Effects of colchicine, cytochalasin-B and papaverine on wound healing in Xenopus early embryos

Surface ectodermal wound healing in the chick embryo

Wound healing has been studied in the surface ectoderm overlying the midbrain region of stages 16-20 chick embryos by light microscopy, scanning and transmission electron microscopy, and immunofluorescent techniques. The embryos were divided into 6 groups, i.e. stages 16-17 for groups I, V and VI, and stages 19-20 for groups II, III and IV. For groups I and II embryos, a longitudinal incision about 0.6 mm was made close to the dorsal midline and the embryos incubated for varying periods of time up to 24 h. To determine the role of actin in the process of healing, selected groups I and II embryos were stained with FITC phalloidin and the wound margins examined using a confocal microscope. Wounds of all embryos in group I and about 20% in group II healed completely within 24 h of reincubation. The process of healing involved a change in the shapes of the ectodermal cells at the wound ends. This appeared as a zipping-up of the wound from both ends. In about 80% of group II embryos where healing did not occur, wound gaping was marked. Intense actin staining (actin cable) was observed at the wound margins of groups I and II embryos suggesting that the actin purse-string mechanism may play a role during wound healing in this epithelial model. The role of tension in wound healing was also determined by placing 2 secondary wounds about 0.5-0.7 mm long close to, and at right angles to the ends of the primary wound in groups III and V embryos. The procedure decreased the tension within the ectodermal cells at the wound ends. Groups IV and VI embryos served as controls for groups III and V embryos, respectively. Healing of both primary and secondary wounds after reduction of tension was rapid. Most primary wounds in group V embryos healed completely within 3 h of reincubation and the rate of reepithelialisation after the reduction of tension was about 160% more than that in group VI (control) embryos. Similarly, most primary wounds in group III embryos were almost closed within 6 h of reincubation. Here, the rate of reepithelialisation was 80 % more than that in group IV (controls). Thus tension is an important factor in wound healing in this model.

Studies on wound healing in the neuroepithelium of the chick embryo

Wound healing has been studied by light microscopy, SEM, and TEM in the neuroepithelium of the early neurula (stages 6 and 8) and advanced neurula (stages 10 and 12) chick embryos. Healing involves two major events: (1) apposition of the wound edges and (2) restitution of the neuroepithelium at the wound site (i.e., restoration of the epithelial integrity of neuroepithelium). Apposition of the wound edges occurs within the first 15 minutes of re-incubation and involves the entire length of the wound. The main event during restoration is a change in the shapes of the rounded cells to elongated forms (i.e., spindle, wedge, and inverted wedge shapes). Wounds of younger embryos heal faster than those of older ones.

An endogenous sodium current may mediate wound healing in Xenopus neurulae

We have studied healing of Xenopus neurulae in order to investigate the role of endogenous sodium-carried electric currents in wound closure. Transected embryos healed completely within 7 hr in an artificial pond water medium. Wound closure was biphasic, with a rapid purse string-like contraction that was independent of sodium concentration followed by a slower sodium-dependent phase. The initial contraction was reversibly inhibited by cytochalasin B. Healing was prevented when neurulae were wounded and left to heal in sodium-free medium. Healing was also prevented in the presence of amiloride, benzamil, or ouabain, drugs that inhibit sodium flux through the epithelium. The transepithelial potential measured in intact neurulae fell rapidly and reversibly by 70% in response to 10 microM amiloride. Currents measured leaving the wound also decreased by 70% following amiloride addition. Our results indicate that at least one phase of wound healing in Xenopus neurulae is dependent upon an endogenous sodium-carried electric current. We hypothesize that the current may act by guiding epithelial cells to the wound site.

A reexamination of the role of microfilaments in neurulation in the chick embryo

Formation of wedge-shaped neuroepithelial cells, owing to the constriction of apical bands of microfilaments, is widely believed to play a major part in bending of the neural plate. Although cell "wedging" occurs during neurulation, its exact role in bending is unknown. Likewise, although microfilament bands occupy the apices of neuroepithelial cells, whether these structures are required for cell wedging is unknown. Finally, although it is known that cytochalasins interfere with neurulation, it is unknown whether they block shaping or furrowing of the neural plate, or elevation, convergence, or fusion of the neural folds. The purpose of this study was to reexamine the role of microfilaments in neurulation in the chick embryo. Embryos were treated with cytochalasin D (CD) to depolymerize microfilaments and were analyzed 4-24 hr later. CD did not prevent neural plate shaping, median neural plate furrowing, wedging of median neuroepithelial cells, or neural fold elevation. However, dorsolateral neural plate furrowing, wedging of dorsolateral neuroepithelial cells, and convergence of the neural folds were blocked frequently by CD. In addition, neural folds always failed to fuse across the midline in embryos treated with CD, and neural crest cell migration was prevented. These data indicate that only the later aspects of neurulation may require microfilaments, and that certain neuroepithelial cells, particularly those that normally wedge with median furrowing and elevation of the neural folds, become (and remain) wedge-shaped in the absence of apical microfilament bands. Thus, microfilament-mediated constriction of neuroepithelial cell apices is not the major force for median neuroepithelial cell wedging and elevation of the chick neural plate. Further studies are needed to localize the motor(s) for these processes.

Epidermal cell migration during wound healing in Dugesia lugubris. Observations based on scanning electron microscopy and treatment with cytochalasin

The epidermal cells that migrate over the surface during the wound closure stage of head regeneration in Dugesia lugubris s.l. were examined by scanning electron microscopy. The effect of cytochalasin B on epidermal cell migration was also examined. During the first few hours after decapitation epidermal cells at the edges of the wound showed significant changes of shape related to the process of migration that was accomplished approximately 10 h after wounding. Flattening of the marginal cells was associated with active epidermal spreading throughout the healing period. Suitable support for migrating cells appeared to be a rhabditic network attached to the wound tissue. Epidermal cell migration was inhibited by cytochalasin B. These results demonstrate that the basis for cell movement in planarians is similar to that of many other systems.

Calcium, microfilaments and morphogenesis

Morphogenesis, the generation of tissue form, is important not only in the embryogenesis of a new individual, but also because a change in morphogenesis may be involved in the establishment of differences between individuals during evolution. Morphogenetic movements are effected in part by coordinated changes in the shapes of individual cells and over the past decade the cellular organelles responsible for cell shape have been identified as microfilaments and microtubules. In non-embryonic systems the contraction of microfilaments is controlled by the level of intracellular free calcium, and so calcium is implicated as an intermediate control mechanism in morphogenisis. Through techniques which perturb the calcium balance of cells, or which measure calcium ion concentration directly, evidence is accumulating that calcium is involved in morphogenetic movements such as gastrulation and neurulation, and related phenomena such as wound healing. Thus fundamental questions about the control of morphogenesis in embryogenesis and evolution may now be couched in more precise terms of the control of intracellular calcium ion balance.