The origin and nature of ramified and amoeboid microglia: a historical review and current concepts

The origin of ramified microglia has been a longstanding controversial issue, with 4 major schools of thought, which state that they are derived (1) from invasion of mesodermal pial elements, (2) from neuroectodermal matrix cells together with the macroglia, (3) from pericytes, and (4) from invasion of monocytes in early development. This paper is in support of the last-mentioned hypothesis. It is known that ramified microglial cells do not divide under normal circumstances, and since our studies in the corpus callosum have shown that these cells do not appear until the fifth postnatal day, it is reasoned that they must be derived from some preexisting mitotically active cells. The putative precursor is the preponderant amoeboid microglia in the same region. Our experimental studies with the carbon labelling technique have demonstrated for the first time that blood monocytes invade into the early postnatal brain to become amoeboid microglia, which then differentiate into ramified microglia. Just like other tissue macrophages, the monocyte-derived amoeboid microglia exhibit features indicative of phagocytic activities. These include the content of hydrolytic enzymes, uptake of carbon, and a characteristic surface morphology, as seen by scanning electron microscopy. The transformation of amoeboid microglia into ramified microglia, which occurs between the second and third postnatal week, is considered to be a regressive phenomenon, as shown by the diminution of their content of hydrolytic enzymes and the downregulation of membrane antigen. Apart from their primary role as active phagocytes, their involvement in Alzheimer's disease (AD) is evidenced recently by the fact that the cells are specifically marked by antibodies present in the cerebrospinal fluid of AD patients. In conclusion, ramified microglial cells are derived from monocytes, but through an intermediate amoeboid microglia stage as active macrophages in the perinatal period.

A quantitative and morphometric study of the transformation of amoeboid microglia into ramified microglia in the developing corpus callosum in rats

The morphometric and quantitative changes associated with the differentiation of amoeboid microglia into ramified microglial cells in the corpus callosum of rats between 21 d postconception (E21) and 15 d postnatally are described. Using lectin labelling, 5 morphological types of labelled cells (R, SP, KLP, TLP, AP) based on cell body shape, the configuration of their cytoplasmic processes and their staining intensity, were recognised. Round cells (R) and cells with stout processes (SP) were aggregated in the central part of the developing corpus callosum whereas the highly branched labelled cells were distributed at its periphery. When the morphometric data and labelling intensities of labelled cells were analysed with the aid of an image analysis system, the values for cell length, area and perimeter increased as the complexity of branching increased, whereas the lectin-labelling intensity became reduced. Quantitative study showed that the proportion of the different morphological types of lectin-labelled cells peaked at different ages. The sequential peaking of R, SP and highly branched cells with advancing age suggests a similar chronological order of differentiation of R into branched cells. The quantitative study also showed a rapid increase in the density of lectin-labelled cells in the postnatal period between P4 and P8, attributed primarily to the active proliferation of the cell type. The consequent reduction of cell density (after P13) was probably due to cell death, a feature which appeared to increase with development.

Studies of activated microglial cells and macrophages using Alzheimer's disease cerebrospinal fluid in adult rats with experimentally induced lesions

Previous investigations have shown that cerebrospinal fluid from Alzheimer's disease patients contains antibodies that recognize the amoeboid microglia--a nascent and active form of microglia in the developing rat brain [McRae et al. (1991) Neuroscience 41, 739-752]. The present study extended this to show that the same cerebrospinal fluid from Alzheimer's disease patients also labeled the activated microglia and macrophages induced experimentally in adult central nervous system. Thus, in the spinal cord, activated microglia were elicited following the destruction of the motor neurons by the toxic lectin, Ricinus communis agglutinin, injected into the sciatic nerve. The activated microglia which were closely associated with the soma of the degenerating neurons were intensely immunostained with the cerebrospinal fluid from Alzheimer's disease patients. The labeling pattern was comparable to some known monoclonal antibodies including OX-42, OX-18 and OX-6 that mark microglia. The microglia cells on the contralateral normal side remained unstained. In the cerebrum, activated microglia and neural macrophages were induced following an epidural application of the excitotoxin, kainic acid or cryolesion. Immunoelectron microscopy of these cells showed that the immunoreactivity was localized at the plasma membrane and its derivatives suggesting that these are the sites where the antigens are associated. The results obtained in this investigation suggest that these experimental models may be a means to gain further insight to antigens recognized by antibodies in the cerebrospinal fluid of Alzheimer's disease patients.

The intraglandular submandibular ganglion of postnatal and adult rats. II. A morphometric and quantitative study

A morphometric study was undertaken on the submandibular ganglion cells in rats of different ages. This showed a direct proportional increase with age in all the variables measured. Mean cross-sectional cell area showed the most dramatic growth, an increase of more than 5-fold between birth and young adulthood. Mean cell diameter and cell perimeter doubled over the same period. The growth of the nucleus, expressed as diameter, was slower when compared with that of the ganglion cells as a whole. The number of intraglandular ganglion cells remained relatively unchanged from birth to young adulthood, ranging from about 3000 to 5000 cells. They were mainly distributed at the hilar region of the submandibular salivary gland, contributing 1/2 to 2/3 of the total ganglion cell population. The second largest cell population was in the intralobular region, which made up about one-third of the population. The least populated region was in the connective tissue of the sublingual salivary gland, which contained only about 5-7% of the total cell number. Cell counts based on the fluorogold labelling method were generally lower than those made after haematoxylin and eosin staining. In the 2-d-old animals, counts of fluorogold-labelled cells were only about half the H & E counts. The discrepancy may be due to the thicker sections used in the fluorogold method, superimposition of cells leading to an underestimation of cell numbers. Nevertheless, the fluorogold labelling method provided rapid and reproducible results. Its main advantage is that the labelled ganglion cells emit a bright yellow fluorescence which is readily identified; the other is the simplicity of the procedure, as labelling of ganglion cells can be achieved by the intraperitoneal route.

