Immunohistochemistry of the Enthesis Organ of the Human Achilles Tendon

Background: The insertional region of the Achilles tendon includes: the fibrocartilage of the enthesis at the tendon-bone junction, the sesamoid fibrocartilage in the deep surface of the tendon, and the periosteal fibrocartilage covering the superior tuberosity of the calcaneus. The mechanism by which compressive stress and low oxygen concentration modulate the phenotypical expression of cartilaginous cells in mesenchymal tissue is still unknown. Tenascin-C isoform, with its anti-adhesive properties, has been hypothesized to have a role in cellular adaptation to compression. The purpose of this study was to define the immunohistochemical localization of tenascin-C in the enthesis organ of the Achilles insertion in man. Methods: Five specimens of Achilles tendon and its insertion were obtained from adult patients during below-knee amputation surgery. To determine the histochemistry, for light microscopy, specimens were stained with hematoxylin-eosin, safranin-O, Verhoeff, alcian blue methods. For immunohistochemistry, the following antibodies were used: anti-S-100 protein; anti-chondroitin sulfate; anti-types I and II collagen, anti-type IV collagen, anti-type X collagen, and anti-tenascin-C. Results: The enthesis, sesamoid, and periosteal fibrocartilages have been defined by a separate region of immunohistochemical labeling for S-100 antibody and type II collagen and chondroitin-sulfate. The sesamoid and periosteal fibrocartilages stained for tenascin-C, which in cartilaginous areas was closely associated with rounded cells in the extracellular matrix. Conclusion: By showing a relationship between chondrocyte-like cells and immunolabeling for tenascin-C in periosteal and sesamoid fibrocartilage, the present data suggest a role in the dynamic equilibrium in the formation of fibrocartilaginous foci in tendon areas exposed to compressive stress. Clinical relevance: The data support the surgical practice of the resection of the diseased tendinous portion in case of chronic Achilles tendinitis. The decompression removed the enthesis organ that in an inflammatory environment could be altered and would not contribute to the distribution of the tensive load. After the decompression, it was hypothesized that in tendon areas exposed to compressive stress there was restoration of the fibrocartilaginous foci that constitute the enthesis organ.