Post-injury hydraulic fracturing drives fissure formation in the zebrafish basal epidermal cell layer

Summary The skin epithelium acts as the barrier between an organism’s internal and external environments. In zebrafish and other freshwater organisms, this barrier function requires withstanding a large osmotic pressure differential. Wounds breach this epithelium, causing a large disruption to the tissue microenvironment due to the mixing of isotonic interstitial fluid with the external hypotonic fresh water. Here we show that, following acute injury, the larval zebrafish epidermis undergoes a dramatic fissuring process that resembles hydraulic fracturing, driven by the influx of external fluid. The fissuring starts in the basal epidermal layer nearest to the wound, and then propagates at a constant rate through the tissue spanning over one hundred micrometers; during this process the outermost superficial epidermal layer remains intact. Fissuring is completely inhibited when larvae are wounded in an isotonic external media, suggesting that osmotic pressure gradients drive fissure. Additionally, fissuring partially depends on myosin II activity, as its inhibition reduces fissure propagation away from the wound. During and after fissuring, the basal layer forms large macropinosomes (with cross-sectional areas ranging from 1-10 µm 2 ), presumably to clear the excess fluid. We conclude that excess external fluid entry through the wound and subsequent closure of the wound through actomyosin purse string contraction in the superficial cell layer causes fluid pressure buildup in the extracellular space of the zebrafish epidermis. This excess fluid pressure causes tissue to fissure, and eventually the fluid is cleared through macropinocytosis.