Immunocytochemical localisation of substance P in vagal ganglion cells and pericellular arborisations in the monkey

The quantitative cell count showed the presence of about 20,000 ganglion cells with associated satellite elements in the nodose ganglion in the monkey. Among these closely packed cells, at least one-third were substance P (SP) immunoreactive, ranging from faint or moderate to intense staining. Substance P immunoreactivity (SP-IR) was localised in the cell bodies and their long extending neurites. Ultrastructural study showed that SP-IR was not associated with any particular organelles or inclusions. A striking feature of the nodose ganglion was the occurrence of SP-positive pericellular arborisations associated with about 0.5% of the ganglion cells which were almost exclusively SP-negative. The pericellular arborisation displayed diverse morphological forms from a simple tortuous fibre to complex glomerular networks or plexuses encircling the soma of SP-negative ganglion cells. The varicose nerve fibres forming the pericellular arborisations appeared to terminate as 'boutons' contacting the soma of the ganglion cells. Electron microscopic study demonstrated the close spatial relation between the SP-IR profiles and the ganglion cell but there was no direct synaptic contact. In some instances, the SP-IR profiles containing agranular and dense-cored vesicles penetrated the cytoplasm of satellite cells, almost reaching the surface of the soma of the ganglion cell. The sources of origin of the nerve plexuses in the pericellular arborisation were either from the small and sparsely distributed jugular ganglion cells which were intensely SP-IR or from the intrinsic SP-IR nodose ganglion cells. The possibility that the efferent neurons in the dorsal motor nucleus of the vagus could also contribute to the pericellular arborisation was also considered. The function of the pericellular arborisations may be related to the modulation of the SP-deficient ganglion cells with which they associate through the release of SP and probably by way of the satellite cells.

Activation and re-expression of surface antigen in microglia following an epidural application of kainic acid in the rat brain

Following an epidural application of kainic acid over the sensorimotor cortex in rats, the ipsilateral hippocampus and the ventrobasal nuclear complex of the thalamus showed extensive neural degeneration. The neuronal death, either as a result of direct neurotoxic destruction or wallerian and retrograde degeneration, elicited a dramatic expression of immunoreactivity on numerous cells bearing the external morphology of microglia. Thus, with the monoclonal antibody OX-42, many amoeboid immunoreactive cells bearing stout processes were observed in the above-mentioned lesioned sites. The present electron microscopic immunocytochemical study confirmed that these OX-42 positive cells were activated microglia characterised by an abundant cytoplasm containing a variable number of lysosomes and phagosomes. The surfaces of these activated microglial cells were thrown into pseudopodial processes engaged in the phagocytosis of cellular debris. Immunoreactivity was also observed in these cells with the monoclonal antibodies OX-18 and OX-6, although in the latter the immunoreactive cells were fewer and less intensely stained. With OX-42, the corresponding areas on the contralateral side showed some widely scattered typical microglial cells bearing extremely fine processes. They were not stained with either OX-18 or OX-6. It was concluded from this study that neural degeneration induced the expression of CR3 receptors (marked by OX-42) and MHC encoded antigens (marked by OX-18 and OX-6) in microglia. The elevation of the former antigen was related to their active phagocytic activity. The latter, on the other hand, would facilitate the capability of interaction between the activated microglia and T lymphocytes in a possible immune response.

The intraglandular submandibular ganglion of postnatal and adult rats. I. A light and electron microscope study

The structure of the intraglandular submandibular ganglion is described in both postnatal and adult rats. The ganglion is localised mainly at the hilum where the majority of the cell bodies are observed. Ganglia are also present in the intralobular septa of both the submandibular and the sublingual glands. Often they are found along the main salivary ducts with the larger ganglia being encapsulated by connective tissue. On electron microscopy, the submandibular ganglion cells show the usual features of autonomic neurons. The cells contain a prominent round nucleus. Numerous short processes project from the soma together with a few long dendrites. The organelles are randomly distributed throughout the soma. Most of the synapses observed were on the short processes with occasional axosomatic synapses. Nonsynaptic desmosome-like contacts are a common feature among the ganglion cells. Especially noteworthy are contacts made by the dendrites which deeply invaginate the soma of an adjacent nerve cell. The ganglion cells of the postnatal and adult submandibular ganglia show minor differences. Ultrastructurally, the postnatal cells show signs of immaturity such as abundant free ribosomes, well developed Golgi complexes and disorganised rough endoplasmic reticulum. Mitotic satellite cells were observed associated with the postnatal ganglion cells. The study has confirmed that all the submandibular ganglion cells show a positive reaction for acetylcholinesterase. Enzyme activity is localised in the cisternae of rough endoplasmic reticulum, the Golgi complex, plasma membrane and nuclear envelope.

Amoeboid and ramified microglia: their interrelationship and response to brain injury

Rio-Hortega's hypothesis that transiently appearing amoeboid microglia might become ramified microglia in the adult and that the latter could differentiate into brain macrophages in the event of brain damage could not be proved because of inherent limitations in existing techniques. The present investigation used a novel method of labelling the rat supraventricular amoeboid microglia with an enduring fluorescent marker, rhodamine B isothiocyanate, introduced intraperitoneally. Observation of their subsequent development showed that they became transformed into the ramified microglia. Both the amoeboid and ramified microglia were OX-42 positive, indicating their macrophage/monocyte lineage. Other microglia in the cerebral neocortex, which were also OX-42 positive, were not derived from any of the rhodamine-labelled cells. Rhodamine-labelled microglia did not migrate toward the site of a superficial cerebral injury. Following a deep lesion reaching the corpus callosum, greatly increased numbers of labelled amoeboid microglia were frequently observed at or near the lesion site. Large rhodamine-labelled cells, which were OX-42 positive, appeared at all lesioned sites, and such were most likely blood derived monocytes. The antigenicity of the ramified microglia became elevated when rhodamine B isothiocyanate was present intracellularly and even more so with the presence of a nearby intracerebral stab wound.

