THE DYNAMICS OF WOUND CLOSURE AND ITS ROLE IN THE PROGRAMMING OF PLANARIAN REGENERATION I—BLASTEMA EMERGENCE

Observations in vivo show that the edges of the wound are brought into close contact by muscle contraction and fuse by first intention immediately after transaction. The wound epithelium forms later by the stretching of the epidermal cells when the muscles relax.
Dorsal and ventral half-thickness fragments were associated in vitro by their anterior or posterior edges. The epidermis only fuses by first intention when the free borders are pressed into close contact. Blastemas of various localizations and sizes are formed from the suture between dorsal and ventral epidermis, in those places where it has been stretched. The opposing forces which cause the stretching are particularly due to the rolling-up of the fragments or to their relaxation after they have been forced to fuse.
Contrary to what was previously assumed, the simple fusion of dorsal and ventral epidermis is not sufficient to initiate blastema emergence. The need for stretching may be explained by the fact that certain epidermal cells are brought close to tissues of the opposite side, forming a transitional epidermis analogous to one edge. As a result of the formation of this distal level close to transection, intercalary regeneration would ensue, whose first step would be blastema emergence.