The organization of a tunicate heart.
Cardiac myoepithelial cells contain three myofibrils adjacent to the lumen. Zonula adhaerentes connect myofibrillar ends; desomosomes and peg-in-socket connections occur laterally with zonulae occludenfes at the pericardial border. Conduction and contraction are vertebrate cardiac muscle type. T-tubules are absent ; atypical sarcoplasmic vesicles run between myofibrils and beneath the sarcoplasmic membrane. Multivesiculate vacuoles, on cytoplasmic lobes protruding into the lumen from sarcomeric positions, induce reversal of heartbeat by a secretory feedback mechanism. Auto-UV-fluorescent dense bodies aooarentlv assemble thick and thin filaments. Both pericardial cavity borders are pinocytotic. Exaggerated interdigitation of adjacent plasma membranes and thick basement membrane of pericardial cells contribute mechanically to heart action.
Kalk
M
M
Biology Department, University of Malawi, Malawi Sarah Berliner Fellow (I F.A.U W.) 1964/65, at University of Melbourne, Australla.