Fig. 4

Representative histological images of subcutaneous injection sites in mice (hematoxylin and eosin stain). a-c show the skin and subcutis of a control animal at 7 days revealing the epidermis (arrow a), hair follicles (arrowhead a) and striated muscle (panniculus carnosus; asterisk a and b). The panniculus carnosus (asterisk c) and an occasional mastcell (arrowhead c) are visible in the subcutis. d-f show the skin 7 days after injection of the hydrogel. The hydrogel is visible as a grey, granular substance (white arrow e) positioned below the panniculus carnosus (arrowhead e) surrounded by a capsule of loosely arranged fibroblasts (asterisk e). Multifocally infiltrates of eosinophils (white arrow f), neutrophils (arrowhead f) and macrophages (black arrow f) separating the hydrogel from the fibrous capsule. g-i show the skin of a control animal at 28 days after injection without significant pathological changes. j-l depict the histological changes in the subcutis 28 days after injection of the hydrogel with the epidermis and panniculus carnosus indicated by an arrow and arrowhead respectively in j. k shows a more compact fibrous capsule (arrow) compared to the loose capsule seen after 7 days postinjection. The cellular reaction directly surrounding the hydrogel show a more granulomatous nature indicated by the presence of macrophages often containing brown pigment l