The formation of the fibrils in the lorica of Poteriochromonas stipitata: Tip growth, kinetics, site, orientation.

The microfibrils of the lorica of Poteriochromonas stipitata Scherffel obviously are formed at the surface of the plasmalemma. They elongate unidirectionally by tip growth. On 1 μm(2) plasmalemma about 1-2×10(6) glycosidic bonds per minute are formed. In the stalk, the primary fibrils are arranged helically. They tend to fasciate; a ribbon-like fibril has the width of about 20 nm and, therewith, nearly that of a microtubule. Every primary fibril coincides precisely with a microtubule. The microtubules lie under the plasmalemma and are connected with it by cross bridges. The secondary fibrils form a more irregular network. They are mainly produced where and when microtubules are absent. They band zipper-like with each other and with the primary fibrils. It is discussed how the different structures which are involved in microfibril formation move and how the formation process is regulated. Microtubules obviously influence morphogenesis of the lorica: they participate in special orientation of microfibrils, but play no role in their synthesis and secretion.