Calcite-resorption in the spine of the echinoid Eucidaris tribuloides

Summary

Reabsorption of calcite is known to occur in echinoid endoskeletons. The structure of the operating cells is described using ‘Prouho's membrane’ as an example, which dissolves the skeleton of cidaroid spines within a thin section. After that the distal part of the spine is shed, and a new shaft grows out of the remaining stump. The calcoclast function is exercised by phagocytes which are also numerous in normal spine tissues. If the spine is highly damaged, however, the phagocytes assemble at a defined level and melt into a single syncytium, called Prouho's membrane. They fulfill three functions: (1) they show an extreme phagocytotic activity and ingest cells of the distal spine part, (2) they block off the surviving stump from the distal part shed later, (3) they etch through the calcite trabeculae in order to detach the distal spine part. The dissolution of the calcite starts with circular bulges, but in the end extremely thin etching lamellae run transversally through the trabeculae.