Muscle Histochemistry of the Planarian Dugesia tigrina (Turbellaria: Tricladida): Implications in the Evolution of Muscle

Abstract

The somatic and pharyngeal musculature of the planarian D. tigrina was studied by histochemical methods. Unfixed frozen sections were incubated for activities of nicotinamide dinucleotide tetrazolium reductase, succinic dehydrogenase, myophosphorylase, non-specific esterase and myofibrillar adenosine triphosphatase preincubated at 4 different pH ranges. Modified Gomori trichrome stain for muscle, hematoxylin and eosin and periodic acid-Schiff stains also were applied. Uniformly strong and specific staining was demonstrated in muscle cells and not in other cells. Different fiber types could not be distinguished. Planarian muscle thus is metabolically differentiated from other tissues, as in higher invertebrates and vertebrates. The histochemical reactions are consistent with ultrastructural evidence that the unique unstriated somatic myofibers of platyhelminths resemble early stages of myogenesis in more advanced phyla, before the organization of sarcomeres or the divergent development of smooth and striated muscle. Dugesia represents an early stage in the evolution and specialization of muscle.