To be Stiff or to be Soft-the Dilemma of the Echinoid Tooth Ligament. I. Morphology.

The morphology of the tooth ligament of the echinoid Eucidaris tribuloides was investigated. The tooth ligament connects the middle part of the rod-shaped tooth to the jaw. It consists of collagen and microfibrils that are synthesized by fibroblasts in the aboral growth zone of the tooth. Histochemical staining showed that the tooth ligament contains highly sulfated acid proteoglycans that seem to connect the collagen fibrils. We did not find any cell bodies in the ligament, but we observed a small number of cell processes. Dense vesicle cells and supporting cells in the jaw near the ligament send these processes among the collagen fibrils. Dense vesicle cells have connections to a newly described jaw nerve. Supporting cells possess a unique podocyte-like filtration structure that probably serves for selective pinocytosis. In the oral region, where the working part of the tooth protrudes from the jaw, collagen fibrils swell and disappear. Sometimes phagocytosis of collagen fibrils could be observed. Autoradiographic data indicate that the collagen of the tooth ligament is not subject to quick turnover. The tooth ligament resembles a catch connective tissue that can change its mechanical properties under nervous control.