An Electron Microscope Study of Protozoan Flagella

Protozoan flagella are otganelles which have been studied extensively; nevertheless, many structural details remain controversial. Because of the limited power of resolution of the light microscope, little of the finer structure of the flagellum was revealed before the advent of the electron microscope. That the organelle is composed of a central axial core (axoneme) and an enveloping sheath is rather generally accepted. Early in the present century, flagella of Euglena and Chilomonas were shown to be separable into fibrils when subjected to pressure (Dellinger, 1909). Other workers (e.g., Korschikov, 1923) pointed out that fibrils occur within the axonemes of various flagellates. More recently, electron micrographs have demonstrated the presence of a number of fibrils within the axoneme-usually nine to twelve (Schmitt, Hall, and Jakus, 1943; Pitelka, 1949; Houwink, 1951). In the euglenoids, these fibrils appear to be arranged in two parallel bundles (Brown, 1945; Pitelka, 1949); however, Pringsheim and Hovasse (1950) interpret this appearance as being due to the ribbon-like structure of the flagellum. Manton and Clarke (1951) present electron micrographs which clearly demonstrate the presence of eleven fibrils in the cilium or flagellum of a fern spermatozoid. The nature of the sheath is more difficult to determine. Apparently it consists of a semi-liquid substance enclosed by a membrane (Pitelka, 1949), and may include fibrils which encircle the axoneme in a helix (Brown, 1945). However, Houwink (1951) has demonstrated that in Euglena the membrane, if present, is very delicate, and that the transverse striations in the Euglena flagellum are not due to the presence of a helical fiber. The functions of the flagellar components are unknown. Some authors (e.g., Pitelka, 1949) assume the axoneme to be the contractile portion, while others (e.g., Hyman, 1940; Houwink, 1951) consider the axoneme merely an elastic supporting structure, and contend that the sheath contains the contractile elements. The excellent electron micrographs of Houwink depict transverse striations in the sheath of the Euglena flagellum which are strikingly similar to the striations of skeletal muscle. Most investigators who have made careful microscopic studies of such organisms as the phytomonads (Chlamydomonas, Sphaerella, etc.) have observed a delicate terminal filament (e.g., Kunstler, 1889; Loeffler, 1889; Fischer, 1894; Gerloff, 1940; Owen, 1949). Pitelka (1949) examined the flagella of Sphaerella by means of the electron microscope, but observed no