ULTRASTRUCTURAL LOCATION OF CALCIUM AND MAGNESIUM DURING MINERALIZATION OF THE CUTICLE OF THE SHORE CRAB, AS DETERMINED BY THE K-PYROANTIMONATE METHOD AND X-RAY-MICROANALYSIS

We have investigated the distribution of Ca2+ and Mg2+ in the new cuti cle of moulting shore crabs (Carcinus maenas), using the K-pyroantimon ate method in combination with X-ray microanalysis in order to identif y antimony precipitates. During the premoult period, Ca2+ and Mg2+ acc umulate in well-defined sites of the new pigmented layer. After moulti ng, mineralisation appears to begin preferentially at these sites. The se form a honeycomb-like structure that quickly increases the rigidity of the new cuticle, with a small recruitment of material from extrane ous sources. Mineralisation of the principal layer, on the other hand, immediately follows deposition of the organic matrix. Our experiments also provide evidence that the epidermal cell extensions associated w ith the pore canals are the means by which Ca2+ and Mg2+ are transferr ed from the epidermis into the mineralising cuticular layers. The plas ma membrane of these cell extensions appears densely lined by particle s of antimony precipitate that probably mark the location of the trans porting sites. Shortly after moulting, the distribution of mineral dep osits is such that the cell extensions cross the mineralised lamellae of the principal layer and constitute preferential access routes to th e pigmented layer, where mineralisation is still in progress.