ishii, 1980a) provided a set of morphological features for the identification of protonephridial structures. Flame cells were defined by the 'filtration weir', which consists of closely apposed strands of cytoplasm surrounding a central cilia bundle, and by numerous microvilli between weir and cilia ( Fig.…”

“…However, substantial disagreements in the underlying light microscopic observations from the late 19th and early 20th century reveal considerable uncertainty regarding this view (Chichkoff, 1892;Wilhelmi, 1906). On the ultrastructural level, electron microscopy studies described protonephridia as having the following components: (1) cylindrical cells located at the tip of the ciliated side branches, with narrowly apposed strands of cytoplasm forming a fenestrated barrel around a central bundle of cilia, the flickering movements of which gave rise to the term 'flame cell' (McKanna, 1968a;Ishii, 1980a); (2) an initial ciliated tubule segment connected to the lumen of the flame cell barrel and purportedly composed of squamous epithelial cells; and (3) nonciliated 'main tubules' composed of a cuboidal epithelium (Pedersen, 1961;McKanna, 1968b;Ishii, 1980b).…”