CELL DIVISION AND MORPHOGENESIS OF THE CENTRIC DIATOM CHAETOCEROS DECIPIENS (BACILLARIOPHYCEAE) II. ELECTRON MICROSCOPY AND A NEW PARADIGM FOR TIP GROWTH

Valve morphogenesis starts when the silica deposition vesicle (SDV) expands across a cleavage furrow covered by an unidentified layer, which may aid in its shaping. A labiate process (LP) is present only in the outer valve of terminal cells in the filament. Before these particular cells form setae, a layered "labiate process apparatus" (LPA) appears on the SDV in the exact center of the forming valve, near the microtubule center arising after cleavage. The LPA thereafter surmounts the lips of the LP as it forms. After the girdle bands separate slightly, two lateral protrusions develop in the corners of the cell. These nascent setae are lined internally by a cylindrical, fibrous band (sleeve), which assembles immediately ahead of the expanding edge of the SDV, very close to the plasmalemma. Then these protrusions, lined by the fibrous band, the SDV, and the forming silica wall, grow through two gaps in the girdle bands. The cytoplasm at the tip of the growing seta is naked. Immediately behind the tip, this fibrous band is adpressed to the plasmalemma and thereby apparently defines the diameter of the seta; it extends to internally ensheath the tipmost edge of the SDV for a short distance, like a tight-fitting inner sleeve. This structure is considered the major organelle involved in seta morphogenesis. Microtubules (MTs), while present, are variable in extent and disposition within the seta. Turgor pressure is considered irrelevant in driving seta growth. Instead, a new paradigm proposed for tip-growing cells generally, may apply to seta morphogenesis, as follows. If, as is suspected, the fibrous band contains actin, cycling of this actin (as in animal cells undergoing ruffling or filopodial extension) could drive seta extension via attachment of the band to the just-formed silica wall. The band is visualized as a molecular treadmill whose support base, the new wall, is being continually extended; extension is controlled and generated strictly at the tip.