Ultrastructural study of the embryonic and larval shell of Anodonta cygnea

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Abstract
An ultrastructural study of the embryonic and larval shell of Anodonta cygnea was carried out by light, polarization, and scanning electron microscopy, and X-ray diffraction analysis. A thin outer layer (cuticle), composed of β-keratin fibrils organized in parallel rows and enveloped by chitin material, was observed. The hooks, spines, and teeth are constituted by cuticular formations only. A calcareous inner layer presenting convergent prismatic monocrystals mainly of aragonite, which form prismatic structures around the pores and inside of irregular polygonal border lines, was also shown. Two clearly differentiated calcareous valves were already observed on very young embryos. The valvular pores totally covered by the cuticle may be involved in the metabolic cell exchange by penetrating cytoplasmatic extensions of the mantle cells. The irregular polygonal configuration observed on the internal surface of young decalcified cuticles supports the hypothesis of a role for the matrix as a regulator of shell formation, inducing the prismatic organization of the calcareous layer. Shell mineralization from the border lines to the center of each polygonal area may occur.