Structural Features of Connective Tissue Formed around Resin Implants Subcutaneously Embedded in Dairy Cows

Foreign body reactions (FBRs) are inadvertently observed in invading or artificially embedded materials, triggering inflammation and subsequent fibrotic processes to occur in situ. Here, we assessed the spatiotemporal formation of connective tissue around implanted materials to establish a technique using connective tissue formed by FBRs as xenografts. An acrylic resin implant, comprising a columnar inner rod and a tubular outer cylinder (OC) with several slits, was embedded in adult dairy cows. Tissues formed in the inner rod and OC groups were histologically analyzed at weeks 2, 4, 8, and 12. Edematous tissues with non-collagenous fibers formed for 2 weeks and showed increased cellularity after 4 weeks. The weight, thickness, amounts of total protein, collagen, DNA, and quantitative scores of α-smooth muscle actin-positive myofibroblasts or elastic fibers notably increased after 8 weeks, with condensed collagen fibers showing orientation. Inflammatory cells were primarily localized in tissues close to the OC, and their numbers increased, with the count of CD204+ cells peaking at 8 weeks and declining at 12 weeks. The count of Ki67+ proliferating cells slightly increased in tissues close to the OC; however, the number and lumen of CD31+ vessels increased. These results may help understand FBR-related tissue remodeling.