Macrophages direct process elongation from adult frog motorneurons in culture

Microglia and astrocytes may participate in the shaping of visual callosal projections during postnatal development

In the adult cat, axons running through the corpus callosum interconnect the border between the visual cortical areas 17 and 18 (A17 and A18) of both hemispheres. This specific pattern emerges during postnatal development, under normal viewing conditions (NR), from the elimination of initially exuberant callosal projections. In contrast, if the postnatal visual experience is monocular from birth (MD), juvenile callosal projections are stabilised throughout A17 and A18. The present study aimed at using such a model in vivo to find indications of a contribution of glial cells in the shaping of projections in the developing CNS through interactions with neurones, both in normal and pathological conditions. As a first stage, the distribution and the morphology of microglial cells and astrocytes were investigated from 2 weeks to adulthood. Microglial cells, stained with isolectin-B4, were clustered in the white matter below A17 and A18. Until one month, these clustered cells displayed an ameboid morphology in NR group, while they were more ramified in MD animals. Their phenotype thus depends on the postnatal visual experience, which indicates that microglial cells may interact with axons of visual neurones. It also suggests that they may differentially contribute to the elimination and the stabilisation of juvenile exuberant callosal fibres in NR and MD animals respectively. Beyond one month, microglial cells were very ramified in both experimental groups. Astrocytes were labelled with a GFAP-antibody. The distributions of connexins 43 (Cx43) and 30 (Cx30), the main proteic components of gap junction channels in astrocytes, were also investigated using specific antibodies. Both in NR and MD groups, until 1 month, GFAP-positive astrocytes and Cx43 were mainly localised within the subcortical white matter. Then GFAP, Cx43 and Cx30 stainings progressively appeared within the cortex, throughout A17 and A18 but with a differential laminar expression according to the age. Thus, the distributions of both astrocytes and connexins changed with age; however, the monocular occlusion had no visible effect. This suggests that astrocytes may contribute to the postnatal development of neuronal projections to the primary visual cortex, including visual callosal projections.

Regenerating descending axons preferentially reroute to the gray matter in the presence of a general macrophage/microglial reaction caudal to a spinal transection in adult zebrafish

We analyzed pathway choices of regenerating, mostly supraspinal, descending axons in the spinal cord of adult zebrafish and the cellular changes in the spinal cord caudal to a lesion site after complete spinal transection. Anterograde tracing (by application of the tracer rostral to the spinal lesion site) showed that significantly more descending axons (74%) regenerated in the spinal gray matter of the caudal spinal cord than would be expected from random growth. Retrograde tracing (by application of the tracer caudal to the spinal lesion site) showed that, rostral to the lesion, most of these axons (80%) extended into the major white matter tracts. Thus, ventral descending tracts often were devoid of labeled axons caudal to a spinal lesion but contained many axons rostral to the lesion in the same animals, indicating a pathway switch of descending axons from the white matter to the gray matter. Ascending axons of spinal neurons were not observed regrowing to the rostral tracer application site; therefore, they most likely did not contribute to the axonal populations analyzed. A macrophage/microglia response within 2 days of spinal cord transection, along with phagocytosis of myelin, was observed caudal to the transection by immunohistochemistry and electron microscopy. Nevertheless, caudal to the lesion, descending tracts in the white matter were filled with myelin debris during the time of axonal regrowth, at least up to 6 weeks postlesion. We suggest that the spontaneous regeneration of axons of supraspinal origin after spinal cord transection in adult zebrafish may be due in part to the axons' ability to negotiate novel pathways in the spinal cord gray matter.

