Scanning electron microscopical observations on epidermal wound healing in the PlanarianDugesia tigrina.

Epidermal wound healing in regeneratingDugesia tigrina (Planaria) has been studied using scanning electron microscopy (SEM). The normal epidermal surface and its differentiations have been descrebed. Observations on living material reveal the highly dynamic state of the wound in invididual animals and its more or less continously changing size due to the state of activity of the animals. These observations show good agreement with the SEM studies, which allow a clear delineation of cellular details of the wound, the wound margins and the apposing epidermal regions. These details are described. The over-all picture of planarian wound healing that emerges is briefly as follows: Epithelization is characterized by absence of proliferation from the "old" intact epidermis. Variable contraction of smooth muscle cells reduces the wound size to a certain extent. Simultaneously with this and also during a longer period epidermal cells adjacent to the wound are extending and some become highly attenuated. These two processes together are only to a certain degree effective in wound closure because of a definite epidermal cell deficit which is reflected in the emergence of an epidermal wound edge reflecting the maximal contribution of these two processes to an attempt to close the wound. Complete epithelization is effected by the operation of a third mechanism: Recruitment of cell through flow of subjacent "blastemal cells" (including rhabdite-forming cells) along the wound border; these cells subsequently occupy a peripheral position in the wound. This process is supplemented by cell immigration and insertion into the adjacent old epidermis and in the wound cell sheet. Rhabdite-forming cells contribute predominantly to this process. Eventually integration between old epidermal cells and the newly recruited cells which differentiate into epidermal cells results in final epithelization. Complete wound healing is based on interactions between the epidermal cell system and the regenerating subepidermal membrane-connective tissue filament-muscle cell system.