AbstractThe concept that flexor tendons have the intrinsic capacity to participate actively in the repair process following laceration has been developed in recent years as the result of experimental studies from numerous laboratories. However, the role of the outer/epitenon and inner/endotenon cell populations with regard to protein synthesis is still controversial. The purpose of this study was to investigate the respective participation of these tendon fibroblast populations in the synthesis of the collagen matrix during in vitro repair of the flexor tendon from nonhuman primates, utilizing immunohistochemical techniques and a Type I procollagen antibody. Zone II profundus flexor tendon segments were obtained from young adultMacaca nemestrinamonkeys. One centimeter segments were cultured either with or without a transverse laceration across 90% of the midsection of the tendon segment. Frozen sections of the cultured tendon segments were reacted with the (mouse monoclonal) Type I procollagen antisera. At all times of culture of the nonlacerated tendon segment, only a few of the epitenon cells along the surface of the tendon and distant from the cut end stained positively for active collagen synthesis. In the lacerated segments, and as early as 9 days of culture and repair, the procollagen reaction product was starting to appear in those cells of the outer epitenon cell layer. These studies support the concept that the inner fibroblasts do actively participate in collagen production. However, it now also appears that a significant degree of collagen synthesis during tendon repair resides in the outer/epitenon layer of cells enveloping the tendon segment, and that the repair response of the flexor tendon in vitro is proportional to the degree of injury.