Electron microscopic studies of planarian regeneration. II. Changes in epidermis during regeneration

Abstract

Abstract Histological changes in planarian epidermis, as well as the differentiation of new epidermal cells during the course of differentiation, were studied with the electron microscope. Immediately after decapitation, pre‐existing epidermal cells of the area surrounding would stretch out over the cut‐surface and cover it completely with a thin film within 16 hours. However, these epidermal cells in the film were replaced by newly differentiated cells during the period extending from the second to sixth day of regeneration. Thus, after injury, the initial compensational closure is formed on the cut‐surface by the pre‐existing epidermal film, but the epidermis which is finally regenerated is formed by complete colonization of pre‐existing epidermis by newly differentiated epidermal cells. New epidermal and rhabdite‐forming cells were found not only in epidermis but also in the interior and sub‐epidermal regions of the blastema and in the old tissue close to the wound surface. Our observations suggest that ectodermally oriented cells, such as epidermal and rhabdite‐forming cells, originate from neoblasts at the anterior of the old body, close to its interface with the regeneration blastema; such neoblasts differentiate into rhabdite‐forming, or epidermal, cells as they migrate from the old somatic stump through the regeneration blastema into its epidermal surface layer. The characteristic polarity of epidermal cells appears in newly differentiated ones when they locate in the epidermal surface.