A light- and electron-microscopic study of enzymes in the embryonic shell-forming tissue of the freshwater snail, Biomphalaria glabrata

Abstract. The mode of formation of the molluscan exoskeleton is still poorly understood, but studies on adult snails indicate that enzymes involved in vertebrate bone formation also participate in mollusc shell formation. The enzymes peroxidase, alkaline phosphatase, and acid phosphatase are expressed in a constant pattern and help to identify the different zones of the adult shell-forming tissue. The present study evaluates whether the expression of these enzymes is also a tool for the identification of the developing zones of the embryonic shell-forming tissue. Thus, we analyzed the temporal and spatial activity of the above-mentioned enzymes and of tartrate-resistant acid phosphatase in the shell forming tissues in Biomphalaria glabrata. Embryos of different age groups and adults were studied; alkaline phosphatase activity was seen in very young embryos in the shell field invagination prior to the secretion of any shell material, while peroxidase activity was present from the start of the periostracum production. Acid phosphatase, found in considerable amounts in yolk granules and albumen cells, appeared in the embryonic shell-forming tissue in relatively few Golgi stacks. Tartrate-resistant phosphatase was not present in embryos, but was found in adults in the same zone of the mantle edge as acid phosphatase. Using the enzymes as cell markers, the differentiation of the embryonic shell-forming tissue to the different zones of the adult mantle edge could clearly be followed.