[Transformation of blood monocytes into microglia and pericytes and their ability to proliferate. Experimental autoradiographical and enzyme histochemical studies with normal and damaged brain tissues of rabbits and rats]

Patterns of vascularization in the developing cerebral cortex

The vascular system of the cerebral cortex can be adapted to changing metabolic requirements which occur during development. Apart from a purely nutritive function the intracerebral vessels influence embryonal gliogenesis and migration of neuroblasts. The internal vascularization of the cerebral cortex starts during embryonic development and continues until the postnatal period. The formation of new penetrating vascular trunks and intracortical capillary branching is terminated before global brain growth reaches a plateau. The information necessary to develop a vascular system designed for functional needs later in development may already be expressed in the basic fetal pattern. The formation of such a system is probably not under direct metabolic control. The cellular composition of the capillary tube changes with the developmental stage and the actual growth rate of the endothelial cells. In the cerebral cortex the maximal growth rate of capillaries proceeds in a regional- and lamina-specific manner according to a defined ontogenetic time-scale. The importance of a local factor in the regulation of vascular growth is strengthened by this observation. The vascular system of the cerebral cortex is evaluated using morphometry and reconstructions of serial sections at different stages of postnatal development. This study aims to provide a morphological basis which may help to define cellular mechanisms associated with vascular patterning during brain development.

Failed central nervous system regeneration: a downside of immune privilege?

Immunity is required to eliminate dangerous or degenerated material and to support regeneration, but also causes significant parenchymal damage. In the eye and the brain, in which cornea and lens poorly regenerate and neurons are hardly replaceable, early transplantation experiments demonstrated remarkable tolerance to various grafts. This "immunologically privileged status" (Billingham and Boswell, 1953) may reflect evolutionary pressure to downmodulate certain actions of immune cells within particularly vulnerable tissues. As an example, tolerating certain "neurotrophic" viruses may often be a more successful strategy for survival than the elimination of all infected neurons. While several constitutive and inducible signals maintaining or re-establishing immune tolerance within the brain have been identified, it has also become evident that the resulting anti-inflammatory environment limits certain beneficial effects of neuroinflammation such as neurotrophin secretion or glutamate buffering by T-cells and the clearance of growth-inhibiting myelin or amyloid. Following spinal cord injury, the costs and benefits of neuroinflammation seem to come close because enhancing as well as suppressing innate or adaptive immunity caused amelioration and aggravation of functional regeneration in similar experiments. Evaluating such balances has also begun in (animal models of) Alzheimer's disease, central nervous system trauma, and stroke, and the appreciation of the beneficial side of neuroinflammation has caused a rethinking of the ill-defined use of immune suppressants. As dual roles for individual molecules have been recognized (Merrill and Benveniste, 1996), we are uncovering an already fine-tuned system, but the challenge remains to further support beneficial immune cascades without causing additional damage, and vice versa.

Capillary pericytes: perspectives and future trends

A complete understanding of the microcirculation requires full knowledge of the structure and function of each of the constituent cells, including pericytes. Vascular endothelium and smooth muscle cells have been investigated intensively during the last two decades, but much less is known about the metabolism and function of capillary pericytes. However, the development of new electron microscopy techniques and the application of new cell culture and molecular biology techniques should allow for the rapid elucidation of the cellular biochemistry and the microvascular function and pathology of this ubiquitous capillary cell.

Purple acid phosphatase of human brain macrophages in AIDS encephalopathy

Brains from AIDS patients with an HIV-induced encephalopathy but without opportunistic infections or indications for an inflammation were studied by immuno- and enzyme-histochemical methods. It was found that the macrophages of these brains expressed a lysosomal tartrate-resistant acid phosphatase which gave a good immunological cross-reaction with an antibody to the well-characterized iron-containing bovine spleen purple acid phosphatase, belonging to the group of purple phosphatases, which are regarded as a marker for a special phenotype of activated macrophages. It was discussed that the numerous brain macrophages found in AIDS encephalopathy derive from latently infected monocytes which are believed to be drawn to the brain from the bloodstream.

