Histochemical studies of pedicellaria of an echinoid, Salmacis bicolor

The tridentate pedicellaria of Salmacis bicolor consists of calcareous head and stalk, connected by a muscular neck regions. Head, neck and stalk are covered by a thin cuticle, which is non-mineralized and soft. Both the cuticle and the organic fraction of the calcareous region are resistant to acids and alkalis. Histochemical studies reveal that both the cuticle and the organic matrix of the calcified head and stalk contain protein, polysaccharide and lipids. The lipid content of the organic fraction of the mineralized matrix recalls the spicules of Pluteus larva reported in an earlier study. The protein component of the cuticle and that of the organic matrix slightly differ in that the former is rich in aromatic amino acids, namely tyrosine and tryptophane. Both cuticle protein and the protein of the organic matrix lack sulphur containing aminoacids. Another feature of similarity between the cuticle and the organic matrix is that both contain acid mucopolysaccharide and neutral mucopolysaccharide components. The role of acid mucopolysaccharide in calcification is discussed. The neutral mucopolysaccharide has been suggested to be similar to chitin, and its significance is discussed. Unlike the skeletal elements of other invertebrates, the cuticle and the organic matrix of pedicellariae are devoid of any phenolic compounds.