MECHANISM OF MOVEMENT OF EPIDERMIS, ESPECIALLY ITS MELANOPHORES, IN WOUND HEALING, AND BEHAVIOR OF SKIN GRAFTS IN FROG TADPOLES

Abstract

1. Immediately following the occurrence of wounds in the skin of tadpoles of Rana clamitans, the surrounding epidermis moves to cover the area. Wounds 5 mm. across may be covered in from 6 to 24 hours. The advancing epidermis is reduced to one or two cells in thickness, thus covering a larger area than previously. The epidermal cells become considerably elongated in the direction of movement. After the advancing edges have met, the cells resume their previous shape. Subsequent mitosis restores the original thickness of the layer. There iS considerable evidence for stereotropism in epidermal migration. 2. In wounds which cause a deficiency in the dermis, the epidermis thickens over the wound area until the smooth contour of the body-surface is restored. 3. In the region surrounding a healing wound there is temporarily a deficiency in the number of epidermal pigment cells due to their movement into the wound. 4. Autotransplants of white ventral skin transferred to the pigmented backs of tadpoles retained their specificity. They remained unpigmented for more than 100 days. Lateral and dorsal autotransplants in which the patches of skin were rotated through 90 or 180° also retained their specificity. There was no change in the pigmentation or the cellular structure of such grafts. 5. In all cases where homoiotransplants of white ventral skin were transferred to the pigmented backs of tadpoles the region of the graft became pigmented. Epidermal pigment appeared at the edge of the graft in an average of 9 days after the graft was made, and in the course of about 6 more days had extended to the center of the patch. Dermal pigment entered the graft area many days after the epidermal pigment had completely covered the patch. 6. The initial pigmentation of grafts results from a replacement of the graft epidermis by that of the host. The pigment cells are carried along by the host epidermis as it covers the patch. There is considerable evidence that the graft epidermis is destroyed by phagocytic action of the host epidermis. However, the epidermal layers of graft and host always unite immediately after the graft is made, indicating some initial affinity between the two layers. 7. Patches of skin which were grafted onto the ventral side of the body always degenerated. The ventral side of the body is perhaps an unfavorable place for grafts to receive nourishment. 8. The replacement of the epidermis of a graft by host epidermis is, essentially, a process of delayed wound healing. 9. In nearly all cases grafts. soon after transplantation, became reddened as the result of an enlargement of the dermal blood vessels of the graft. There is considerable evidence that the enlargement of the blood vessels is due to blood pressure which mechanically stretches the vessels in the absence of the normal tonus, the nerve connections having been cut at the time of transplantation. Later, and probably in consequence of restoration of nerve connections, the dermal vessels became reduced to normal proportions. 10. Radial arrangement of the epidermal melanophores occurs around homoiotransplants and skin wounds—that is, wherever there is translatory movement of the epidermis. In any migrating epidermis in which the melanophores are expanded, their long axes tend to become parallel to the direction of migration. The position of the individual melanophore, in radial or parallel alignment, is a consequence of the movement of the epidermal cells near a wound and is not due to independent orientation of the melanophores. The epidermal cells nearest the wound move first and most rapidly, therefore tending to rotate the elongated melanophores into the line of movement of the epidermis. Since the movement of the epidermis is centripetal, the melanophores become arranged radially around the point toward which the epidermis moves.