HPC-7: A novel oligodendrocyte lineage protein which appears prior to galactocerebroside

The identification of cell type-specific molecules expressed at different developmental stages can help to elucidate the regulatory mechanisms governing the survival, differentiation, and development of cells in the central nervous system (CNS). A cell surface protein, HPC-7, was detected on rat oligodendrocytes (OL) in culture by a monoclonal antibody generated against adult rat hippocampal membranes. Adult rat brain and sciatic nerve sections showed selective labeling of white matter and other myelinated fibers in both the CNS and peripheral nervous system (PNS). Double-labeling of secondary cultures of OL, O-2A, and type-2 astrocytes and primary cultures of type-1 astrocytes with independent cell type-specific antibodies confirmed that HPC-7 was expressed only by the OL lineage. By using a series of OL stage-specific antigenic markers (A2B5, O4, OL-1, galactocerebroside, myelin basic protein) HPC-7 was found to appear at the time when OL precursors became A2B5 negative and began their terminal differentiation in OL. On immunoblots, anti-HPC-7 antibody recognized a single 66 kDa band in rat OL and a single band at 100 kDa in adult myelin. N-glycosidase treatment showed that the HPC-7 protein did not contain substantial amounts of N-linked carbohydrate. Thus, HPC-7 appears to be a cell surface protein of the OL lineage that marks the important transition from proliferative precursor to postmitotic OL. GLIA 23:169–179, 1998. © 1998 Wiley-Liss, Inc.     

White matter ultrastructural pathology of experimental Creutzfeldt-Jakob disease in mice

Summary Creutzfeldt-Jakob disease (CJD), previously regarded as a neurodegenerative disorder strictly of the gray matter, occasionally occurs as a panencephalopathic form which is characterized by severe white matter damage. An ultrastructural study of the white matter pathology in mice experimentally infected with the Fujisaki strain of CJD virus revealed: (1) vacuoles within myelin sheaths, formed by splitting either at the major dense or intraperiod lines, or within axons; (2) macrophages filled with numerous myelin figures, lipid droplets and paracrystalline inclusions; (3) astrocytes actively digesting myelin debris; (4) unusual wrapping of several axons by a common myelin sheath; (5) vesicular degeneration of myelin sheaths; (6) close contact between numerous coated pits and outer myelin lamellae; and (7) proliferation of inner mesaxons. Our data indicate that the damage to myelinated axons in the panencephalopathic type of CJD is accomplished primarily by active degradation of myelin by macrophages and astrocytes and by formation of intra-axonal and intra-myelin vacuoles. The myelin vacuolation is most consistent with that produced by leukolysins released from activated macrophages and astrocytes.Presented in part at the 40th annual meeting of the American Academy of Neurology, held in Cincinnati, Ohio, April 17–23, 1988. Dr. Pawel P. Liberski is a recipient of a fellowship from the Fogarty International Center and a grant from the Polish Academy of Sciences (771-VI)     

The pathology of multiple sclerosis is location-dependent: no significant complement activation is detected in purely cortical lesions

Complement activation is known to occur in white matter multiple sclerosis (MS) lesions. It is thought to mediate oligodendrocyte/myelin damage and to be a marker of pathologic heterogeneity among individuals. Less is known about complement deposition in the gray matter in MS. The aim of this study was to characterize the presence and distribution of complement activation products in cortical MS lesions. Immunohistochemical staining was performed on cryostat sections from the brains of 22 MS patients and 5 nonneurologic control patients obtained at autopsy. Deposition of the complement activation products C1q, C3d, and C5b-9 (membrane attack complex) was detected on and within macrophages/microglia and astrocytes and in blood vessel walls in white matter MS lesions. C3d and C4d were detected along myelin sheaths at the edge of the lesions. In the gray matter part of combined gray matter/white matter lesions complement activation was less frequent, but increased immunopositivity was detected for C3d on blood vessels, and for C3d and C4d on myelin at the border of lesions, when compared with control areas. In contrast, in the purely cortical lesions, the extent of complement deposition in general was low. In conclusion, the role of complement in MS pathogenesis seems lesion location-dependent.     

