Participation of human superficialis flexor tendon segments in repair in vitro

We analyzed the healing capability of the human superficialis (sublimus) flexor tendon in response to injury in vitro by performing a morphologic study of tendon repair. Tendon segments were cultured in cell-free medium for 2, 4, or 8 weeks, and the cut-end repair sites were compared with those in fresh control segments on light and electron microscopy. Tendon encapsulation had occurred at 2 weeks, by the proliferation and migration of elongated fibroblasts from the epitenon and from collagen bundles of the endotenon to cover the surface of the cut tendon ends. As migrating epitenon cells approached the cut margin, they consistently shifted from a parallel to a circumferential orientation with respect to the tendon axis. By 4 weeks, the encapsulating surface cells had lost their membranous ruffles and had become flattened. Within the capsule, phagocytic cells engulfed collagen debris, and fibroblasts containing extensive rough endoplasmic reticulum profiles and secretory vesicles participated in protein synthesis. The surface of the capsule was smooth and, at 8 weeks, was almost indistinguishable from the epitenon surface of uncultured controls. The capsule now had a collagen-synthesizing layer of fibroblasts superimposed on layers of maturing collagen bundles that separated resting-phase fibroblasts. These findings demonstrate that segments of human superficialis flexor tendons participate in an intrinsic repair response without contributions from extrinsic cells.