Origin, proliferation, and fate of cerebrospinal fluid cells. A review on cerebrospinal fluid cell kinetics

Four aspects of cytokinetic investigations of cerebrospinal fluid cells were reviewed: the origin of these cells, their ability to divide, their fate, and the alterations occurring under pathologic conditions. Animal experiments and in vitro reactions of human cerebrospinal fluid cells indicated that, under normal and pathologic conditions, blood lymphocytes and blood monocytes are capable of leaving the vascular system inside the subarachnoid space. When these cells are located extravascularily, they seldom divide in the subarachnoid space under normal conditions. Migration via lymphatic pathways is possible, at lest under pathologic conditions. Cell turnover, as a whole, is extremely low under normal conditions. It is markedly altered by meningeal inflammation and meningeal tumor cell dissemination. Diagnostic significance other than morphology cannot yet result from cytokinetic investigations of cerebrospinal fluid cytology; prognostic significance may be possible.

Acidic protein in macrophages

An antiserum to unstimulated rat peritoneal macrophages was produced in rabbits. The antibodies were directed against an acidic protein with a molecular weight of 35,000 and with an isoelectric point at 4.6. The macrophage acidic protein (MAP) was purified by gel filtration of rat lung soluble proteins, followed by preparative isoelectric focusing. The preparation of MAP was pure as assayed by agar gel electrophoresis and showed one precipitation peak in crossed immunoelectrophoresis against the crude antiserum directed against peritoneal macrophages. The purified MAP was used for immunization of rabbits, and the antiserum obtained was monospecific, assayed by crossed immunoelectrophoresis and Grabar-Williams immunoelectrophoresis. The titre was 4 times higher in the anti-MAP antiserum (1:80) than in the crude antimacrophage antiserum (1:20), tested against MAP by counter-current immunoelectrophoresis. The antigen (MAP) was demonstrated by direct and indirect immunofluorescence microscopy in rat blood monocytes, in spleen and lung monocytic cells, in clusters of cells in the thymus, and in adventitial macrophages around larger blood vessels in liver, kidney, lung and brain. Scattered meningeal macrophages showed fluorescence in the normal, brain. In stab-wounded areas of rat brain MAP was localized to perivascular and perineuronal macrophages with a morphology similar to that of microglial cells. The localization of the fluorescence was the same both for the antiserum against MAP and for the antiserum raised against crude peritoneal macrophages.

Non-specific esterase activity in reactive cells in injured nervous tissue labeled with 3H-thymidine or 125iododeoxyuridine injected before injury

Tritiated thymidine (3H-TdR) injected before a stab wound of the spinal cord or transection of the hypoglossal nerve has resulted in many labeled reactive cells in the CNS after injury, most of which have the ultrastructural features of microglia. To test for the possible origin of these labeled cells from monocytes, we examined them for the presence of sodium fluoride- (NaF) sensitive non-specific esterase (NSE), an enzyme characteristic of monocytes. Some of the labeled cells in stab wounds had NaF-sensitive NSE, but no such cells were found in the nucleus of the injured hypoglossal nerve. To test for the possibility that the NSE-negative labeled cells had been labeled by reutilization of 3H-TdR, we used 125I-5-iodo-2'-deoxyuridine (125I-UdR), a thymidine analogue with a much lower rate of reutilization, to label blood mononuclear cells prior to either a spinal cord stab wound or hypoglossal axotomy. The number of labeled cells was decreased in the spinal cord wound, but more than half were NSE-negative. No labeled blood mononuclear cells were found in the hypoglossal nucleus, although there was no decrease in the hyperplasia of unlabeled non-neuronal cells. When 125I-UdR was injected on the fourth day after hypoglossal axotomy, or when both 3H-TdR and 125I-UdR were injected simultaneously before hypoglossal axotomy, many labeled cells were found in the hypoglossal nucleus, indicating that 125I-UdR can be used by the reactive cells and that it did not inhibit their proliferation. Therefore, the microglial cells that proliferate in response to peripheral nerve injury are not recently derived from any type of circulating large blood mononuclear cell. The most likely explanation for the presence of the 3H-TdR-labeled cells in the nucleus of the injured hypoglossal nerve is that they were proliferating intrinsic cells labeled by reutilization of 3H-TdR.

Dynamic aspects of glial reactions in altered brains

We investigated cellular reactions in altered brain with electron microscopy, 3H-thymidine autoradiography and immunohistochemistry. Comparing the results to those of classical studies with silver-impregnation method, following conclusions were obtained: 1. Full-blown macrophages, "amoeboid microglia", "rod cells" in acute viral encephalitis and "true" inflammatory cells in retrograde degeneration are derived from circulating mononuclear leukocytes which enter into brain parenchyma after the injuries. 2. Microglia, pericytes and other indigenous cells in brain parenchyma do not contribute to the macrophage formation. 3. Silver-impregnated resting microglia are definite cell group existing in the normal brain parenchyma. They are separate kind of cells from oligodendroglia or from mononuclear leukocytes. 4. In response to brain damage the resting microglia show marked swelling in the nucleus and cytoplasm, then, proliferate actively. After division they transform into reactive, fibrous astroglia. 5. Therefore, resting microglia are considered to be the reserve cells of fibrous astroglia.