IMMUNOCYTOCHEMICAL CHARACTERIZATION OF PRIMARY GLIAL CELL CULTURES FROM NORMAL ADULT HUMAN BRAIN

Primary cultures were established from autopsy or biopsy samples of normal adult human brain and characterized by immunocytochemical techniques. Initially, macrophages were the predominant cell type adhering to the substratum, but as their number fell that of glial cells increased. Oligodendrocytes comprised 30% of the glial population in white matter cultures, and their perikarya and elongated processes were immunostained with antibodies directed against galactocerebroside and four myelin proteins. In white and grey matter cultures, process-bearing astrocytes and small numbers of polygonal astrocytes were stained with antibodies against glial fibrillary acidic protein and glutamine synthetase. Fibroblasts started to appear at 3 weeks and proliferated to form a monolayer beneath glial cells by 5 weeks. Glia began to die in the 6th week. These primary cell cultures of white or grey matter can be used to study the properties of glial cells from normal or pathological adult human brain.     

Monoclonal antibody 14E identifies the oligodendrocyte cell body in normal adult human and rat white matter

A monoclonal antibody, 14E, which detects immunocytochemically the perikarya of interfascicular oligodendrocytes in sections of normal adult human and rat white matter, was obtained using isolated normal human oligodendrocytes as the immunogen. 14E is an IgM antibody that reacts with a polypeptide of approximate molecular weight 23,000 on immunoblots of the particulate fraction of normal white matter. In sections of active multiple sclerosis plaques and recent infarcts, 14E immunostained the cell bodies and processes of only a small number of hypertrophic glial cells which could be oligodendrocytes undergoing pathological changes or a subpopulation of reactive astrocytes.     

Immunocytochemical localization of 5'-nucleotidase in oligodendroglia and myelinated fibers in the central nervous system of adult and young rats

Rat central nervous system (CNS) tissue sections were immunostained by the peroxidase-anti-peroxidase (PAP) method using a rabbit serum directed against rat liver 5'-nucleotidase. In paraffin sections from the brains of 60-day-old rats 5'-nucleotidase immunoreactivity occurred in the same white-matter regions as myelin-basic protein immunoreactivity and histological staining of myelin. The immunostaining of cerebral white matter for 5'-nucleotidase was more intense and wide-spread at the age of 120 days than at 60 days, and the choroid plexus and blood vessels were stained consistently. In the paraffin sections from the brains of younger (20-day-old) rats the staining of 5'-nucleotidase in the white matter was faint and patchy. In paraffin sections from spinal cord, 5'-nucleotidase immunoreactivity was observed throughout the lateral white-matter columns and, frequently, in the cell bodies of interfascicular oligodendroglia. Interfascicular oligodendroglia also showed 5'-nucleotidase immunoreactivity in vibratome sections from the CNS tissue of young and adult rats. The findings were consistent with histochemical and biochemical evidence for 5'-nucleotidase in rat brain myelin and oligodendroglia, with substantial increases in activity in the myelin as rats develop from the ages of 20 to 120 days. 5'-Nucleotidase immunoreactivity was not observed in any astrocytes or in oligodendrocytes in the gray matter; however, the enzyme may occur in those glial cells at levels lower than were detectable using the present method.     

Myelination and glial ensheathment of purkinje cells in cerebellar cultures are not inhibited by antibodies to the neural cell adhesion molecule, N-CAM

Mouse cerebellar cultures were exposed to anti-N-CAM antibodies throughout their in vitro development. Some cultures were stripped of myelinating oligodendrocytes and functionally competent astrocytes by treatment with cytosine arabinoside (Ara C), while others were left untreated and were potentially capable of forming myelin around axons and astrocytic sheaths around Purkinje cell somata and dendrites. As expected, the antibodies inhibited axonal fasciculation in the Ara C treated cultures. However, the same antibodies had no discernible effect on formation of myelin or astrocytic sheaths in cultures not treated with Ara C. N-CAM is expressed on the surfaces of neurons, oligodendroglia and astrocytes, and has been proposed as the signal molecule governing both kinds of neuron-glia interactions. The observations of the present study strongly suggest, however, that N-CAM does not have an indispensable role in such interactions.     