Expression of major histocompatibility complex antigens and CR3 complement receptors in activated microglia following an injection of ricin into the sciatic nerve in rats

The ventral horn motor neurons in the lower lumbar cord underwent rapid degeneration following an injection of Ricinus communis agglutinin-60 (RCA) into the sciatic nerve. The cell death which was most drastic between the fifth and seventh post-injection day elicited a significant increase in the number of microglia. The activated microglia were scattered throughout the neuropil but the dramatic feature was their close association with the somata of the degenerating neurons. Often several microglial cells were seen surrounding the soma of a degenerating neuron. Immunocytochemical study showed that both the interstitial as well as the perineuronal activated microglia were labelled with the monoclonal antibodies OX-18 and OX-42 for the detection of MHCI encoded antigen and type three complement receptors, respectively. Intense immunoreactivity was observed especially in the perineuronal microglia with OX-18. Electron microscopic study confirmed the identification of the activated microglia. Although the activated microglia closely apposed the neuronal soma, there was no sign of a direct endocytosis. The cytoplasm of the activated microglia, however, contained massive lipofuscin bodies in longer survival animals. Electron microscopic immunocytochemical study showed that the immunoreactivity of the activated microglia was localized along their plasma membrane facing the neuronal soma. Since the microglia cells on the contralateral side of the ventral horn were not marked by the antibodies used, it was postulated that the vigorous expression of MHCI antigen and CR3 receptors on the activated microglia was induced by the neuronal degeneration resulting from the application of the toxin ricin.

Ultrastructural changes of the nodose ganglion cells following an intraneural injection of Ricinus communis agglutinin-60 into the vagus nerve in hamsters

Virtually all the ganglion cells in the nodose ganglion in hamsters underwent rapid degeneration following an intraneural injection of RCA-60 into the vagus nerve in the cervical region. The earliest signs of neuronal degeneration were evident in animals which survived 5 days after the ricin application. A remarkable feature was the appearance of a variable number of granular dense bodies measuring 1-4 microns in diameter in the cytoplasm. They were composed of closely stacked cisternae which were continuous at the periphery with those of the rough endoplasmic reticulum. Associated with the membranous cisternae were large accumulations of glycogen. With longer survival time, these glycogen-membrane complexes appeared to disintegrate. Numerous vacuoles and neurofilaments accumulated in their vicinity. Satellite cells were activated between the 7th and 10th postoperative days. These penetrated deeply into the degenerating neurons dividing them into numerous fragments by their extensive cytoplasmic prolongations. The cytoplasmic fragments of the RCA-poisoned neurons eventually became necrotic and disintegrated in the satellite cells, suggesting a rapid mode of neuronophagia. The biosynthesis of acetylcholinesterase was inhibited by the ricin injected as shown by the drastic reduction of the enzyme activity in the rough endoplasmic reticulum and nuclear envelope. Some isolated ganglion cells apparently survived the RCA injection as shown by their occurrence in long surviving animals (30-90 days). A few of them displayed an enhanced density of their cytoplasm and neurites. It is postulated that this was induced by the RCA released from the RCA-poisoned neurons.

Expression of major histocompatibility complex and leukocyte common antigens in amoeboid microglia in postnatal rats

The expression of major histocompatibility complex (MHC) antigen and leukocyte common antigen (LCA) was observed in the amoeboid microglial cells in postnatal rat brain. Considerable MHC class I surface antigen was detected at the plasma membrane and its tubular invaginations in the amoeboid microglia in the corpus callosum using the monoclonal antibody OX-18. In early postnatal (2 and 5 day) rats, the OX-18 positive cells were mostly round but a few possessed stout processes. With increasing age (9 and 15 days) the OX-18 positive amoeboid microglia assumed an oval or elongated form. By the time of weaning (21 days) and in older animals, the immunoreactivity was extremely weak and was detectable only on some branched microglia bearing fine processes. The presence of MHC class Ia antigens with OX-3 and OX-6 was hardly detectable except for a few weakly stained cells in the corpus callosum and cavum septum pellucidum in early postnatal rats. The expression of LCA was observed in amoeboid microglial using the monoclonal antibody OX-1 and this followed a similar temporal pattern to that with OX-18. The additional phenotypic features of amoeboid microglial cells in the present study support their monocytic origin. These cells are endowed with MHC class I antigens which may serve as the restriction elements for T lymphocytes, at least in the early postnatal brain.

Vagus nerve and spinal cord projecting neurons demonstrated by horseradish peroxidase and different fluorescent dyes

Vagal projecting (VP) neurons were localized by intraneural injections of fluorescent dyes or cholera toxin conjugated horseradish peroxidase (CT-HRP) or by intraperitoneal injection of fluorescent dyes. Spinal projecting (SP) neurons were localized by injecting CT-HRP or contrasting dyes into the C4/C5 cord segments. No doubly labelled neurons were seen in the three nuclei known to project to both vagus nerve and spinal cord, viz., dorsal nucleus of the vagus (DNV), nucleus ambiguous complex (NAc) and the intermediate region (NI) between DNV and NAc. VP and SP neurons intermingled in the caudal parts of the NAc and DNV. In the middle part of the NAc, VP neurons congregated mostly dorsal to the SP neurons. In the rostral extremity of the NAc, SP neurons were rarely encountered. No SP neurons were seen in the rostral end of the DNV. In contradistinction to the few VP neurons in the NI, there were many SP neurons in this region. The ratios of VP to SP neurons in DNV were on the average 20 to 1 and those of VP to SP neurons in the NAc, 1.35 to 1.

Antibodies in the cerebrospinal fluid of some Alzheimer's disease patients recognize amoeboid microglial cells in the developing rat central nervous system

Previous investigations have demonstrated that the cerebrospinal fluid from Alzheimer's disease patients contains antibodies that recognize specific neuronal populations in the adult rat central nervous system. These findings suggest a pathogenic role for immunological aberrations in this disorder. In the present report the investigation of antibodies in the cerebrospinal fluid of Alzheimer's disease patients was extended to developing rat central nervous system. The antibody in cerebrospinal fluid from Alzheimer's disease patients recognized entirely different types of antigens in the developing rat central nervous system as compared to adult rat central nervous system. One of the most remarkable differences was the recognition of amoeboid microglial cells. Diverse morphological forms of amoeboid microglial cells were observed, located mainly in the cavum septum pellucidum and in the corpus callosum. Electron microscopy revealed that the cerebrospinal fluid antibody from the Alzheimer's disease patients recognized specific membrane receptors in the macrophagic microglia. The unexpected recognition of amoeboid microglia by antibodies in Alzheimer's disease-cerebrospinal fluid is particularly interesting since these cells proliferate in response to nervous system disease and also engulf debris. The results add further support to the concept that inflammation and similar immune mechanisms may contribute to Alzheimer's disease pathogenesis.

