Ultrastructural organization, ontogenesis, and regeneration of the periostracum in the slug Deroceras reticulatum (Mollusca)

Abstract

Electron microscopy studies show that the periostracum of the internal reduced shell in Deroceras reticulatum is composed of two layers: an inner dense layer and an outer fibrous one. The inner dense layer, the thicker part of the periostracum, consists of stacked lamellae formed in the Golgi apparatus of all the mantle edge cells. The structure and the synthesis of the inner dense layer support the hypothesis that the reduced shell in Deroceras may be considered a typical molluscan shell. The outer fibrous layer appears as a peculiar characteristic of this species and it seems to be synthesized by the dorsal epithelium of the shell sac. Shell regeneration begins with a rapid restoration of the periostracum which forms an uninterrupted sheet enclosing a mineralization chamber. Rapid synthesis is related to the involvement of not only the mantle edge cells but also of all the mantle cells, indicating that the mantle cells that produce only the mineralized layers during shell ontogenesis can form the periostracal units also. This potentiality is expressed only when the closed mineralization chamber must be restored in a short time.