CYCLOLEUCINE ENCEPHALOPATHY

Greco C. M., Powell H. C, Garrett R. S. & Lampert P. W. (1980) Neuropathology and Applied Neurobiology 6,349–360
Cycloleucine encephalopathy
Cycloleucine, a non-metabolizable amino acid analogue produces status spongiosus in cerebral white matter of rats and mice as well as a distinctive lesion of astrocytes. Its mechanisms of action include competition with natural amino acids from transport across the blood-brain barrier leading to inhibition of entry of circulating amino acids into brain, interference with ribosomal RNA maturation, and blockage of transmethylation reactions, including the conversion of homocystine to methionine. Cycloleucine also affects the kidney, producing aminoaciduria. Electron microscopy of cerebral white matter reveals spongiform changes of myelin sheaths caused by separation of myelin lamellae along intraperiod lines and accumulation of whorls of filaments in astrocytes. The myelinopathy is dose related and its toxicity is cumulative due to its long half-life in animals. The findings are discussed with reference to other spongiform myelinopathies, including status spongiosus observed in homocystinuria and other aminoacidurias.     

Glial proliferation in the irradiated rat spinal cord

Summary The identity of mitotic cells in the ventral half of the irradiated spinal cord in 13-day-old rats was studied by light and electron microscopy. At this post-irradiation interval, astrocytes as well as oligodendrocytes are markedly reduced in both gray and white matter, and few myelin sheaths are present. Earlier studies showed incorporation of³H-thymidine into cells identified light-microscopically as neuroglia. In the present study, a number of mitotic cells were identified in thick plastic sections. When adjacent thin sections were examined by electron microscopy, these mitotic cells were identified ultrastructurally as astroglia on the basis of the bundles of filaments in their cytoplasm and the irregular outline of the cell body and its processes. It is apparent from this study that astroglia proliferate prior to the delayed myelination that occurs later in the glial cell deprived ventral irradiated cord.Supported in part by NIH grants NS-15320 and NS-04761, by a grant from the Folger Foundation, and by the Medical Research Service, V. A.     

Adrenoleukodystrophy — Early ultrastructural changes in the brain

Summary A light and electron microscopic study was performed on the cerebral white matter in a case of adrenoleukodystrophy (ALD) with peculiar symptoms of olivopontocerebellar atrophy. The affected white matter on the light microscope had many macrophages containing characteristic membranebound linear inclusions. The unaffected white matter on the light microscope demonstrated the following ultrastructural changes: (1) slight but definite degeneration of myelin sheaths scattered among apparently normal myelinated axons; (2) oligodendroglia-like cells containing membrane-free intracytoplasmic inclusions; and (3) many swollen astrocytes containing the same membrane-bound linear inclusions as those in the macrophages within the affected white matter. The mechanism of demyelination in ALD is also discussed.Supported in part by a grant from the Ministry of Health and Welfare of Japan     

Immunocytochemical study of Myelin-associated Glycoprotein (MAG) and Basic Protein (BP) in acute Experimental Allergic Encephalomyelitis (EAE)

To compare distributions of oligodendroglial myelin-associated glycoprotein (MAG) and myelin basic protein (BP) during demyelination in acute experimental allergic encephalomyelitis (EAE). Lewis rats were sensitized with an emulsion containing guinea pig spinal cord and complete Freund's adjuvant. The rats were examined clinically and groups were perfused with fixative before symptoms appeared (7d), within 48 h of symptom onset (10d) and during more severe illness (14d, 21d). Paraffin sections of cerebrum, brainstem, and spinal cord were immunostained with antisera to MAG and BP. Other sections in each serially mounted series were stained with luxol fast blue, hematoxylin and eosin or with the Bodian method to correlate MAG and BP results with histological changes. No abnormalities were detected 7d after sensitization. After 10d, small perivenular inflammatory cell infiltrates were present in white matter, displaced myelinated fibers, but their MAG stained periaxonal regions and BP-stained myelin sheaths appeared normal. In pontine grey matter lesions, there were focal abnormalities in a few myelin sheaths. After 21d, demyelinating lesions were present that were largest in pontine grey matter. Decreased MAG staining was present in areas of myelin sheath loss. MAG-stained fragments were found in zones of active myelin breakdown but no decrease or other change in MAG staining extended beyond the margins of demyelinating lesions into areas with normally stained myelin sheaths. Thus, in contrast to multiple sclerosis (Itoyama et al. 1980), changes in periaxonal oligodendroglial MAG are not present in acute EAE before inflammatory cell infiltrates or myelin sheath changes appear. Our findings suggest that myelin sheaths are the primary targets in EAE-induced demyelination. Oligodendroglia appear to be relatively unaffected and are available to remyelinate axons.     