Study of the transformation of amoeboid microglial cells into microglia labelled with the isolectin Griffonia simplicifolia in postnatal rats

The transformation of amoeboid microglial cells into ramified microglial cells in the brain of postnatal rats has been studied by labeling the cells with the isolectin Griffonia simplicifolia (GSA1-B4). The latter served as a specific membrane marker of the cell type. Thus, at the light-microscopic level, the amoeboid microglial cells in 1- to 5-day-old rats were intensely stained with GSA1-B4. All the stained cells appeared round. In 10-day-old rats, while most of the stained cells were round, some had assumed an oval appearance. In older rats, i.e. 15-22 days, all the stained cells became flattened or fusiform with long cytoplasmic processes. The present electron-microscopic study confirmed the above features but also added the fact that the reaction for GSA1-B4 was localized at the plasma membrane in the amoeboid microglial cells in all the age groups studied. The reaction for the isolectin was also detected in some vacuoles in the cytoplasm of the round cells. It was concluded from this study that the round amoeboid microglial cells differentiate to become the ramified microglia with age. In the course of this transformation, they retained specific membrane receptors for the isolectin which distinguished them from other glial cell types.

Lectin labelling of amoeboid microglial cells in the brain of postnatal rats

The labelling of amoeboid microglial cells in the postnatal (2-10 days old) rat brain was studied by intracerebral injection of various lectins, including peroxidase-labelled Ricinus communis agglutinin (RCA), peroxidase-labelled isolectin Griffonia simplicifolia (GSA1-B4) and gold-labelled concanavalin A (Con A). Three to six hours after the injection of RCA and GSA1-B4, the amoeboid microglial cells in the supraventricular corpus callosum were selectively labelled. Most of the labelled cells were round, showing dense black reaction products. With the electron microscope the reaction of the binding sites for RCA and GSA1-B4 was localised on the plasma membrane, in the plasmalemmal invaginations, in the limiting membrane of the cytoplasmic vacuoles and in the dense granules identified as lysosomes. The binding sites for gold-labelled Con A were initially (one hour) observed at the plasma membrane. With time (3-6 hours) the gold particles occurred in the invaginations of the plasma membrane and consequently in the cytoplasmic vacuoles and in the dense granules. It appears therefore that the lectins first bind to their specific carbohydrate receptors on the plasma membrane and are later internalised by the cells. It is suggested that the receptors play an active role in phagocytic function. Furthermore, the fact that the amoeboid microglial cells show similar membrane lectin receptors as the monocyte-derived tissue macrophage supports the hypothesis of their origin from blood monocytes.

Immunocytochemical localization of CR3 complement receptors with OX-42 in amoeboid microglia in postnatal rats

The present study described the labelling of amoeboid microglial cells in the postnatal rat brain with OX-42, an antibody that recognizes type 3 complement receptors CR3 in mononuclear phagocytes. Of the diverse morphological forms of amoeboid microglia present in the corpus callosum in early postnatal (2-5 days) rats, cells with a round regular outline, or showing short stout processes, were the most intensely stained. When traced from the main cell colony into the borderline zone with the cortex, the immunoreactivity of amoeboid microglia that assumed a ramified form was drastically reduced. Examination of materials from the late postnatal (8-12 days) age group showed that the majority of the OX-42 positive cells in the corpus callosum became oval, elongated and ramified. Immunoelectron microscopy confirmed the above observations, and also showed that the immunoreactivity in the round amoeboid microglia was localized in their plasma membrane, surface projections and invaginations, as well as in some of the subsurface vacuoles. The immunoreactivity was reduced in the oval cells, and diminished in the elongated or ramified form. It is proposed that the presence of CR3 membrane receptors in amoeboid microglial cells is related to their active role in endocytosis. These, however, diminish with the growth of the brain.

Synaptic junctions between sympathetic axon terminals and pinealocytes in the monkey Macaca fascicularis

The distribution of axon terminals in the pineal gland of monkeys was studied by electron microscopy. Numerous terminals bearing small pleomorphic agranular and dense-cored vesicles were localized in the perivascular space and among the pinealocytes in the parenchyma in normal monkeys. Following bilateral superior cervical ganglionectomy, they underwent degenerative changes, including the accumulation of glycogen masses, appearance of dense residual bodies and the displacement of synaptic vesicles. Some of these degenerating terminals showed synaptic contacts with the cell bodies of pinealocytes. At the synaptic junction the postsynaptic membrane was thickened asymmetrically. Examples of synaptic contacts were most frequently observed in 5 and 7 days postoperative animals. In the longer surviving (30 days) monkey, most of the axon terminals showed round agranular vesicles, and they were mainly presynaptic to the intrapineal ganglion cells with some of the pinealocytes. They remained structurally unchanged following the resection of both the superior cervical ganglia. A few axon terminals containing small dense-cored vesicles appeared to have survived the initial insult, but some of their vesicles appeared swollen 30 days after the operation. It is concluded from this study that some of the pinealocytes are under the influence by the postganglionic neurons in the superior cervical ganglia through direct synaptic contacts. The intrapineal ganglion cells are postsynaptic to fibres originating exclusively from the central nervous system. Some of these fibres, however, may be presynaptic directly to pinealocytes.

Labelling neurons with fluorescent dyes administered via intravenous, subcutaneous or intraperitoneal route

Fast blue, true blue and fluorogold injected into neonatal and subadult rats via subcutaneous or intraperitoneal route labelled certain forebrain nuclei and the central and peripheral neurons whose axons form the spinal and most cranial nerves. The A1 and A5 noradrenergic neurons, nucleus of the tractus solitarius and some neurons in the nucleus raphe medianus, nucleus reticularis gigantocellularis were also labelled. In addition, occasional fluorescent neurons were seen in the vestibular nucleus, locus coeruleus, fastigial nucleus and Purkinje cell layer of the cerebellum. Some nuclei labelled in neonatal rats hardly showed any labelled neurons in subadult rats. After injection the subarachnoid space, blood vessels in the spinal cord, brain and peripheral ganglia, the circumventricular organs and the choroid plexuses were labelled much faster than the neurons. A similar pattern of labelling was seen in subadult rats receiving intravenous injection of fluorogold. Evans blue and rhodamine injected intravenously failed to label any neurons in the brain or spinal cord.

