Calcification initiation sites in the crab cuticle: the interprismatic septa. An ultrastructural cytochemical study.

In the crab cuticle the interprismatic septa (IS), which correspond to imprints left in the cuticle by the margins of the epidermal cells, penetrate the twisted structure of the chitin-protein matrix. The ultrastructure and geometric relationship between the fibrous architecture and the pattern of the prisms is described. The cytochemical characterization of the IS, by pronase treatment and ruthenium red staining, supports the hypothesis that this material corresponds to cell-coat glycoproteins released in the cuticle during secretion of the organic matrix. Calcification begins after ecdysis in the external laminae of the pigmented layer and along the IS. The presence of cation-binding glycoproteins in the sites where calcification is initiated could induce the nucleation of the mineral phase by concentrating calcium. The extracellular distribution of carbonic anhydrase, which favours carbonate deposition, is observed on ultrathin sections over the IS.