An Additional Ultrastructural Component of Flagella

Electron microscopic observations of flagella and cilia have indicated a constancy in the number of peripheral and axial fibrils seen in cross-sections (1-3) and a variability in the structure and form of the basal bodies and accessory structures in different organisms (4-8, 17). During recent electron microscope studies of plastids in colorless flagellates (9), observations were made of an additional structural component in cross-sections of the flagellar bases of Polytoma obtusum and P. uvella (10). Bacteria-free cultures of P. obtusum (L791) and P. uvella (L19) from the Culture Collection of Algae at Indiana University (11) were grown in flasks containing 125 ml of Polytomella medium (12) at 20 41 °C. Harvesting by low speed centrifugation after 2 to 3 days of growth was followed by fixation in buffered 1 per cent osmium tetroxide and embedding in a mixture of Araldite and Epon 812 (H. H. Mollenhauer, unpublished). Ultrathin sections were cut and examined in the usual manner in an RCA EMU-3D. Cross-sections of the 2 flagellar shafts of these colorless, unicellular members of the Chlamydomonadaceae show the typical 9 + 2 pattern of fibrils embedded in an apparently structureless matrix, the whole enclosed in a trilaminar sheath. Each of the 9 peripheral fibrils is composed of 2 tubular subfibrils as is commonly found in plant and animal cilia or flagella. In cross-sections of the flagellar shaft immediately distal to the basal body, the 2 central fibrils are missing. The exact point of the basal termination of the central fibrils is not so clear in this material as in other flagellates (13), but in longitudinal sections the 9 peripheral fibrils are seen to continue into the anterior cytoplasm as the basal body. As yet, the possible triple form of the fibrils comprising the basal body comparable to that of Pseudotrichonympha (8) and other forms has not been clearly seen in Polytoma. The basal body of 1 flagellum is connected to that of the other by a broad fibril with periodic striations which arches across the anterior papilla (Fig. 4), as described previously in Polytoma (9). A few favorable cross-sections of the transition region between the flagellar shaft and the basal body indicate the presence of what appears to be a complex pattern of thin fibrils (about 90 A wide) interconnecting the 9 peripheral fibrils. The pattern is a 9-pointed star in which each star point contacts 1 subfibril of each peripheral fibril (Figs. l and 2). The connections forming the star contact alternate peripheral fibrils so that subfibril A (arbitrary designation) of peripheral fibril I (arbitrary designation) is connected with subfibril A of peripheral fibril 3; similarly, 2-4, 3-5, 4-6, 5-7, 6-8, 7-9, 8-1, 9-2. It is possible, for example, that the connection between peripheral fibrils 1 and 3 is continuous with the connection between the second and fourth peripheral fibrils at the point of intersection. This pattern is more elaborate than the "pin-wheel" found in the basal body itself by Noirot-Timothefi (14) and Gibbons and Grimstone (8). Directly opposite each star point, a dense thickening appears in the connections. Gibbons and Glimstone (8) interpreted similar dots as representing cross-sections of another set of thin longitudinal fibrils. The 2 central fibrils are absent at this level, thus leaving the central region of the 9-pointed star devoid of structure. Furthermore, in this material there is an indication of attachment of each of the 9 peripheral fibrils to the flagellar sheath in the form of electron-opaque, pyramid-shaped structures (Figs. 1 and 2). Longitudinal sections of these flagellar bases