Degenerative changes of neurons in the superior cervical ganglion following an injection of Ricinus communis agglutinin-60 into the vagus nerve in hamsters

The present study describes neuronal changes in the superior cervical ganglion of hamsters following injection of Ricinus communis agglutinin-60 (RCA-60) into the ipsilateral vagus nerve in the cervical region. There were no noticeable structural changes in the ganglion 1 day after injection. Between 3 and 15 days after injection, a small number of neurons located in the caudal part of the ganglion underwent degenerative changes including disappearance of rough endoplasmic reticulum and cytoplasmic vacuolation. The structural alterations were most acute 7 days after the injection when some neurons showed signs of total vacuolation and lysis. A second phase of neuronal change occurred after longer survival periods extending from 60 to 120 days after injection. The most striking feature of such neurons was darkening of their dendrites associated with abnormally high density cytoplasm that contained mitochondria with disrupted cristae. As distinct from the early phase in which cell necrosis was observed, there was no evidence of cell death of neurons bearing darkened dendrites. Since examples of exfoliation of the affected dendrites and their phagocytosis by satellite cells were extremely rare, it is postulated that these structural alterations are probably reversible but over an extended period. The significance of the two phases of degenerative change is discussed in connection with the acute and possible chronic effects of the toxic lectin. The present study also confirms the presence of postganglionic sympathetic axons in the cervical vagus nerve.

Response of intraventricular macrophages to crotoxin-coated microcarrier beads injected into the lateral ventricle of postnatal rats

Crotoxin-coated microcarrier beads were injected into the lateral ventricles of 5 days old postnatal rats. The morphology of the cells attached to the beads at various time intervals was studied by scanning and transmission electron microscopy. Scanning electron microscopy showed that very few cells were associated with the surface of the beads 18 hours after the injection. After 2 days a large number of spherical cells showing blebs and filopodia were attached to the surface of the beads. One week after the injection, these cells became oval and, in longer survival periods between 2 weeks and 30 days after the injection, the cells developed a flattened or angular cell body bearing a number of radiating slender processes. Transmission electron microscopy of the re-embedded materials from animals killed 2 days after the injection showed many cells with an eccentric nucleus containing dense chromatin masses. Their abundant cytoplasm was endowed with a variable number of lysosome-like dense granules and vacuoles. In longer surviving animals, the cells became elongated with scanty cytoplasm showing relatively fewer dense granules and cytoplasmic vacuoles. It is postulated from this study that the cells attached to the crotoxin-coated beads are derived from the intraventricular macrophages. These are functionally active initially in response to the beads injected. With time, however, they undergo morphological alteration and regress into quiescent cells which are microglia-like.

Ultrastructure of interstitial cells of Cajal at the gastro-oesophageal junction of the monkey (Macaca fascicularis)

The ultrastructure of the interstitial cells of Cajal (ICC) in the oesophagus of the monkey resembled that described in the oesophagus of other mammalian species but differed in their paucity and almost lack of smooth endoplasmic reticulum, caveolae and filaments. The plasmalemma of the ICC was in close contact (20- to 30-nm gaps) with that of smooth muscle cells. This may occasionally take the form of a desmosome, but gap junctions have not been observed. Vesiculated axon profiles, containing large granular or agranular vesicles were in close contact (20- to 30-nm gaps) with the plasmalemma of ICC. In a few vesiculated profiles a presynaptic density could be recognized. The intercalation of the ICC between the vesiculated axon profiles and the smooth muscle cells suggest a role in oesophageal motility. Between 3 and 21 days following bilateral vagotomy some ICC showed regressive changes such as increased electron density and shrinkage of the cytoplasm, crowding of the organelles and dissolution of the nuclear chromatin material. Axon profiles in the vicinity of the affected ICC contained glycogen granules suggesting injury. In late stages, the number of ICC and smooth muscle contacts was reduced. The results suggest that the vagus nerves exert a trophic influence on the ICC and that the intercellular relationships between ICC and smooth muscle cells possess a degree of plasticity. It is tentatively suggested that these vagal effects may be mediated via the oesophageal myenteric ganglia.

Neural degeneration and non-neuronal cellular reactions in the hypoglossal nucleus following an intraneural injection of toxic ricin

The present study describes neuronal degeneration and its accompanying non-neuronal cellular reaction in the hypoglossal nucleus following an intraneural injection of Ricinus communis agglutinin-60 (RCA-60) into the hypoglossal nerve. The first noticeable structural changes were observed in neurons in hamsters killed 3 days after the RCA injection. Drastic alterations occurred in the period extending from the 5th to the 15th postoperative day. Two forms of neuronal degeneration were observed: light and dark types. In the light type, masses of free ribosomes were observed; other changes included the dilation of Golgi saccules and the presence of abnormal mitochondria. In the dark type of degeneration, the cells became condensed with vacuoles in their cytoplasm. Axon terminals presynaptic to the degenerating cells during this period appeared to be normal. A massive influx of mononuclear leucocytes by diapedesis occurred at the large venules. Some of the infiltrated cells were clearly lymphocytes, while others were monocytes which became indistinguishable from indigenous microglia once they were in the neuropil. Neural macrophages, most probably derived both from microglia and the infiltrated monocytes, were engaged in the phagocytosis of neuronal debris. A remarkable finding in the present study was the wide-spread occurrence of dark axon terminals in the neuropil in longer surviving animals (90 and 120 days). The structural alterations, e.g., clumping and swelling of some of the synaptic vesicles in the enhanced cytoplasmic density, suggest that these were undergoing atrophic changes resulting from the long period of dysfunction following the death of postsynaptic neurons induced by RCA.

