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Immunohistochemical staining of Thy-1 antigen was studied in rat peripheral nerves shortly after a complete crush lesion and after an epineural depot injection of lysophosphatidylcholine-palmitate (lysoPC). The Thy-1 antigen was expressed in endoneurial capillaries after 20-30 h and was visible previous to alterations in the axolemma or other signs of neural damage with the exception of endoneurial oedema formation. Thy-1 positive capillaries were restricted to the vicinity of degenerating nerve fibres in partially damaged nerves following epineural depot injection of lysoPC. It is therefore concluded that expression of the Thy-1 antigen in response to peripheral nerve damage in reactive endoneurial capillaries is an active process and not due to diffusion from degenerating axolemmata. Thy-1 staining could be used as a marker of reactive endoneurial capillaries in pathological conditions of peripheral nerves.

Sciatic nerves undergoing Wallerian degeneration are subject to massive invasion by monocytes bearing Fc receptors. The present experiments were done to explore the potential role of the Fc receptors. The effect of suppressed immunoglobulin levels on the rate of Wallerian degeneration was studied in the n. medioulnaris of the chick and in murine sciatic nerve. In the avian model, total immunoglobulin production was eliminated by bursectomy. The mice were injected after birth with antibodies against IgM, causing a selective reduction of immunoglobulin M levels. Myelin degeneration was measured in both groups by computer-assisted line sampling and corrected for intrafascicular edema. In both models, there were only minor differences with the controls. Immunoglobulins, accordingly, do not seem to play a significant role in the efficiency of myelin phagocytosis by monocytes during Wallerian degeneration. The study also includes data on the degree and the distribution of edema in nerves undergoing Wallerian degeneration.

Changes of myelin proteins in mouse sciatic nerves were studied comparing nerves degenerating in situ with nerves enclosed in millipore diffusion chambers which eliminate invasion of non-resident cells. Nerves kept in chambers showed nearly complete preservation of myelin sheaths with a very slow degradation of myelin proteins. Nerves degenerating in situ showed rapid myelin phagocytosis by macrophages with almost complete disappearance of myelin proteins after 28 days. These data elucidate the role of macrophages for removal of myelin proteins.

Previous experiments with nerves enclosed in millipore diffusion chambers had shown that myelin degradation during Wallerian degeneration depends on invasion by non-resident cells. The present study was aimed at a more precise identification of the invading cell population. Monoclonal antibody studies of degenerating nerves showed many cells with the Fc marker; cells having the Lyt-1, Lyt-2, Ia or Mac-1 markers were sparse or absent. Nerves transplanted into mice of the Chediak-Higashi bg/bg strain were invaded by cells lacking the bg/bg marker (giant lysosomes), while cotransplanted muscle tissue was invaded by cells with the bg/bg marker. Blocking monocytes with silica reduced both cell invasion and myelin degradation in degenerating nerves. These observations show that Wallerian degeneration of peripheral nerve fibers involves a subset of monocytes which are silica-sensitive and have Fc receptors but no bg/bg giant lysosomes.

Wallerian degeneration was studied in the phrenic or sciatic nerves of mice following transplantation into Millipore diffusion chambers of 0.22 micron pore size which were implanted in the peritoneal cavity and kept for up to eight weeks. This method positively eliminates the access of nonresident cells to the tissue, at the same time providing proper conditions for tissue survival. Such nerves showed no proliferation of Schwann cells and no evidence for their active role in the removal or digestion of myelin. Schwann cells rejected their sheaths and the latter persisted for weeks, leading either to sheath distension (the sheath becoming wider and thinner) or to collapse (the sheath becoming thicker, collapsing upon the empty axis cylinder). The outer envelope of Schwann cytoplasm separated into pseudopodia rich in microtubules. Sheath rejection led to a slow decay of the myelin in the absence of active phagocytosis. There was profuse fibroblastic proliferation from the epineurium and perineurium, from which cells migrated into the chambers developing fatty change. No evidence was found to link the fatty change in fibroblasts to sheath decay. Diffusion chambers of 5.0 micron pore size were invaded by leukocytes and monocytes. Nerves kept in such chambers showed active phagocytosis of myelin leading to its removal, similar to Wallerian degeneration in situ. Phagocytes were shown to attack selectively the rejected myelin sheaths, distinguishing the latter from the surviving Schwann cells, even though both structures derive from the same cell. The activity of phagocytes in digesting myelin was mediated by a signal which diminished in intensity with time; there was very little active phagocytosis of myelin in nerves that had been predegenerated in 0.22 micron pore chambers. Various modifications of the experiment, including studies with co-cultured peritoneal macrophages or bone marrow, indicate a need for additional activating factors to induce myelin phagocytosis.