Ultrastructural studies on migrating epidermal cells during the wound healing stage of regeneration in the adult newt, Notophthalmus viridescens

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Abstract
The ultrastructure of the epidermal cells which migrate over the wound surface of the amputated limb of the adult newt, Notophthalmus viridescens, was observed with transmission (TEM) and scanning (SEM) electron microscopy. In order to aid in the visualization of polyanionic surface materials on the wound epithelium and wound surface with TEM, the basic dye, ruthenium red, was introduced into the fixatives and buffer. Control limbs were processed without ruthenium red. Shortly after amputation, basal cells at the wound margin possessed elongated, flattened profiles with long pseudopodial projections (lamellipodia and filopodia) that appeared to make contact with the fibrin exudate covering the stump tissues. Epidermal cells proximal to the site of amputation were also in a state of mobilization. Large intercellular spaces and a reduction in the number of desmosomes were observed in the migrating cells. Epidermal cell nuclei became characteristically euchromatic with well‐developed nucleoli. Microfilaments were seen within the cytoplasm, extending toward the plasma membrane of cellular processes. Phagocytosed material was also present in the migrating cells. By approximately 9 hours post‐amputation, wound closure was complete, and the wound epithelium consisted of three to four cell layers of a non‐cornified epidermis. Generally, the amount of extracellular material present on the surface and in the enlarged intercellular spaces of migrating epidermal cells remained the same throughout the period of wound closure. A layer of polyanionic material was observed consistently over the fibrin meshwork covering the wound surface with TEM.