Features and distribution of intracerebrally injected peritoneal macrophages

Radioactively labeled macrophages were injected intracerebrally in order to acquire additional criteria for the identity of certain intracerebral cell types with cells of the monocyte-macrophage series. The intracerebral distribution of the labeled cells, their reactive ability following silver impregnation, and the formation of processes were considered as indications for ameboid motility. The localization, reactive ability, and structure of the cells were similar to that found in cells inside the brain which are considered to be monocyte derivatives, i.e., intraventricular cells such as epiplexus cells and supraependymal cells, progressive microglia, free subarachnoidal cells, and perivascular cells of intracerebral vessels. A survival time of 2 months was assumed for the cells since isolated, intracerebrally administered, peritoneal macrophages can still be demonstrated inside the subarachnoid space 2 months after the injection.

Initial response of silver-impregnated "resting microglia" to stab wounding in rabbit hippocampus

Adult rabbits received stab wound in the cerebrum and were sacrificed at intervals of 20, 30, and 39 h thereafter. Each animal was injected intracerebrally with 3H-thymidine 2h before fixation. Altered brain tissues of the stratum radiatum of hippocampus were taken for examination. Response of "resting microglia" to stab wounding was investigated by electron microscopic autoradiography and by autoradiography applied on silver-impregnated materials. Following results were obtained: (1) Resting microglia undergo marked swelling shortly after the brain damage. We designate these cells as "swollen microglia". (2) Swollen microglia form the only cell population that proliferate actively in the initial stage of glial response to the brain injury, and (3) astroglia do not proliferate during the same experimental periods, in the rabbit hippocampus.

Fine structure of reactive cells in injured nervous tissue labeled with 3H-thymidine injected before injury

To examine the fine structure of blood mononuclear cells in injured nervous tissue, mice were given repeated injections of 3H-thymidine with the last injection at least 16 hours before injury. Under ether anesthesia the animals either were given a stab wound to the spinal cord or had their left hypoglossal nerve transected. The animals were killed at 2, 4, 8, or 16 days after injury. Tissue sections containing the spinal cord wound or both hypoglossal nuclei were prepared for electron microscopic radioautography, and all labeled cells were photographed. About half the labeled cells in the injured spinal cords and almost all the labeled cells in the nuclei of the injured hypoglossal nerves had nuclei with dark staining peripheral heterochromatin, dark cytoplasm with long cisternae of granular endoplasmic reticulum, and other ultrastructural features characteristic of the cells usually identified as microglia. The remaining labeled cells in the injured spinal cords were macrophages, fibroblasts, cells with pale nuclei, some of which contained cytoplasmic filaments, and vascular cells. Since uninjured nervous tissue has extremely few labeled cells and since 3H-thymidine should be available for only a short time following injection, most of the labeled cells in this experiment should be derived from blood mononuclear cells. However, the possibility is discussed that some or all of the labeled cells may be intrinsic cells proliferating in response to the injury and labeled through reutilization of labeled DNA precursor material.

Ultrastructural cytochemical demonstration of peroxidase-positive monocyte granules: an additional method for studying the origin of mononuclear cells in encephalitic lesions

Unlike lymphocytes, blood monocytes possess in their cytoplasm peroxidase-positive (azurophil) granules (ppg) which largely correspond to the homonymous organelles of neutrophil granulocytes. We tested whether ppg, demonstrated cytochemically at the submicroscopic level, could serve as markers of monocyte-derived reactive mononuclear cells in encephalitic lesions. Samples of cerebrocortical tissue from adult albino mice with experimental yellow fever virus encephalitis were incubated in a medium containing diaminobenzidine and H2O2 for localization of peroxidatic activity. Mononuclear cells exhibiting ppg were found (1) in the lumen of brain venules, (2) in different stages of migration through the walls of such vessels, (3) in perivascular areas, (4) in the glioneuropil, either loosely scattered or forming small clusters, (5) in a satellite position to neurons, and (6) in leptomeningitic inflitrates. Several mononuclear elements harboring ppg had assumed an elongated, rod cell-like outline. Amongst the peroxidase-negative mononuclears were fully developed brain macrophages and elements showing morphologic features characteristic of activated lymphocytes. Most mononuclear cells without ppg resembled the peroxidase-reactive ones. The results of this study provide direct evidence in favor of a monocytic origin of, at least, numerous reactive mononuclear elements in encephalitic lesions. The approach followed in the present study is not suitable for quantitative investigations of the histogenesis of mononuclear cells responding to brain injuries, since emigrated blood monocytes rapidly lose their ppg, particularly, when they display enhanced phagocytic activity.