Rat CNS white matter, but not gray matter, is nonpermissive for neuronal cell adhesion and fiber outgrowth

In adult mammalian CNS, axons mostly fail to regenerate after injury, while in the PNS they often succeed in reaching their previous targets. Crucial differences are present in the local tissue microenvironment of CNS and PNS. To investigate the substrate properties of nervous tissue for neuronal adhesion and fiber growth, we used frozen sections of rat CNS and PNS as culture substrates for neuroblastoma cells and for sympathetic and dorsal root ganglia. The results showed that CNS white matter from adult rat spinal cord, cerebellum, forebrain, or optic nerve did not allow cell adhesion and axonal elongation. In contrast, gray matter areas, sciatic nerve, and also trout CNS white and gray matter were permissive substrates. To delineate the tissue components of white matter involved in this nonpermissive substrate effect, newborn rats were injected for 13 d with the antimitotic agent 5-azacytidine. This treatment strongly reduced the oligodendrocyte population and the myelin content of the spinal cord. The immunoreactivity for specific oligodendrocyte and astrocyte markers confirmed the selective suppression of oligodendroglia in these rats. Neuroblastoma cells plated on spinal cord sections taken from these animals were no longer exclusively localized on the gray matter but were also found on regions normally rich in myelin. A significant reduction of the white matter nonpermissive substrate effect was also obtained by the monoclonal antibody IN-1 directed against 2 defined myelin proteins with inhibitory substrate properties (Caroni and Schwab, 1988b). Our results, therefore, show that, in the adult mammalian CNS, cell adhesion and axonal elongation are prevented by white matter components, which are, at least in part, associated with oligodendrocytes and myelin.     

Visualization of oligodendrocytes and astrocytes in the intact rat optic nerve by intracellular injection of lucifer yellow and horseradish peroxidase

The morphology of glial cells in the intact rat optic nerve, a central nervous system (CNS) white matter tract, was analysed by filling over 500 macroglial cells intracellularly with horseradish peroxidase (HRP) or Lucifer yellow (LY). Two main cell types were distinguished: fibrous astrocytes and cells presumed to be oligodendrocytes. Intracellularly stained astrocytes were highly complex, with 50-60 long branching processes which passed radially from the cell body and terminated in end-feet at the pial surface or on blood vessels; some processes ended freely in the nerve parenchyma. Astrocytes filled with LY were usually dye-coupled to other astrocytes after the first week of life. Filled oligodendrocytes had a unique appearance that unmistakably distinguished them from astrocytes and were occassionally dye-coupled to nearby oligodendrocytes. These cells had 20-30 longitudinally oriented processes 150-200 microns long, which passed exclusively along the long axis of the nerve parallel to axons; the longitudinal processes were connected to the cell body by thin branches 15-30 microns long. The longitudinal processes probably represent the tongue processes of the internodal myelin sheaths, and thus each oligodendrocyte appears to myelinate 20-30 axons with sheaths that are 150-200 microns in length.     

Sensory neurite outgrowth on white matter astrocytes is influenced by intracellular and extracellular S100A4 protein

The central nervous system (CNS) is considered a nonpermissive environment for axonal regeneration because of the presence of myelin and associated repulsive molecules. However, neural cells transplanted to the CNS preferably migrate and extend their fibers in white matter areas. We previously showed that white matter astrocytes in vivo express the calcium-binding protein S100A4, which is strongly up-regulated in areas of white matter degeneration. To investigate the role of white matter astrocytes and their specific protein S100A4 in axonal regeneration, we developed white matter astrocyte cultures with strong S100A4 expression and grew dissociated adult dorsal root ganglion (DRG) cells on top of astrocytes for 24 hr. By using small interfering S100A4 RNA, we were able to eliminate S100A4 expression and compare growth of DRG cell neurites on S100A4-silenced and S100A4-expressing astrocytes. In addition, we studied whether extracellular S100A4 has an effect on neurite growth from adult DRG cells cultured on S100A4-expressing white matter astrocytes. Our data show that white matter astrocytes are permissive for neurite growth, although high levels of S100A4 in white matter astrocytes have a negative effect on this growth. Extracellular application of S100A4 induced extensive growth of DRG cell neurites on white matter astrocytes. These findings suggest that white matter astrocytes are able to support axonal regeneration and, furthermore, that administration of extracellular S100A4 provides strong additional support for axonal regeneration.     

