Abstract :
Morphological studies based on specific cytochemical staining were carried out in young embryos and larvae of the freshwater bivalve Anodonta cygnea and analyzed by light microscopy. X-ray analyses were also accomplished and compared with previous x-ray diffraction results in order to understand the early shell formation. Supported by the present morphological cytochemical results we can speculate a new functional hypothesis for the early shell formation, during the first stages of the embryo development in Anodonta cygnea. Thus, instead of the classic theory concerning the “shell gland†(on its secretion restrict meaning), the embryos’ body present some specific dorsal ectodermal cells, around the invagination pore, which differentiates and migrates to the shell formation region followed by cellular expansion. Further cell disintegration will occur in this region, releasing protein-like β-keratin and chitin material (secreted together) and forming later the organic cuticle, under a similar mechanism usually described for the nails structure. Simultaneously, a calcareous layer is developed on the inner face of the cuticle by deposition of crystalline calcium carbonate on its biomineralizing organic matrix, mainly under the aragonite form. Eventually the early shell calcium supply will be processed across the vitelline membrane, moving from the external rich calcium compartment of the mother gills on a gradient dependence. This larval shell structure and elementary properties will be maintained until juvenile shell starts to be constructed by the mantle epithelia outside the mother gills.
Keyword :
Anodonta cygnea, Embryonic shell, Embryos, Gastrula stage, Light microscopy