Scanning electron microscopy of transitory subependymal cysts in the developing midbrain of postnatal rats

Transitory cystic cavities, associated with the subependymal region of the aqueduct in the midbrain of postnatal rats aged 1-15 days, were studied by light and scanning electron microscopy. The walls of these cysts, as observed in scanning electron microscopy, were lined by a dense feltwork of nerve fibres. Two types of cells were identified in the cysts: smaller glioblasts and larger amoeboid microglial cells. The glioblasts were characterized by a smoother cell body with radiating long processes. The amoeboid microglial cells showed blebs and pseudopodia on their surface. They either adhered to the walls or floated freely in the lumen. It is postulated that the formation of the subependymal cysts in the developing brain resulted following the cleavage or breakdown of the nervous tissue due to the expansion of the aqueduct and the brain as a whole. The amoeboid microglial cells in the cysts were probably derived from the extravasated blood monocytes in response to the physical damage ensuing during the formation of the cysts.

Ultrastructure of the pineal gland of the monkey, Macaca fascicularis, with special reference to the presence of synaptic junctions on pinealocytes

The present study described the normal ultrastructure of the monkey pineal gland. The gland was composed of the principal pinealocytes, intramural neurons and glial cells. The nucleus of the pinealocytes was deeply infolded with evenly distributed chromatin materials. The abundant cytoplasm was rich in organelles including the well-developed Golgi apparatuses, multivesicular bodies, dense-cored vesicles and widely scattered free and polyribosomes. A variety of axon terminals was observed and the majority of them contained pleomorphic agranular vesicles with a few large dense-cored vesicles. A few terminals showed flattened vesicles or small dense cored vesicles. Some of the axon terminals formed synaptic contacts with the cell bodies of pinealocytes. These synapses were mainly concentrated in the posterior third of the gland. The occasional intramural neurons observed were postsynaptic to axon terminals containing round agranular vesicles. The sources of the nerve fibres and terminals forming synaptic junctions with pinealocytes and intramural neurons were discussed.

Ultrastructure of the chronically vagotomised atrial myocardium in the monkey (Macaca fascicularis)

The ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis) was studied after bilateral cervical vagotomy and survival times of 100, 175 and 367 days. Changes were observed in the nucleus and the cytoplasm of the myocyte. Sequestered within the nuclei of the affected myocytes were cytoplasmic organelles and inclusions. In the late stages, there was a tendency towards condensation and margination of the heterochromatin. Changes in the cytoplasm included increased glycogen, mitochondrial degeneration and myofibrillar disorganisation and degeneration. There was increased collagen and mononuclear cell infiltration in the extracellular space in the later stages. This study has shown that the long term structural integrity of the atrial myocyte depends on an intact vagal innervation. The survival of the monkey after chronic bilateral vagotomy suggests that this nonhuman primate is a suitable model for functional studies of the parasympathetically denervated heart.

Neuroglial response to neuron injury. A study using intraneural injection of ricinus communis agglutinin-60

The present study has shown the selective destruction of large ventral horn neurons in the lumbosacral cord segments following a single injection of RCA-60 into the sciatic nerve. The neurons appeared to undergo structural alteration beginning 3 days after the RCA application. In the postoperative period extending from 1 to 60 days, degeneration of neurons was progressive and irreversible and this elicited a rapid increase in the number of microglial cells. They were most numerous in the 7 days postoperative animals. The massive microglial cells penetrated the neuropil and appeared to strip off the axon terminals from the postsynaptic somata. Occasional axon terminals were phagocytosed by microglia. The numerous microglial cells often formed a multilayered 'barrier' encircling the somata of the RCA-poisoned neurons which eventually became totally disorganised. It is postulated that in the course of neuronal degeneration induced by RCA, microglial cells serve to prevent the leakage or diffusion of the toxic lectin from the neuronal somata into the neighbouring neuropil. They also function as scavenger cells in the removal of degenerating myelinated axons in the longer surviving rats. Oligodendrocytes do not appear to react actively to the degeneration process. However, astrocytes showed a significant increase in the 7 and 15 day postoperative rats and this coincided with the presence of mitotic astrocytes in the same period.

Development of the various glial cell types in the cerebral cortex of postnatal rats

The present quantitative study in the postnatal rats showed the rapid growth of the various glial cell types in the cerebral cortex. Among them, the increase of microglia was most dramatic. The increase was about 15 times, covering a period of 15 days extending from 5 days of age to 20 days. The majority of the microglia observed were in the outer third of the cortex. During the same period, the number of oligodendrocytes and astrocytes also showed a steady but moderate increase. The increase of oligodendrocytes was most significant between 5 and 10 days. Their density was greater in the inner third of the cortex. Astrocytes were distributed uniformly throughout. Examination of the cerebral cortex in 1- to 3-day-old rats by electron microscopy showed sporadic ameboid microglia cells and glioblasts. The possibility that they served as the precursor cells of microglia and macroglia (astrocytes and oligodendrocytes), respectively, was considered.

An electron microscopic study of the nodose (inferior vagal) ganglion cells in the monkey

The present study described the normal ultrastructure of the monkey nodose ganglion cells. Furthermore, experimental monkeys were subjected to supranodose vagotomy in order to ascertain if the present cell bodies would undergo degeneration following severance of their central processes. In the normal materials, most of the ganglion cells possessed a single neurite. However, occasional cells bearing more than one process in a sectioned profile were observed. The neurites, ranging between 2-4 microns in diameter, displayed a relatively regular contour. Their cytoplasm contained parallel arrays of microtubules, ribosomes, endoplasmic reticulum and slender mitochondria. The electron density of some of these neurites was abnormally high. Embedded in these darkened neurites were a variable number of swollen mitochondria characterized by disrupted cristae. Axon terminals containing round agranular and a few large dense cored vesicles formed synaptic contacts primarily with the neurites of some of the ganglion cells. Three days after superanodose vagotomy, darkened neurites were more commonly observed but their incidence was comparable to that of the normal ganglion in longer survival animals. Another reactive change was the appearance of axon terminals undergoing various degrees of degeneration. There was no evidence of cell death in the duration studied. It was concluded from this study that the occasional darkened neurites from the normal ganglion cells was probably undergoing 'spontaneous degeneration' which appeared to be accentuated when their central process was severed by supranodose vagotomy. The degeneration of axon terminals associated with some of the ganglion cells following the vagotomy suggested that they were derived from vagal descending fibres which were undergoing anterograde degeneration. The presence of synapses on some of the ganglion cells was also discussed and the possibility considered that the latter may represent 'aberrant' or displaced autonomic neurons.

