The Fine Structure of an Elementary Contractile System

Vorticella are ciliated protozoa that live in water, sometimes free, and sometimes attached by a long contractile peduncle (or stalk) to floating plant material. Early light microscope studies of the animal revealed the existence of longitudinal and circular myoid fibrils linked with the contractile axis of the stalk. Observers reported (1, 15) that this system has the dual capacity of dosing the oral extremity (cytosome), and of provoking the animal's characteristic shape change, from campaniform to spheroidal (while the peduncle itself contracts into a helical form. In this Brief Note, it is not our intention to describe the morphology of Vorticella, but only to describe the fine structure of the myoid fibrils. It is sufficient to note that most of the myofilament bundles run near the cell periphery, some in close contact with the cell membrane, some inside the cell cytoplasm. At the (proximal) origin of the stalk, these longitudinal fascicles join to form a thick bundle (Figs. 1 and 3). In the electron microscope, each observably discrete fibril in the system is made up of many elemental components, each 30 to 40 A in width, and of indefinite length (Fig. 2). Differences between contracted and elongated myofibrils are not clearly detectable, and we presume that a certain degree of contraction may always occur as a consequence of fixation (Fig. 4). Electron micrographic profiles show that the fibrillar bundles exist in a close relationship with a membrane-limited canalicular system that resides both inside the bundles and paralleling and bounding them. Both the inner and outer canaliculi are integrated in the same system by connecting tubular bridges, but a striking difference distinguishes the two (Figs. 1 and 2). The outer surface of the membrane limiting the inner canaliculi is in intimate contact on all sides and solely with the myofibrils; the outer canaliculi, on the other hand, are in contact with the myoneme on one side only, and while the contacting face is smooth and flattened, the remainder of the membrane surface is rounded and associated with particulate material of the cytoplasmic matrix. A wealth of canaliculi intermingle with the myofilaments of the axial bundle of the stalk, and most of these appear to be connected with the canalicular network in the myonemes of the pro-tozoan body. Cross-sections of the stalk show the spongy aspect of the myoneme (Fig. 4). A canalicular system, comparable to that lined up along the bundles …