Wound Healing Insights from Flies and Fish

Abstract

All organisms from single-cell amoebae through to Homo sapiens have evolved strategies for repairing wounds as an essential homeostatic mechanism for rebuilding their outer barrier layers after damage. In multicellular animals, this outer barrier layer is the skin, and, for more than a century, scientists have been attempting to unravel the mechanisms underpinning skin repair because of its clear clinical relevance to pathologies that range from chronic nonhealing wounds, through to excessive scarring. Most of these studies have been in rabbits and rodents, or in in vitro scratch wound models, but in the last decades, two newcomer model organisms to wound healing studies-flies and fish-have brought genetic tractability and unparalleled opportunities for live imaging to the field. These two models are complementary to one another, and to mouse and in vitro approaches, and thus offer different insights into various aspects of the wound repair process.

Figure 1.

Schematic of a typical mammalian wound in the early stages of repair, with a clot plugging the wound gap, epidermal tongues migrating down between clot and healthy granulation tissue, and connective tissue fibroblasts invading the wound and beginning to lay down scar collagen. Inflammatory cells have also arrived at the wound site (many having extravasated from nearby vessels), and are orchestrating other aspects of the repair process, including sprouting of further vessels to drive wound angiogenesis.

Figure 2.

Drosophila as a model of wound healing. Schematics to show the typical wound models in embryonic (Ai), larval (Bi), and pupal (Ci) Drosophila stages. Embryos are generally wounded with a focused laser beam and have been used to live image the earliest calcium damage “flash” (Aii), and how the simple epithelium seals by means of an actomyosin purse string (arrows), zippering filopodia (arrowhead), and lamellae (asterisk) (Aiii), and also as a very simple model of inflammation (Aiv) where the dashed line indicates the wound edge. Larvae are less amenable to whole organism live cell imaging but can be “pinch” wounded, and their imaginal discs can be wounded and cultured in vitro (Bi). Embryo and larval tracking/imaging studies reflected in a schematic with cell rows indicated in different colors to show their movements as wound closure progresses. The contractile actin cable (red) constricts the leading edges of front row (yellow) cells so that some fall back from the front row. In rows further back, there is considerable fluidity and shuffling of cells to release tension (Bii). Drosophila pupae have recently been developed as a wound model and have revealed fat body cells (orange) within the pupal tissues (Cii) and how these cells (green) migrate to seal the epithelial (red) gap; wound indicated by arrowheads (Ciii). Pupae also enable imaging of the extravasation step of inflammation by correlative light microscopy (red hemocytes) (Civ) and transmission electron microscopy (pink hemocytes); h-ex and h-w are hemocytes in the act of extravasating from the wing vein versus those that have already migrated to the wound (Cv). (Images in Aii–iii and iv are courtesy of William Razzell and Helen Weavers, respectively. Schematic Bi is adapted from Razzell et al. 2014 and Tetley et al. 2019. Images in Cii–iii and iv–v are courtesy of Anna Franz and Leila Thuma, respectively.)

Figure 3.

Zebrafish as a model of wound healing. Schematics to show the typical wound models in larval (Ai) and adult (Bi) zebrafish. Aii and iii show the rapid and sustained recruitment of fluorescently tagged neutrophils and macrophages to larval needlestick and tail amputation wounds. Aiv is a time-lapse series of collagen deposition after wounding a collagen:GFP larva. Scale bar, 15 μm. Adult fish have proven to be a good model for investigating scarring because they both lay down a collagenous scar (Bii), and subsequently resolve it. They are less amenable to live imaging, but multiphoton microscopy enables viewing of the wound angiogenic response to a wound or foreign body (Biii). (Images in Aii, iii, and iv are courtesy of Anne George, Luke Deane, and Josie Morris, respectively. Images in Bii and iii are courtesy of Rebecca Richardson and David Gurevich, respectively.)