A 25-year perspective of peripheral nerve surgery: evolving neuroscientific concepts and clinical significance

In spite of an enormous amount of new experimental laboratory data based on evolving neuroscientific concepts during the last 25 years peripheral nerve injuries still belong to the most challenging and difficult surgical reconstructive problems. Our understanding of biological mechanisms regulating posttraumatic nerve regeneration has increased substantially with respect to the role of neurotrophic and neurite-outgrowth promoting substances, but new molecular biological knowledge has so far gained very limited clinical applications. Techniques for clinical approximation of severed nerve ends have reached an optimal technical refinement and new concepts are needed to further increase the results from nerve repair. For bridging gaps in nerve continuity little has changed during the last 25 years. However, evolving principles for immunosuppression may open new perspectives regarding the use of nerve allografts, and various types of tissue engineering combined by bioartificial conduits may also be important. Posttraumatic functional reorganizations occurring in brain cortex are key phenomena explaining much of the inferior functional outcome following nerve repair, and increased knowledge regarding factors involved in brain plasticity may help to further improve the results. Implantation of microchips in the nervous system may provide a new interface between biology and technology and developing gene technology may introduce new possibilities in the manipulation of nerve degeneration and regeneration.

Modulation of macrophage and microglial responses to axonal injury in the peripheral and central nervous systems

OBJECTIVE

After axonal injury, macrophages rapidly infiltrate and become activated in the mammalian peripheral nervous system (PNS) but not the central nervous system (CNS). We used the dorsal root pathway to study factors that modulate the response of macrophages to degenerating axons in both the PNS and the CNS.

METHODS

Lewis rats underwent transection of dorsal roots (Group 1), stab within the spinal cord (Group II), crush at the dorsal root entry zone (Group III), transection of dorsal roots combined with a CNS lesion (Group IV), or systemic administration of a known activator of macrophages, lipopolysaccharide, alone (Group V) or combined with transection of dorsal roots (Group VI). ED-1 antibody stained for macrophages and activated microglia at 7, 14, and 42 days postinjury.

RESULTS

At early time points, Group I demonstrated ED-1 cells in the PNS but not the CNS portion of the degenerating dorsal roots. Group II revealed ED-1 cells near the stab lesion. Group III demonstrated ED-1 cells adjacent to the dorsal root entry zone crush site. Group IV revealed ED-1 cells along both the PNS and the CNS portions of the degenerating dorsal roots when the CNS lesion was placed near the transected roots. Group V demonstrated few ED-1 cells in the PNS and the CNS, whereas Group VI revealed a marked ED-1 cellular response along both the PNS and the CNS portions of the transected dorsal roots.

CONCLUSION

Local CNS trauma and systemic administration of lipopolysaccharide can "prime" macrophages/microglia, resulting in an enhanced response to degenerating axons in the CNS. Such priming might prove useful in promoting axonal regeneration.

Promotion of regeneration and axon growth following injury in an invertebrate nervous system by the use of three-dimensional collagen gels

We describe the application of three-dimensional collagen matrices to the study of nerve cord repair in the leech. Our experiments show that ganglia and connectives of the leech ventral nerve cord can be maintained for up to four weeks embedded in 3D gels constructed from mammalian type I collagen. Severed nerve cords embedded in the collagen gel reliably repaired within a few days of culture. The gel was penetrable by cells emigrating from the cut ends of nerves and connectives, and we consistently saw regenerative outgrowth of severed peripheral and central axons into the gel matrix. Thus, 3D gels provide an in vitro system in which we can reliably obtain repair of severed nerve cords in the dish, and visualize cell behaviour underlying regenerative growth at the damage site: and which offers the possibility of manipulating the regenerating cells and their extracellular environment in various ways at stages during repair. Using this system it should be possible to test the effect on the repair process of altering expression of selected genes in identified nerve cells.

Role of macrophages in the stimulation and regeneration of sensory nerves by transposed granulation tissue and temporal aspects of the response