Radioautographic investigation of gliogenesis in the corpus callosum of young rats. II. Origin of microglial cells

Microglial cells are absent from the corpus callosum of newborn rats. In the hope of finding out when and how microglial cells appear with age, 3H-thymidine was given intraperitoneally as single or three shortly spaced injections to 5-day-old rats weighing about 15 g; and these animals were sacrificed at various time intervals from 2 hours to 35 days later. Pieces of corpus callosum were taken near the superior lateral angle of the lateral ventricles; and semithin sections were radioautographed and stained with toluidine blue. The corpus callosum of 5-day-old rats is composed of loosely arranged unmyelinated fibers and scattered cells. Among these cells, microglia are rare; there are a few astrocytes, many immature glial cells, rare pericytes, and 6--7% of phagocytic "ameboid cells" consisting of a few monocytes and many macrophages. In the animals sacrificed two hours after 3H-thymidine administration, label is present only in immature cells and "ameboid cells." As time elapses and the fibers of corpus callosum become myelinated, oligodendrocytes and, later, microglial cells appear. At the age of 12 days, microglial cells are present in substantial number; and by 19 days, the number doubles to reach a plateau. Many of the new microglial cells are labeled, e.g., 78.1% in 12-day-old animals (7 days after 3H-thymidine administration). The labeled microglial cells must have come from the transformation of cells that acquired label early, that is, from the immature cells or the "ameboid cells." The height of the peaks of labeling--59.8% at nine days for immature cells and 77.8% at 12 days for "ameboid cells"--points to the latter as precursors of the highly labeled microglial cells. Furthermore, the "ameboid cells" disappear as microglial cells appear and there are transitional elements between these two cell types. Cell counts suggest that about a third of the "ameboid cells" transform into microglial cells, while the others degenerate and die. Thus, the microglial cells which appear in the corpus callosum during the first three weeks of life result from transformation of the "ameboid cells"--a group of macrophages showing various stages of transition from monocytes. As for the occasional microglial cell appearing after the third week or in the adult, they presumably come directly from monocytes. In either case, monocytes would be the initial precursors.

Lack of labelling of microglial cells following microinjection of [3H] beta-alanine: an electron microscopic autoradiographic study

Following axotomy of the facial nerve, the uptake of [3H] beta-alanine into different types of glial cells in the facial nucleus was studied by autoradiography. A marked proliferation of microglial cells, predominantly in a satellite position to neurons, was accompanied by a localization of [3H] beta--alanine over astrocytes and oligodendrocytes but not over microglial cells. Microglial cells therefore appear to be a functionally distinct cell type and should not be classified with the macroglia.

Electron microscopic features of the resting microglia in the rabbit hippocampus, identified by silver carbonate staining

Vibratome sections of hippocampus of adult rabbits were stained by a modified Hortega's silver-carbonate method. Impregnated materials were examined by electron microscopy to decide fine-structural characteristics of the resting microglia. Comparing their characteristics with those of macrophages, we came to the following conclusions: (1) Impregnated resting microglia in the hippocampus of adult rabbits can be identified as cells having distinct fine structures. (2) Resting microglia are morphologically different from macrophages or their precursor cells, and, therefore, seem not to be hematogenous cells sojourning in the normal brain parenchyma.

Proliferation of blood vessels and stroma in brain tumours. An enzyme-histochemical study

Proliferation of the vascular endothelium occurring in brain tumours is accompanied by a proliferation of histiocytes in the peripheral part of the vessel wall. These histiocytes infiltrate the tumour tissue in a very regular pattern. Enzyme-histochemically, there are marked differences between the activities of alkaline phosphatase, 5-nucleotidase, and ATPase in the normal and proliferating blood vessels. The whole process encompasses reactive changes evoked by the destroyed perivascular sheath of astroglial foot processes and the subsequent oedema in the tumour and the surrounding parenchyma. There are often tumour areas where diminished vascular permeability is established by proliferation of perivascular connective tissue. Here the oedema has completely disappeared. A clearcut influx of monocytes from the blood into the vessel wall is seen only in the vicinity of necrotic foci; the number of histiocytes is increased and their turnover is observed in swollen macrophages. In the rest of the tumour influx of monocytes and activity of macrophages are inconspicious.

