ULTRASTRUCTURE OF PSAMMOBATIS-EXTENTA (RAJIDAE) ELECTROCYTES AND CYTOCHEMICAL-LOCALIZATION OF ACETYLCHOLINESTERASE, ACETYLCHOLINE-RECEPTOR AND F-ACTIN

1. The electrocytes from the electric organ of the Patagonian ray P. e ntenta are very unusual cells: semicircular in shape, multinucleated a nd highly polarized. They have an anterior, concave, innervated face, which exhibits positive histochemical reactions for acetylcholinestera se and for the nicotinic acetylcholine receptor. Multiple nerve-ending s 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 electro cytes possesses abundant filamentous actin (F-actin), orderly distribu ted in the cell and exhibiting intense fluorescence with NBD-phallacid in. The F-actin is in contact with Z-lines as in muscle, and in contra st with Torpedo (Kordeli et al., 1986, 1987) and Discopyge (Vidal er a l., 1986, 1989a) electrocytes, where it is confined to the non-innerva ted face. 3. Electrocytes of this Rajidae are an ideal model for the s tudy of F-actin because of the similar embryological origin with skele tal muscle tissue and also because of the peculiar characteristics of their cytoplasm, packed full with F-actin. In addition, electric organ s could constitute an alternative biological source for the study of t he cholinergic synapse.