Functional micro-anatomy of the digestive gland of the scallop Pecten maximus (L.)

The digestive gland of Pecten maximus consists, as in other lamellibranchs, of numerous blind-ending tubules which communicate with the stomach by partially ciliated main ducts and non ciliated secondary ducts. The non-ciliated cells of the main ducts are characterized by a well developed brush border constituted by high and dense microvilli and a strong pinocytotic activity. Ciliated and non ciliated cells have a very similar fine structure. The digestive tubules have a large lumen and contain digestive cells at different stages of absorption, digestion and excretion, one part of the tubules being functional while the other is disintegrating. The dark crypts contain the flagellated secretory cells, characterized by a well developed granular endoplasmic reticulum, and the young immature cells which may replace both the secretory and digestive cells. The numerous lipid droplets occurring in the digestive duct cells and in the digestive cells reveal the lipid storage function of the digestive gland. Several enzyme activities involved in digestion have been localized in the digestive gland. High amylase activity and cellulase and lysozyme activities have been found in the ducts and in the tubules, whereas no proteolytic activity could be detected histochemically. Some intracellular peptidases and glycosidases have been localized in the cells of the digestive gland, especially in the brush-border cells of the ducts and in the functional part of the tubules. High alkaline and acid phosphatase activities are displayed by the duct brush-border cells and the digestive and secretory cells. These results show the main role of the digestive gland, both in extracellular digestion (secretion of the digestive enzymes) and in absorption and intracellular digestion and provide information on the respective functions of the different cells within these processes.