Wound healing of the brain of rats after cryonecrosis. Autoradiographic investigations with 3H-thymidine

Histologic and autoradiographic studies were performed to investigate the cellular reactions during wound healing of the brain of rats after cryonecrosis. A parietal lobe was frozen for 30 s with a cryoprobe at a temperature of -196 degrees C. The survival time ranged between 12 h and 21 days. One h before killing, tritiated thymidine was injected intraperitoneally. Stripping film autoradiograms of the brain sections were made in the usual manner. A typical cryonecrosis develops 12 h after local freezing surrounded by an edematous tissue layer with activated mesenchymal and neurologlial cells. After 10 days a pseudocyst develops surrounded by highly cellular glial tissue. The pseudocyst is still recognizable 3 weeks later but without any remarkable cellular reaction. Autoradiographically the labeling indices of the fibroblasts, leptomeningeal,and endothelial cells in the periphery of the necrosis and the labeling indices of neuroglial, perivascular connective tissue, and microglial cells in the perinecrotic zone increase after 12 h and have their maximum between the 2nd and 3rd postoperative day. In the nonfrozen contralateral cerebral hemispheres, the labeling indices of neuroglial and mesenchymal cells show small peaks in the first 3 postoperative days,also. This can be explained by accompanying edema. There are no remarkable differences between frozen and nonfrozen parts of the brain 3 weeks after local freezing. The results underline the rapid repair of cryonecrosis.

Resting and reactive macrophages in the developing cerebellum: an experimental ultrastructural study

The whole heads of 6 days old rats were exposed to 150R of X-ray irradiation to induce cell-death in the external granular layer of the cerebellum. The animals were sacrificed in a developmental sequence, and the tissue obtained from the cerebellum was prepared for electron microscopy. On the basis of the cytological criteria established for the macrophages in the cerebellum of the normal animals, characteristics of the resting and reactive macrophages and their transformation from one state to the other in the experimental animals are described. In their reactive state the macrophages showed hypertrophic and nuclear changes which were found identical to those seen in the reactive microglial elements. Issues pertaining to the ultrastructural characteristics of the macrophages, their endogenous presence in the developing cerebellum, and their transformation into the microglia cells are discussed.

Non-glial phagocytes within the degenerating optic nerve of the newt (Triturus viridescens)

Two non-glial phagocytes were found to participate along with ependymoglial cells in Wallerian degeneration of the severed optic nerve of the newt (Triturus viridescens). The first type of non-glial cell (polymorphonuclear phagocyte) was positively identified as a neutrophil and participates in the early stages of degeneration. Cells of this type make a brief appearance, reaching a peak by the second postoperative day (2 p.o.d.), and quickly diminish until few can be found by 4 p.o.d. Neutrophils invade the degenerating optic nerve from surrounding connective tissue spaces, most likely, through channels which penetrate the nerve parenchyma. The second type of non-glial cell is an invading mononuclear phagocyte which exhibits characteristics of microglial cells reported in other vertebrate species. Such cells appear in the nerve much later than the neutrophils and towards the end of Wallerian degeneration (6-10 p.o.d.). Their mode of entry and exit appears to be the same as that reported for neutrophils. The neutrophils and microglial-like, mononuclear phagocytes may serve to supplement the histolytic action of the ependymoglial cells, picking up scattered fragments of degenerating myelin and axons.

Macrophages in brain tumours induced transplacentally by N-ethyl-N-nitrosourea in rats: an electron-microscope study

The fine structure of macrophages has been studied in experimental brain tumours induced transplacentally in BD-IX rats by a single intravenous injection of 30 mg of N-ethyl-N-nitrosourea per kg of body weight on the 15th day of gestation. The tumours, depending on their localisation and size, caused various lesions in the brain, namely axonal degeneration, loss of myelin, oedema, haemorrhage and cell necrosis. The tumours and the resulting alterations elicited a strong reaction by macrophages: activation of microglial cells in situ and infiltration of the brain by leucocytes, chiefly by monocytes. Since both microglial cells and monocytes underwent morphological changes, it was difficult, or impossible, to establish the origin of these reacting cells. In a few cases, however, microglial cells and monocytes could be distinguished; this indicated that microglial cells were still being activated and leucocytes were still entering the brain. Various stages of activity of macrophages have been described: the number of lysosomes and cytoplasmic inclusions were thought to indicate activation, phagocytosis and repletion. Activation is characterised by an increase of lysosomes and frequent cell divisions. Phagocytic activity is accompanied by the appearance of inclusions which varied in different lesions: protein-like material in oedema, remnants of erythrocytes in haemorrhages and myelin-lamellae with lipid droplets in demyelination. These various inclusions were frequently present in the same cell, since the different lesions not uncommonly occurred at the same time. In the stage of repletion macrophages contained mainly lipid droplets and unidentifiable debris in their abundant cytoplasm and thus corresponded to the compound granular corpuscles.

