The spine tissues in the echinoid Eucidaris tribuloides

Summary

The tissues of a fully grown echinoid skeleton are described using the primary spines of E. tribuloides as an example. Cidaroid spines are covered with an external, polycrystalline cortex, but as long as they are still growing they are covered with an epithelium. The mineral skeleton is embedded in the mesodermal stroma tissue which largely consists of fluid. Different types of mesodermal cells float within this fluid, but the sole characteristic stroma cells are the sclerocytes which are anchored to the calcite trabeculae by means of a cytoplasmic stalk. The latter spreads over the surface of the young trabeculae as a thin, continuous sheath, but on fully grown trabeculae it ramifies into numerous filiform processes (dp). The sclerocyte cell body is surrounded by a boundary layer which, however, is absent in the distal sheaths or filiform processes. The cytoplasm of the sclerocytes is electron-translucent and contains numerous free ribosomes. Sclerocytes which lie below the epithelium produce the cortex layer, and in the end the extracortical stroma as well as the epithelium vanish, and the cortex becomes external.

Phagocytes within the stroma are at least as numerous as sclerocytes. They have a dense cytoplasm with long, straight pseudopodia protruding from it and running through the midst of the pore space. In normal conditions the pseudopodia do not touch the trabeculae. In a single instance, however, a phagocyte was demonstrated to etch a trabecula. Its etching face was crowded with clear vesicles which are not found elsewhere.