Infiltration of carbon-labelled monocytes into the dorsal motor nucleus following an intraneural injection of ricinus communis agglutinin-60 into the vagus nerve in rats

A marked increase in the number of non-neuronal cells occurred in the neuropil of the ipsilateral dorsal motor nucleus (DMN) 6 days after an intraneural injection of Ricinus communis agglutinin-60 into the vagus nerve in the cervical region of rats. Other structural changes in the DMN were the hypertrophy and reduction in number of the neurons. In order to verify the origin of the non-neuronal cells, a single intravenous injection of carbon was administered into these rats 4 days before, simultaneously, or 4 days after, the injection of the RCA-60. Thus, in rats given carbon 4 days before the RCA-60 injection, none of the non-neuronal cells were labelled. A few labelled cells, however, were observed in rats given carbon and RCA-60 simultaneously. Labelled non-neuronal cells were most common in rats given carbon 4 days after the RCA-60 injection. They were located in the neuropil as well as in the walls of blood vessels. Some blood elements in the lumen of blood vessels in the DMN were also labelled by carbon. Histochemical study at the electron microscopical level showed that some of the non-neuronal cells present in the neuropil of DMN were stained positively for non-specific esterase. They were located in the perivascular region and in the neuropil far removed from the blood vessels. Occasional non-specific esterase-positive mononuclear cells were observed seemingly in their passage through the endothelium of blood vessels. It was concluded from this study that a small proportion of non-neuronal cells which appear in the DMN following a RCA-60 injection into the vagus nerve are derived from blood monocytes. The infiltration of these cells, which had been labelled by intravenous carbon injection, is probably elicited by the degenerating neurons destroyed by the retrograde transport of RCA-60.

Electron-microscopical study of the choroid plexus and epiplexus cells in cats following a cisternal injection of crotoxin complex

The choroid plexus and its associated epiplexus cells in the fourth ventricle in cats were studied with scanning and transmission electron microscopy (SEM, TEM) following a cisternal injection of crotoxin complex (phospholipase A2). In SEM, the epiplexus cells of the control animals were predominantly stellate with long radiating processes. At 2 h after the administration of crotoxin complex, these radiating processes flattened out forming sheet-like membranes covering the ventricular surface of the choroid epithelial cells. The membranous coverings remained extended in 5-hour-survival cats. Numerous blebs of different sizes were observed in areas that were not covered by the cytoplasmic membrane in 5-hour animals. Some of the blebs appeared to have ruptured. In TEM, the microvilli of the choroid epithelial cells in crotoxin complex-treated rats were dilated. The luminal surface of the epithelial cells showed eruption of blebs filled with amorphous materials. Pinocytotic vesicles increased in number in the apical cytoplasm. The lumen of the ventricle often contained portions of cytoplasm believed to be derived from the extrusion of the blebs. These appeared to be engulfed by the overlying epiplexus cells. It was concluded that the injected crotoxin complex stimulated both the secretory as well as pinocytotic activity of the choroid epithelial cells. The phagocytosis of the secretory products from the epithelial cells by epiplexus cells suggests a close functional relationship between the two cell types.

Origin and fate of neural macrophages in a stab wound of the brain of the young rat

Colloidal carbon was injected intravenously into young rats to label circulating monocytes before making a stab wound in the brain. The rats were killed 3-16 days after the stab wound. Demonstration of non-specific esterase, thiamine pyrophosphatase and 5'-nucleotidase was carried out on the carbon-labelled macrophages at the site of lesion at various survival times. In rats killed 3-5 days after the injury numerous carbon-labelled macrophages were present in the needle passage as well as in the marginal area of the lesion and they showed a positive reaction for non-specific esterase. The reaction of the enzyme was found in some of the dense bodies in the form of punctate precipitates. The reaction for thiamine pyrophosphatase was seen in the Golgi saccules as well as on the plasma membrane, although in the latter the reaction was weaker. Intense reaction for 5'-nucleotidase was localised over the plasma membrane as well as over the dense bodies. The carbon-labelled macrophages displaying the activities of the above enzymes in the 3-5 days postoperative group were of the round type. However, in the 8-16 postoperative days animals, the cells were either oval or had assumed an elongated outline resembling the microglial cells seen in the tissue taken from the normal side. It is concluded that circulating monocytes are a main source of brain macrophage in traumatic brain lesions. In the healing process of the wound some of the cells regress to become microglial cells as shown by the presence of the carbon particles as well as non-specific esterase, thiamine pyrophosphatase and 5'-nucleotidase activity in the various stages of structural transformation.

The dorsal motor nucleus of the vagus nerve of the hamster: ultrastructure of vagal neurons and their responses to vagotomy

The dorsal motor nucleus of the vagus nerve of the hamster was studied electron microscopically. Two types of neuron distinguished by their ultrastructural features were identified. Type I cells, which had an average dimension of 20 X 12 microns, were rich in cytoplasmic organelles. The smaller Type II cells (15 X 9 microns) had scanty cytoplasm and a deeply indented nucleus. Type I cells were labelled by horseradish peroxidase (HRP) following its injection into the vagus nerve, whereas Type II cells were not. Seven days after cervical vagotomy, Type I neurons underwent typical retrograde degeneration. By the fourteenth postoperative day, the density of the cells was greatly enhanced, an early sign of cell death. Type II cells were not affected by vagotomy. With the electron microscope, three types of axon terminals were identified in the neuropil of the DMN: (1) boutons containing round agranular vesicles, (2) boutons containing round and elongated dense-cored vesicles, and (3) boutons containing pleomorphic agranular vesicles, a few of which were dense-cored. All of them showed synaptic contacts with HRP-labelled dendrites. Occasional boutons containing pleomorphic vesicles showed degenerative changes after vagotomy. Moreover, a few of them were seen to contain HRP granules. The observations made in the present study strongly suggest that the Type I neurons send efferent fibres to the vagus nerve. The Type II cells which are not labelled by HRP nor affected by vagotomy, probably serve as interneurons, or they may be cells projecting to other areas of the brain.