Autoradiographic investigations of glial proliferation in the brain of adult mice. II. Cycle time and mode of proliferation of neuroglia and endothelial cells

The cycle time of the proliferating glial cells outside the subependymal layer of the lateral ventricle as well as that of endothelial cells was studied autoradiographically in the brains of adult and untreated mice. To determine the mean cycle time two independent methods were used. A mean cycle time of about 20 hours was obtained for glial and endothelial cells from the decrease of the mean grain number/nucleus as a function of time after tritiated thymidine (3H-TdR) injection. Another group of experiments utilized the "method of labeled S phases". With this method the passage of labeled cells through successive S phases is observed. Passing through S phase following 3H-TdR injection the 3H-labeled cells are double labeled by an additional 14C-TdR injection. This method again resulted in a cycle time of 20 hours for glial and endothelial cells. From the present work and a former study (Korr et al., '73) the following cell cycle parameters were derived: Cycle time 20 hours; S phase 9.4 hours; G2 less than three hours; (G2+M) five hours; G1 five hours. The growth fraction of glial cells related to all glial cells is only 0.004. Furthermore, the present experiments show that in the case of glial cells 17% of the daughter cells after mitosis become pyknotic and are eliminated from the glial cell population. Apart from this cell loss, after mitosis about one-fourth of the daughter cells do not enter the next S phase. These cells leave the growth fraction and are replaced by a corresponding number of non-proliferating glial cells. There is a relatively extensive permanent exchange of cells between the growth fraction and non-growth fraction of glial cell.

The ultrastructure of Wallerian degeneration in the severed optic nerve of the newt (Triturus viridescens)

Wallerian degeneration in the severed newt's (Triturus viridescens) optic nerve is complete between the 10-14th post operative day (p.o.d.). Consequently, the newt optic nerve displays one of the most rapid degenerative responses yet reported for the central nervous system of vertebrates. In most cases it also exhibits the speed of degenerative phenomenon in the vertebrate peripheral nervous system. The degeneration of unmyelinated axons is most rapid and is completed by 2-3 p.o.d., compared to myelinated axons, most of which degenerate between 2-10 p.o.d. Myelin ring formation (vesicular transformation) is the principal form of lamellar breakdown and occurs in a highly organized manner which can be clearly staged. The glial cell response to Wallerian degneration is two-fold: cytoplasmic hypertrophy and myelin-lytic. Glial hypertrophy subsides by the 10-14 p.o.d. with the ingrowth of numerous regenerating nerve fibers. The myelin-lytic response accounts for most of the myelin destruction. Leukocyte-like and microglia-like cells also participate in myelin breakdown but to a lesser degree.

The relationship between microglia and brain macrophages. Experimental investigations

A series of experiments are reported which indicate that the microglia are endogenous cells which may constitute the only source of phagocytes in certain mild degenerative conditions, such as Wallerian degeneration and retrograde nerve cell disintegration. In more extensive lesions with increased vascular permeability a substantial number of the phagocytes are derived from the blood monocytes.

Experimental studies on kinetics and functions of monuclear phagozytes of the central nervous system

The described experimental studies on rabbits gave evidence of the hematogenous - obviously monocytic - origin, the lymphatic drainage, and the IgG- and complement-receptor sites of monocytoid CSF-cells, epiplexus cells, perivascular cells of the intracerebral vessels, and of some cells within damaged brain tissue - so-called progressive microglia. - Becuase of their identical kinetics and functions these types of mononuclear cells of the CNS were placed in a system known as 'Mononuclear Phagocyte System".

The ultrastructure of normal and reactive microglia

Normal microglia have a distinct morphology. In rapidly, but now in slowly evolving pathological states the features used to identify the resting cell are often lost. When there is invasion by haematogenous monocytes, phagocytes develop whose origin - cerebral or haematogenous- cannot be ascertained on morphological features alone. These observations stress that microglia are part of the reticulo-endothelial system.