Astrocytes, oligodendrocytes, and Schwann cells share a common antigenic determinant that cross-reacts with myelin basic protein: identification with monoclonal antibody

We have produced a monoclonal antibody against myelin basic protein that reacts with astrocytes, oligodendrocytes, and Schwann cells. This antibody was generated by fusion of mouse myeloma cells with spleen cells from BALB/c mice immunized with delipidated white matter from adult rat corpus callosum. The antibody was characterized via solid-phase radioimmunoassay, immunoblot of SDS-PAGE, and by indirect immunofluorescence staining of monolayer cultures containing oligodendrocytes, astrocytes, and Schwann cells. Myelin basic protein (MBP) was shown previously to be present only in myelin producing cells in CNS and PNS (oligodendroglia and Schwann cells) and not in astrocytes. The binding of this monoclonal antibody to all 3 cell types suggests that these cells share a common epitope. This epitope may be related to a common progenitor cell.     

Myelin breakdown and elimination in the posterior funiculus of the adult cat after dorsal rhizotomy: a light and electron microscopic qualitative and quantitative study

Adult cats were subjected to unilateral dorsal L6, L7, and S1 rhizotomy. After survival times of 1-1,552 days the degeneration of axons and myelin sheaths and the elimination of degenerating myelin was studied qualitatively and quantitatively with light and electron microscopy and in Marchi-stained sections in the posterior funiculus in T12-L2. Degeneration was first observed as swollen or shrunken nerve fibers. Somewhat later there was an increased occurrence of collapsed myelin sheaths. The latter lost their myelin periods and appeared to be transformed into myelin bodies. The occurrence of myelin bodies coincided temporally with the presence of many Marchi-positive bodies. Later, an increasing number of intracellularly located lipid droplets occurred, paralleled by the occurrence of a great number of Marchi-positive granules and crystalline structures. Profiles of collapsed myelin sheaths, myelin bodies, and lipid droplets were frequently seen in the cytoplasm of microglial cells. Later, astrocytes and perivascular cells became filled with numerous lipid droplets. The findings suggest that microglial cells take up collapsed myelin sheaths and within these cells the sheaths become transformed into myelin bodies and subsequently into lipid droplets. These two products of myelin disintegration appear to correspond to the Marchi-positive structures seen during the degeneration process. The lipid droplets appear to be transported to astrocytes and perivascular cells.     

Apoptosis of astrocytes with enhanced lysosomal activity and oligodendrocytes in white matter lesions in Alzheimer's disease

Cerebral white matter lesions in Alzheimer's disease (AD) consist of subcortical degeneration and ischaemic-hypoxic changes. Glial changes are intimately associated with the white matter lesions, and regressive changes in astrocytes and loss of oligodendroglial cells have been reported. We quantitatively compared glial changes including apoptosis and enhanced lysosomal activity in the frontal and temporal white matter by using terminal dUTP nick end labelling (TUNEL) and immunohistochemistry for glial markers, lysosomes and apoptosis-regulating proteins in non-familial AD brains. The degree of myelin pallor and axonal loss varied considerably in both the frontal and temporal white matter but fibrillary gliosis in demyelinated lesions tended to be less prominent in the temporal white matter in AD cases. A morphometric study with planimetric methods for cross-sectional areas of frontal and temporal white matter revealed that the white matter of AD cases manifested atrophy with significant reduction in frontal (11.9%) and temporal (29.4%) white matter compared to normal controls. Double immunolabelling for glial fibrillary acidic protein (GFAP) and KP1 (CD68) revealed KP1-positive fragmented structures within the weakly GFAP-labelled astrocytes. These KP1-positive structures correspond to process fragmentation and cytoplasmic vacuoles, which in turn indicate enhanced lysosomal activity during regressive changes in astrocytes. The KP1-modified astrocytes were not found in Pick's disease and corticobasal degeneration. The density of apoptotic glial cells, largely oligodendroglial, was significantly higher in the temporal than in the frontal white matter, and most GFAP-positive astrocytes with regressive changes were apoptotic. GFAP-positive astrocyte density was statistically the same in the frontal and temporal white matter, but the density of KP1-modified astrocytes was higher in the temporal than in the frontal white matter. The rate of white matter shrinkage was significantly correlated with the density of apoptotic glial cells and the density of KP1-modified astrocytes in the temporal lobe in AD cases. An increase in apoptotic glial cell density was found to contribute to GFAP-positive astrocytes with regressive changes in temporal white matter, while apoptosis of vascular smooth muscle cells did not show topographical accentuation. Astrocytes labelled with beta amyloid protein were not apoptotic, and the density of apoptotic cells labelled with CD95 and caspase-3 was too low in both types of white matter to be statistically evaluated. Our results imply that regressive changes in astrocytes and glial apoptosis are, to some extent, associated with white matter lesions, particularly of the temporal lobe in AD brains. The presence of apoptotic astrocytes with evidence of regressive change could therefore be a histological hallmark for white matter degeneration in AD.     

