Fine structure of the gap junction in the tunicate heart

Summary The plasma membranes of the tunicate heart exhibit an abundance of macular gap junctions distributed widely over the membrane surface. A study of these junctions by the freeze-etch technique was undertaken in an effort to elucidate the fine structure of this important membrane modification in a primitive heart. In cross or near-cross fractured junctions the junctional particles in contiguous membranes appear to be paired in register and to meet in the midline. In favorable face views, the junctional particles are seen to be disposed in hexagonal array. The individual particles display a distinct rosette-like substructure consistent with a six-membered ring of globular protein molecules clustered around a central channel. Similar junctional-type particles can be found in nonjunctional areas of membrane suggesting that the transport mechanism which they may represent is not restricted to the gap junction.Career Investigator of the American Heart AssociationWe wish to thank Dr. J.B. Jillett for use of the facilities of the Portobello Marine Biological Station; Mr. W.S. Bertaud, Physics and Engineering Laboratory, D.S.I.R., Lower Hutt, who kindly supervised the preparation of some of the freeze-etch replicas; Dr. R.H. Millar of the Dunstaffnage Marine Research Laboratory, Oban, Argyll, Scotland, who identified the tunicate used in the present (and previous) study; Prof. W.D. Trotter who made facilities in the Department of Anatomy, University of Otago Medical School, Dunedin, available to one of us (V.L.); and Mrs. S.M. O'Kane for excellent technical assistance. Generous support from the American Heart Association (to V.L.) and from the Medical Research Council of New Zealand (to D.G.R.) is gratefully acknowledged