ABSTRACT
1. The electrocytes from the electric organ of the Patagonian ray P. extenta are very unusual cells: semicircular in shape, multinucleated and highly polarized. They have an anterior, concave, innervated face, which exhibits positive histochemical reactions for acetylcholinesterase and for the nicotinic acetylcholine receptor. Multiple nerve-endings are covered with Schwann cell projections, similar to those present in skeletal muscle. Their posterior face is convex, non-innervated and is in contact with collagen fibres. 2. The cytoplasm of these electrocytes possesses abundant filamentous actin (F-actin), orderly distributed in the cell and exhibiting intense fluorescence with NBD-phallacidin. The F-actin is in contact with Z-lines as in muscle, and in contrast with Torpedo (Kordeli et al., 1986, 1987) and Discopyge (Vidal et al., 1986, 1989a) electrocytes, where it is confined to the non-innervated face. 3. Electrocytes of this Rajidae are an ideal model for the study of F-actin because of the similar embryological origin with skeletal muscle tissue and also because of the peculiar characteristics of their cytoplasm, packed full with F-actin. In addition, electric organs could constitute an alternative biological source for the study of the cholinergic synapse.