Schwann cell-like myelination following transplantation of an olfactory bulb-ensheathing cell line into areas of demyelination in the adult CNS

In this study we have transplanted a clonal olfactory bulb-ensheathing cell line into focal areas of the rat spinal cord which contain demyelinated axons but neither oligodendrocytes nor astrocytes. The cell line was created by retroviral incorporation of the temperature-sensitive Tag gene into FACS-sorted 04+ cells from 7-day-old rat pup olfactory bulb. The spinal cord lesions were obtained by injecting small volumes of ethidium bromide into the dorsal white matter of spinal cord previously exposed to 40 Grays of X-irradiation. Many of the axons were remyelinated by P0+ myelin sheaths 21 days after transplantation. Light and electron microscopy revealed cells engaging and myelinating axons in a manner highly reminiscent of Schwann cells within similar lesions. GFAP+ cells were also present within the lesion. This study provides the first in vivo evidence that olfactory bulb-ensheathing cells are able to produce peripheral-type myelin sheaths around axons of the appropriate diameter. © 1996 Wiley-Liss, Inc.     

Effects of dorsal root transection on morphology and chemical composition of degenerating nerve fibers and reactive astrocytes in the dorsal funiculus

The morphology and chemical (elemental) composition of the dorsal funiculus of the rat spinal cord were examined 1 and 7 days after unilateral transection (rhizotomy) of the L4 and L5 dorsal roots, using light and electron microscopy as well as X-ray microanalysis. Changes were observed only in the dorsal funiculus on the side of injury and included disintegration of the axonal cytoskeleton, enlargement of axonal mitochondria, and widening of the myelin lamellae of the injured axons. X-ray microanalysis demonstrated a significant increase in intraaxonal sodium at 1 day after injury. This increase was abolished at 7 days, but at this stage there was a significant lowering of potassium in axons and myelin sheaths and of phosphorus in myelin as well as a marked increase in calcium in the axoplasm of the degenerating axons. The nonneuronal cell compartment, largely composed of astrocytes, showed elevated sodium, chlorine, and calcium and lowered potassium levels. The changes in chemical composition paralleled an increase in immunoreactivity for the calcium-binding Mts1 (S100A4) protein, which is exclusively expressed by white matter astrocytes. The influx of calcium is likely to play a crucial role in the loss of axonal integrity after rhizotomy, while the alterations in potassium, and perhaps also phosphorus, may contribute to activation of the nonneuronal cells, including the up-regulation of Mts1 expression in astrocytes.     

Fine structure of astrocytic processes during serum-induced demyelination in vitro

Astrocytic changes were examined ultrastructurally during serum-induced demyelination in organotypic cultures of mouse spinal cord tissue. The myelin sheaths of most myelinated fibers showed myelin breakdown. Sheet-like astrocytic processes completely and closely surrounded the demyelinating fibers and frequently engulfed myelin fragments. These processes were virtually devoid of glial filaments or microtubules and contained flocculogranular material, round and elongated cisterns and glycogen granules. Penetration of the myelin sheath by astrocytic processes was only rarely found. The cell body of the astrocyte was never insinuated between the myelin sheath and the axon. No endocytosis of myelin droplets via coated pits on the surface of astrocytes was observed. In comparison with phagocytic mononuclear cells in vivo, astrocytes in vitro dispose of myelin debris less actively. Astrocytes in serum-induced demyelination probably play a nonspecific but fundamental role during degeneration in segregating damaged nerve fibers from surrounding neuronal elements.