Observations on planarian epithelization after wounding.

Wound closure of planarian regenerates was studied by light and electron microscopy. Quick response of epidermal cells after amputation depended on muscular contraction near the open wound. Early epithelization involved both active and passive cell migration phenomena. First, the old cells from the dorsal and ventral epidermis facing the wound simultaneously spread to close the wound surface, joining first near the wound center. This was facilitated by the contraction. Cell stratification at the wound margin was the result of active cell migration at the time of amputation. Secondly, these cells were passively stretched from the wound margin so that the stratification disappeared. Partial relaxation of the wound was a causative factor for extreme elongation of the wound epidermal cells. Disappearance of hemidesmosomal attachments was initiated by the collapse of the basal lamina. This was obviously linked to cell locomotion. Further development of the wound epidermis was accomplished by the successive supplementation of precursor cells from the newly formed regeneration blastema (mainly rhabdite-forming cells).