Collagen and elastin histochemistry of the teleost bulbus arteriosus: false positives

This report analyzes the localization of collagen and elastin in the teleost bulbus arteriosus by histochemistry and by transmission electron microscopy. Martin's trichrome staining shows widespread distribution of collagen in the wall of the bulbus. However, Sirius red indicates that collagen is mostly restricted to the valves and to the subepicardial layer. This is confirmed by transmission electron microscopy. Trichrome staining gives false positives that may be related to the chemical characteristics of both matrix components and dyes. By contrast, Sirius red constitutes a highly reliable method to detect collagen distribution. On the other hand, orcein heavily stains the bulbus of all teleosts examined. This includes the bulbus of the Antarctic teleosts, which do not show structurally discernable elastin fibers. In these cases, orcein may be staining non-elastin components, or basic elastin components not assembled into larger units. In the teleost bulbus, accurate identification of collagen and elastin cannot be based solely on histochemistry, but should be accompanied by structural identification of the components under study.