. The Biological bulletin. Biology; Zoology; Marine biology. ASCIDIAN HEART 411 regionâhave been investigated. We report here that dif- ferentiated cardiomyocytes of the ascidian heart, like those of higher chordates, retain the capacity for DNA synthesis. Unlike the myocardium of vertebrates, however, the as- cidian myocardium possesses a permanent cambial zone in the raphe regions. Materials and Methods Live specimens of the solitary tunicate Ascidia ohliqua Alder, 1863, were supplied by the Institute of Marine Bi- ology at the University of Bergen. The animals, ranging in weight from 8 to 1
. The Biological bulletin. Biology; Zoology; Marine biology. ASCIDIAN HEART 411 regionâhave been investigated. We report here that dif- ferentiated cardiomyocytes of the ascidian heart, like those of higher chordates, retain the capacity for DNA synthesis. Unlike the myocardium of vertebrates, however, the as- cidian myocardium possesses a permanent cambial zone in the raphe regions. Materials and Methods Live specimens of the solitary tunicate Ascidia ohliqua Alder, 1863, were supplied by the Institute of Marine Bi- ology at the University of Bergen. The animals, ranging in weight from 8 to 12 g, were kept in an aquarium with seawater at about 15°C. Tritiated thymidine (State Insti- tute for Applied Chemistry, St. Petersburg, Russia) with a specific activity of 16 Ci/mmole was injected into the body cavity of the animal at a dose of 10 mCi per gram body weight. The hearts were fixed 4 h later in a modified version of Karnovsky fixative, in which distilled water had been replaced by Ringer solution (Nylund, 1985). After fixation for h, the tissue was washed in a phos- phate-buffered solution of 8% sucrose and postfixed for h in a phosphate-buffered 1% solution of osmium tetroxide. Dehydration through a graded series of alcohols and acetone followed. Finally the tissues were embedded in Epon 812. For electron microscopy (EM), thin sections were cut with a diamond knife, stained with uranyl acetate and lead citrate, and examined under a transmission micro- scope (JEM 7A, Japan). EM autoradiographs were pre- pared by the method of Larra and Droz (1970), with an exposure time of 60 days. The share of [â 'H]Tdr-labeled nuclei (from the total number of nuclei) was calculated from autoradiographs of semithin sections (1 mm thick) stained with methylene blue and azure II after 30 days of exposure to photoe- mulsion. For each of six animals, 20 sections were ex- amined. The number of counted nuclei averaged 5000 in the myocardium, 3500 in the pericardium, and 80
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