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Differentiated epidermal outgrowths in the planarian Dugesia gonocephala: a model for studying cell renewal and patterning in single-layered epithelial tissue.

Large deep wounds on the ventral side of a flatworm (Planaria) will not heal. Instead, the damage to the parenchyma in the wound's roof will result in a differentiated swelling in the dorsal epidermis, above the wound which will eventually disappear with the disintegration of the underlying damaged tissue and a ventrodorsal hole appears in place of the wound. The dorsal epidermal outgrowth is formed by a number of excrescences, the development of which involves four successive stages. Their analysis suggests that epidermal cells are continuously produced by their own stem cells which remain unnoticed because their nuclei are hardly stainable. The daughter cells differentiate without information from either the underlying tissues or the basal epithelial membrane. During the first stage of this differentiation the cells become ciliated and motile, with some embryonic features. They then produce rhabdites and take up a columnar shape as they may become attached to the basal membrane. After wound setting the production of epidermal cells increases and the overcrowding of the basal membrane results in (1) detachment of stem cells and motile ciliated cells from the basal tissues, i.e. outgrowths; (2) stretching of columnar cells at the base of the outgrowths. When in the process of tissue disintegration the basal membrane of the epithelium also disappears, the cells remain in a single-layered epithelial configuration and retain their original polarity. These results are at variance with the generally accepted hypothesis that, in planarians, epidermal cells originate from the parenchyma and the epidermis is not an autonomous tissue.