Inhibitions to this results in reversed polarity across the blastema. Wilms' tumor is a malignant tumor containing metanephric blastema, stromal and epithelial derivatives. Instead of originating from the metanephrogenic blastema, the collecting duct originates from the ureteric bud. Segmental regeneration in these animals is epimorphic and occurs through blastema formation. These fibroblasts multiply to form a blastema, the progenitor for a new limb. In vertebrates, epimorphosis relies on blastema formation to proliferate cells into the new tissue. First, the local cells dedifferentiate at the wound site into progenitor to form a blastema. Blastema is absent whereas calcospherites may be present. Blastema cells can differentiate into any cell type with the exception of neurons.