Ultrastructural studies on the formation of the periostracum in Helix aspersa (Mollusca).

The ultrastructure of the periostracal gland in Helix aspersa and the sequence of formation of the periostracum is described. The periostracal units which measure about 9-12 nm wide and 0.4-0.6 mum long are first detected in the Golgi cisternae. The cisternae containing the unit(s) eventually pinch off from the rest of the Golgi. Microtubules are seen in the space separating the nascent secretory inclusion and the Golgi. Cross-bridges are seen between the microtubules and the secretory inclusions, suggesting that the microtubules are involved in the transport of nascent inclusions away from the site of synthesis. Many periostracal units unite to form periostracal sheets. The mature inclusions containing periostracal sheets migrate to the apical part of the cell where they fuse with the lysosomes before being extruded externally in the lumen of the gland. Perhaps the lysosomal enzymes somehow modify the periostracal units before their extrusion or digest the excess periostracal units. The periostracal sheets released in the lumen disperse randomly to produce a fibrous sheet, which is non-uniform in texture at first but is entirely homogeneous by the time the periostracum is secreted outside.