The microscopic morphology of orthotopic free muscle grafts in rabbits.

Abstract

The flexor digitorum superficialis muscle was free grafted (without neurovascular anastomoses) in 122 rabbit forelimbs. Histologic nd histochemical examinations through 6 months after grafting were performed. An early ischemic necrosis of the entire graft, except for a few percent of fibers at the very surface, was consistently seen. Subsequently, there occurred a regeneration of muscle with reconstitution of up to 100 percent of normal numbers of fibers. There was a wide variation in the numbers of fibers regenerated; however, the fiber-free areas were then being replaced by connective tissue. Muscle grafts in 1-month-old rabbits regenerated faster and yielded muscle with evidence of more extensive reinnervation and less connective tissue than 3-month-old animals. In the early postgraft period, minced grafts appeared to be as good as whole ones, but after 1 month, they developed far more connective tissue. Differentiation into fast-twitch and slow-twitch muscle fibers and into high- and low-oxidative fibers began at 2 to 3 weeks after grafting but was not extensive until 3 months. At 6 months, grafts showed areas of normal-appearing muscle interspersed with areas that lacked signs of reinnervation. The earliest sign of regeneration is the appearance of several very elongated nuclei encircling each previously anucleate necrotic muscle fiber. A small amount of basophilic cytoplasm then appears around each new nucleus. As blood vessels grow into the graft, a centripetal wave of phagocytosis is seen, taking 2 to 3 weeks and leaving a bed of immature muscle fibers. We believe this to be the first documentation of regeneration's commencing prior to and thus independently of phagocytosis.