Ultrastructure and Cytochemistry of Periostracum and Mantle Edge of Biomphalaria glabrata (Gastropoda,Basommatophora)

Abstract Three layers of different electron density can be distinguished in the periostracum. Periostracal units of up to 900 nm length are merged into the outer fibrous layer and binding of gold-labelled lectin-WGA indicates the presence of chitin because it is labile to chitinase treatment. The periostracum is formed by the epithelia of the groove and the belt at the mantle edge. The distal and basal epithelium of the groove consists mainly of type A cells with an extended Golgi apparatus and apical vesicles. The presence of peroxidase and phenol oxidase indicates a function in tanning of the periostracum. In the proximal epithelium of the groove, type B cells with protruding apices add more material for periostracum formation. Type C cells secrete single periostracal units which are formed within single vesicles or larger vacuoles. Type D cells secrete electron-dense vesicles which also contain WGA-positive material. The distal cells of the belt are characterized by predominating strands of the rER while subapical vacuoles, to some of which WGA binds, dominate in the cells of the central part. In the belt, phenol oxidase and peroxidase can be localized in cisternae of the rER and the Golgi apparatus. Numerous control incubations indicate that, indeed, two different enzymes are localized.