THE DYNAMICS OF WOUND CLOSURE AND ITS ROLE IN THE PROGRAMMING OF PLANARIAN REGENERATION. II — DISTALIZATION

When the dorsal and ventral epidermal layers join by first intention during the closure of the wound, the cells of their borders (M-cells) do not meet in the same manner in all sections. In anterior sections the dorsal M-cells attach themselves to the ventral basement membrane, so that only the dorsal epidermis is stretched. In posterior sections the dorsal and the ventral M-cells join by their apical edges without being closely apposed to the wound surface. Only the ventral cells are stretched because of their specific motility. In longitudinal sections the dorsal and the ventral M-cells also join by their apical edges, but since they are closely apposed to the wound surface both epidermal layers are stretched. The stretching is a process equivalent to distalization. The junction between the dorsal and the ventral epidermis is shifted ventrally in the anterior wounds (as in the intact heads) and dorsally in the posterior wounds (as in the intact tails). Some abnormalities of wound closure have been observed at levels where heteromorphic regeneration frequently occurs. These findings are consistent with the hypothesis previously advanced (3) that the modalities of wound closure establish the programme for regeneration.     

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.     

Plastid terminal oxidase (PTOX) has the potential to act as a safety valve for excess excitation energy in the alpine plant species Ranunculus glacialis L.

Ranunculus glacialis leaves were tested for their plastid terminal oxidase (PTOX) content and electron flow to photorespiration and to alternative acceptors. In shade‐leaves, the PTOX and NAD(P)H dehydrogenase (NDH) content were markedly lower than in sun‐leaves. Carbon assimilation/light and C_i response curves were not different in sun‐ and shade‐leaves, but photosynthetic capacity was the highest in sun‐leaves. Based on calculation of the apparent specificity factor of ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco), the magnitude of alternative electron flow unrelated to carboxylation and oxygenation of Rubisco correlated to the PTOX content in sun‐, shade‐ and growth chamber‐leaves. Similarly, fluorescence induction kinetics indicated more complete and more rapid reoxidation of the plastoquinone (PQ) pool in sun‐ than in shade‐leaves. Blocking electron flow to assimilation, photorespiration and the Mehler reaction with appropriate inhibitors showed that sun‐leaves were able to maintain higher electron flow and PQ oxidation. The results suggest that PTOX can act as a safety valve in R. glacialis leaves under conditions where incident photon flux density (PFD) exceeds the growth PFD and under conditions where the plastoquinone pool is highly reduced. Such conditions can occur frequently in alpine climates due to rapid light and temperature changes.