The mammalian anti-alpha-smooth muscle actin monoclonal antibody recognizes an alpha-actin-like protein in planaria (Dugesia lugubris s.l.).

The presence of an alpha-smooth muscle (alpha-sm) actin-like protein in planaria (Dugesia lugubris s.l.) is reported. The protein shows a 42 kDa molecular weight determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis and is specifically recognized by the mammalian anti alpha-sm actin monoclonal antibody. When a planarian is induced to regenerate by head amputation, the immunostaining of the alpha-sm actin-like molecule becomes important in the area of growing blastema, reaching a maximum between 70-120 hours after injury. Conventional electron microscopy at the 4-day-regeneration stage shows that blastema-forming cells are a homogeneous population whose morphological features resemble those of migrating mesenchyme-like cells; only the myoblasts show a recognizable phenotype. The immunocytochemical localization of alpha-sm actin-like molecule by immunoperoxidase (light microscopy) and immunogold stains (electron microscopy) was carried out on both intact and injured worms. The antigen was localized mainly at the basal portion of the epidermal cells and in the undifferentiated mesenchyme-like cells. Myoblasts, but not differentiated myofibers, were also labelled by this antibody. The results indicate that in the lower Eumetazoan planarians, as well as in vertebrates, the alpha-sm actin can be considered to be a marker for myoid differentiation. The suggestion that alpha-sm actin can be used as a marker for mesenchyme-like cells in vertebrates and in invertebrates is also discussed.