Ultrastructure of cephalopod chromatophore organs

Abstract

The chromatophore organs of Loligo opalescens are composed of five different types of cells: the chromatophore proper; radial muscle fibers; neuronal processes (axons); glial cells; and chromatophoral sheath cells. The surface of a retracted chromatophore is extensively folded, but upon contraction of the radial muscle fibers it becomes flattened and the folds of the surface disappear. The cell membrane cannot be responsible for the elasticity of the chromatophore as claimed by earlier investigators. The pigment granules are confined within a filamentous compartment (cytoelastic sacculus) throughout the cycle of expansion and retraction. The sacculus decreases in thickness during expansion and increases in thickness during retraction and does not become folded. The elastic properties of the chromatophore are attributed to the cytoelastic sacculus. Primary infoldings of the chromatophore surface are anchored to the sacculus at points called focal haptosomes. The periphery of the sacculus attaches to the plasmalemma of the equatorial part of the chromatophore, opposite the area of attachment of the radial muscle fibers (myochromatophoral junction) by a zonular haptosome. The regular, obliquely striated muscle fibers that expand the chromatophore are associated with axons and glial cell processes. Adjacent muscle fibers may be electrically coupled through close junctions. The entire chromatophore and the muscle fibers are surrounded by sheath cells.