Application of granulation tissue, which is rich in macrophages, to a peripheral nerve induces a conditioning effect, in that it enhances the regeneration capability of peripheral nerves after a test crush lesion. The temporal aspects of this response and the role of macrophages and interleukin-1 beta (IL-1 beta) were studied in the sciatic nerves of 71 rats. Granulation tissue was implanted close to the sciatic nerve and test crush lesions were applied after various periods of time (0-21 days). Regeneration was evaluated after an additional two, three, four, or six days. Regeneration distances were longer in granulation-treated nerves than in nerves treated with subcutaneous tissue. Furthermore, in animals in which the test crush lesion was made at the same time as the granulation tissue was implanted (n = 6), regeneration distances were longer, 8.1 (0.8) mm compared with 7.2 (0.6), than those in which the crush was made after conditioning intervals of 3 (n = 6, 7.6 (0.4) compared with 6.9 (0.4), p = 0.03); 7 (n = 6, 7.4 (0.4) compared with 6.6 (0.1), p = 0.03); and 21 days [(n = 8, 7.2 (0.6) compared with 6.4 (0.5)]. Inactivation of the granulation tissue by freezing suppressed the conditioning effect. There were numerous ED1 and ED2 positive macrophages as well as positive staining for IL-1 beta in the granulation tissue on day 0. Positive staining for IL-1 beta was also seen in nerve fibres as well as in non-neuronal cells after a conditioning interval. The results suggest that regeneration is stimulated by factors released from the cells of the granulation tissue, and that the amount of factors released or the responsiveness of the regenerating nerve change during the conditioning interval.

First appearance, distribution, and origin of macrophages in the early development of the avian central nervous system

A phagocytic cell system of hemopoietic origin exists in the early avian embryo (Cuadros, Coltey, Nieto, and Martin: Development 115:157-168, '92). In this study we investigated the presence of cells belonging to this system in the central nervous system (CNS) of chick and quail embryos by using both histochemical staining for acid phosphatase and immunolabelling with antibodies recognizing cells of quail hemangioblastic lineage. The origin of these cells was traced in interspecific chick-quail yolk sac chimeras. Hemopoietic cells were detected within the CNS from developmental stage HH15 on, and steadily increased in number at subsequent stages. Analysis of yolk sac chimeras revealed that most of these cells were of yolk sac origin, although some hemopoietic cells of intramebryonic origin were also found in the CNS. Immunocytochemical, histochemical, and ultrastructural characterization allowed us to identify hemopoietic cells in the CNS as macrophages. These cells were consistently found in the brain vesicles and spinal cord, appearing (1) between undifferentiated neuroepithelial cells at dorsal levels of the CNS; (2) in areas of cell death; (3) in the marginal layer in close relationship with developing axons; (4) in large extracellular spaces in the subventricular layer; (5) on vascular buds growing through the marginal and subventricular layers; and (6) in the ventricular lumen. Macrophages in different locations varied in morphology and ultrastructure, suggesting that in addition to their involvement in phagocytosis, they play a role in other processes in the developing CNS, such as axonal growth and vascular development. The first macrophages migrate to the CNS independently of its vascularization, apparently traversing the pial basal lamina to reach the nervous parenchyma. Other macrophages may enter the CNS together with vascular buds at subsequent stages during CNS vascularization.

Role of macrophages in peripheral nerve degeneration and repair

A cut or crush injury to a peripheral nerve results in the degeneration of that portion of the axon isolated from the cell body. The rapid degeneration of this distal segment was for many years believed to be a process intrinsic to the nerve. It was believed that Schwann cells both phagocytosed degenerating axons and myelin sheaths and also provided growth factors to promote regeneration of the damaged axons. In recent years, it has become apparent that the degenerating distal segment is invaded by monocytes from the blood. We will review the evidence that these recruited macrophages play a role in both degeneration and regeneration of peripheral nerve axons after injury and consider whether the slow degeneration and poor monocyte recruitment in the central nervous system may contribute to the poor regeneration there.

Regeneration in the Xenopus tadpole optic nerve is preceded by a massive macrophage/microglial response

Changes in the optic nerve following a crush lesion and during axonal regeneration have been studied in Xenopus tadpoles, using ultrastructural and immunohistological methods. Degeneration of both unmyelinated and myelinated axons is very rapid and leads to the formation, within 5 days, of a nerve which consists largely of degeneration debris and cells. Immunohistological analysis with monoclonal antibody 5F4 shows that there is a rapid and extensive microglial/macrophage response to crush of the nerve. Regenerating axons have begun to enter the distal stump by 5 days and grow along the outer part of the nerve in close approximation to the astrocytic glia limitans. Between 5 and 10 days after nerve crush, regenerating axons reach and pass the chiasma. Macrophages are seen in the nerve at the site of the lesion within 1 h, and the response peaks between 3-5 days, just before axonal regeneration gets under way.