SITE OF BETA-CHITIN FIBRIL FORMATION IN CENTRIC DIATOMS .2. CHITIN-FORMING CYTOPLASMIC STRUCTURES

β-Chitin fibril formation was investigated in the centric diatoms Cyclotella cryptica, Cyclotella nana, Cyclotella meneghiniana, and Thalassiosira fluviatilis with transmission electron microscopy. The pores in the silica valves from which the fibrils originate were examined in serial sectioning to identify the underlying cytoplasmic structures involved in chitin formation. In all species synthesizing chitin, the fibrils originate from special invaginations of the plasma membrane. These invaginations are conical and bear a special coat on their cytoplasmic face. This coat shows a hexagonal arrangement. It is assumed that this coat plays a role in chitin fibril formation, which seems to be a transmembrane event. Kinetic data on fibril formation and calculations on the package density of synthetase molecules which should be localized in the conical plasma membrane invagination are discussed in relation to the special coat subunits. A relationship between the conical invaginations and a chitosome-like type of vesicles was observed only in Thalassiosira. © 1979 Academic Press, Inc.