Reference:/;/!'/ Hull 191: 1-4. (August. Contractile Connective Tissue in Crinoids RUDIGER BIRENHEIDE AND TATSUO MOTOKAWA Tokyo Institute of Technology, Faculty ofBioscience & Biotechnology, Biological Laboratory, Ookuyama 2-12-1, Meguro, Tokyo. 152 Japan Active movements in animals are usually attributed to cellular protein engines, e.g., the actin-myosin system of muscle cells. Here we report the first evidence of an extra-cellular contractile connective tissue, which we have found in .sea lilies and feather stars (Echinodermata. Cri-noida). These marine animals have arm muscles that are antagonized, not by other muscles, but by ligaments con-sisting of extracellular fibrils interspersed with neuron-like cell processes. Contractile cells are lacking, yet these arm ligaments actively contracted upon stimulation. The ligaments stayed in a contracted condition even after the stimulus had stopped. The stresses generated were lower than those of typical skeletal muscles. Additional data from crinoid cirri, which lack muscles entirely, corrobo-rate the hypothesis that the connective tissue of the liga-ments is contractile. Crinoids use their arms for filter feeding, crawling, and swimming. The arms consist of a series of ossicles inter-connected by mobile joints (Figs, la, 2a). Muscles bend the arms orally (upwards). The aboral (downward) pow-erstroke has been thought to be effected by the passive elastic recoil of aboral ligaments ( 1 ); but we provide evi-dence here that the ligaments contract actively and thus contribute to the movements of the arms. At rest, crinoids are anchored by a different set of ap-pendages the cirri (Fig. 1 ). Each cirrus consists of a row of ossicles joined by ligaments that lack any muscles (2, 3): yet these appendages move actively (3). We show in this study that the cirri can develop forces that are prob-ably responsible for their movements. Specimens of crinoids were trawled from a depth of 1 30 m off Numazu, Suruga Bay. Japan (sea lily Metacri-mis rotundus) or collected in shallow waters near Komi-nato, Chiba Prefecture, Japan (feather star Comanthus Received 14 February 1996; accepted 29 May 1996. japonicus). The animals were kept in circulating seawa-ter aquaria. Pieces of arms consisting of 5 to 10 ossicles were prepared as follows: The oral half of each ossicle was removed along with its connecting ligaments and muscles; thus the remaining aboral half ossicles were connected only by their aboral ligaments (Fig. 2b). The mechanical responses i.e., displacement (Fig. 3a) and generated forces (Fig. 3b) of these half-arm prepara-tions were examined. Transmission electron microscopy revealed no evi-dence of any muscle cells in the ligaments of the arms or cirri; these ligaments appeared to be conglomerates of collagen fibrils, microfibrils, and neuron-like cell pro-cesses (Fig. Ib). The neuron-like cell processes were dis-tributed randomly among the fibrils; the cell bodies asso-ciated with these processes are located in the pore space of the ossicles and are connected to the nervous system ( 1 ). They are likely to be the site of the mechanism that triggered the contraction of the ligaments described be-low. These findings reconfirm earlier reports that the lig-aments of crinoid arms (1, 4, 5, 6) and cirri (2. 3) lack muscle cells. When arm pieces were connected to a holder at one end and a metal plate was fixed to their other end (Fig. 2b), they showed a constant slow bending under the in-fluence of gravity. But Figure 3a shows the strong flexion against the force of gravity caused by K + depolarization; the ligaments shortened to less than half of their length. Contraction of the aboral ligament is the only possible means by which this flexion could have been accom-plished. K + depolarization had no effect on samples anesthetized with seawater containing menthol, but re-activity was restored after washing in seawater. Probably the K + ions acted by depolarizing neural elements (7). The possibility that the neuron-like cell processes con-tract upon stimulation has to be considered. Many echi-
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