3-DIMENSIONAL RECONSTRUCTION OF THE CONTRACTILE SYSTEM OF THE NEMATOMORPHA MUSCLE-FIBER

The body-wall muscle fiber of Nematomorpha (flattened circomyarian with central cytoplasmic axis) is characterized by very long (about 120 μm) bipolar, paramyosinic filaments with a periodic asymmetrical organization. They are arranged in longitudinal contractile fields and form paired systems of very elongated semihelices regularly staggered along the fiber axis, concave toward the periphery and alternatively enantiomorphic. A three-dimensional reconstruction of the contractile system was performed considering the morphology of the contractile fields, the position in these latter of the central reversal zones and tapered extremities of thick filaments, and the peculiar distribution in the direction of the filament polarity. The validity of the possible models was checked comparing the actual images visible under the electron microscope with those obtainable by means of a computer. The possible functional meaning of this contractile structure is briefly discussed. © 1979 Academic Press, Inc.