FIG. 4.

Representative histological images for H&E, Masson's Trichrome, and Picrosirius Red. (A) Representative H&E stained histological images (Row 1; 20 × magnification; scale bar 50 μm; n = 4). Only the bioprinted skin group had developed an epidermis. Bioprinted skin-treated groups appear to have a dermis structure similar to healthy skin (thicker bundles, basket weave organization), while hydrogel only and untreated wounds had sparse or incomplete epidermis and a thick unorganized dermis. Representative Trichrome staining of tissues (Row 2; 20 × magnification; scale bar 50 μm; n = 4). Bioprinted skin showed the most similar staining intensity for keratin (red) and collagen (blue), compared to healthy skin. Hydrogel only treated wounds had some staining for keratin (red) and sparse dermal collagen staining (blue). Untreated wounds were highly disorganized, highly cellular (brown), and were not representative of healing skin. Representative Picrosirius Red staining of tissues viewed with polarizing light (Row 3; 20 × magnification; scale bar 50 μm; n = 4). Healthy skin is dominated by red mature collagen type I in a basket weave orientation. Bioprinted skin appeared most like healthy skin with this basket weave organized collagen fibers, while control hydrogel treated wounds had thick orange fibers organized in parallel, typical of scar formation. Untreated wounds appeared to show increased levels of immature green collagen type III fibers and were also organized more in parallel. (B) Summary table of histological findings. (C) MATLAB was used to quantify the relative percentage of each collagen color type by dividing the pixel count of each color by the total pixel count for each image, shown here as the average percentage for each treatment group (n = 4). Color images are available online.