Effects of intraneural injection of Ricinus communis agglutinin-60 into rat vagus nerve

The dorsal motor nucleus (DMN) of the rat was studied at various survival periods following an intraneural injection of Ricinus communis agglutinin-60 (RCA-60) into the vagus nerve at the mid-cervical region. No obvious structural changes were noted in the DMN 2 and 4 days after the injection of RCA-60. At 5 and 6 days after the RCA-60 injection, the larger neurons (measuring 19 X 12 microns) in the DMN underwent chromatolytic degeneration whereas the smaller ones (measuring 10 X 6 microns), characterized by their infolded nuclei, remained unaffected. The majority of the degenerating DMN neurons became pale and crenated in outline. Other structural changes included swollen mitochondria with disrupted cristae and profiles of rough endoplasmic reticulum denuded of ribosome particles. A few of the degenerating neurons became extremely condensed and darkened. Axon terminals which showed synaptic contacts with these cells remained normal. Both pale and darkened degenerating dendrites, derived from the degenerating neurons, were present in the neuropil. In addition to these, degenerating axon terminals with clumping or swelling of synaptic vesicles were also present. They were presynaptic to dendrites of various sizes. Massive infiltration of mononuclear cells occurred in the DMN. These cells reached the DMN by diapedesis and were actively engaged in the phagocytosis of degenerating neuronal elements. While most of the invading cells transformed into active neuronal macrophages, some of them eventually died in the neuropil of the DMN. Light microscopic study by Fink-Heimer's method for degenerating fibres and terminals revealed their distribution to the DMN, nucleus of the tractus solitarius, nucleus commissuralis, dorsolateral and lateral part of the hypoglossal nucleus and the area postrema. It was concluded from this study that RCA-60, when injected into the cervical vagus was retrogradely transported to the cell body of the DMN neurons of the larger category. The selective destruction of the DMN neurons by RCA-60 elicited a massive infiltration of mononuclear cells which gave rise to the neural macrophages. The RCA-60 injected also killed the vagal sensory neurons as demonstrated by the numerous degenerating fibres and axon terminals in the DMN which would represent their central processes.

Localisation of thiamine pyrophosphatase in the amoeboid microglial cells in the brain of postnatal rats

The activity of TPPase in amoeboid microglial cells has been studied in postnatal rats. When examined with the light microscope such cells in 1-10 days old rats perfused with 4% paraformaldehyde were round and showed a dark brown reaction in their cytoplasm. In older rats (10-30 days), the reactive amoeboid microglial cells were oval, flattened or branched. Electron microscopic examination revealed that the reaction product was seen on the plasma membrane, in the subplasmalemmal vacuoles, in tubular invaginations of plasma membrane and in the transface of the Golgi saccules. In rats perfused with the mixed aldehyde solution, the amoeboid microglial cells did not show a positive TPPase reaction with the light microscope but at the ultrastructural level a weak reaction was seen in some cytoplasmic vacuoles and in the Golgi saccules.

Effects of bilateral vagotomy on the ultrastructure of the cardiac ganglia in the monkey (Macaca fascicularis)

This study describes the effects of bilateral vagotomy on the ultrastructure of the cardiac ganglia of the monkey (Macaca fascicularis). One to three days after bilateral vagotomy there is widespread glycogen accumulation in the cytoplasm of the principal cardiac neurons. This is associated with distension of the granular endoplasmic reticulum and the loss of ribosomes from the cisternae. Between five and ten days after operation, about 10% of the neuronal profiles show an overall increase in electron density and intense darkening of the dendrites. From twenty one to twenty eight days postoperatively, the majority of the neuronal profiles have pale cytoplasm with reduction in granular endoplasmic reticulum and polyribosomes. The plasma membrane of the neuron is ruffled over the major portion of its surface. The satellite cells, which are reactive throughout the course of the experiments, exhibit a phagocytic capacity at this stage by removing portions of the neuronal cytoplasm. Vacuolation of the neuronal cytoplasm to a variable degree occurs in a small number of profiles between five to twenty eight days. The results suggest that bilateral vagotomy causes a widespread disturbance in the metabolic activity of the cardiac neurons. This is followed by transneuronal degenerative changes that are of a prolonged nature.

Effects of vagotomy on the ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis)

The ultrastructure of the atrial myocardium in the monkey (Macaca fascicularis) was studied after bilateral cervical vagotomy and survival times of 1, 3, 5, 7, 10, 21 and 28 days. During the first week after vagotomy, a few atrial cells showed a reduction in the sarcoplasm, crowding of the myofibrils, peripheral dispersion and reduced intercristal density of the mitochondria and increased sarcoplasmic reticulum and glycogen particles. In some profiles, there was increased electron density and granularity at the I bands and the intercalated discs. The number of such affected cells increased in the subsequent days such that by 21 to 28 days about 50% of the cells were estimated to be affected. During the latter stages further changes included, the degradation of the myofilaments and increased electron density, disorganisation and disintegration of the digital extensions at the intercalated discs. Throughout the experiments there was a leucocytic infiltration, more evident in the longer survival times.

Ultrastructure of the area postrema of the monkey, Macaca fascicularis

The area postrema of the monkey, Macaca fascicularis, were a pair of oval organs at the caudal end of the floor of fourth ventricle. Their ependymal lining was covered by well-developed microvilli with occasional overlying supraependymal cells. Two types of lining cells were present: pyramidad- and flattened cells. The pyramidal cell showed a long extending basal process resting on the underlying blood vessels. In transmission electron microscopy, the organ showed numerous fenestrated sinusoids characterized by a distinct perivascular space containing mast cells, macrophages and collagen fibrils. The parenchyma of the organ was composed of neurons and glial elements. Only one type of neuron ranging from 9.5 to 15 microns could be distinguished. The neurons contained an indented nucleus surrounded by organelle rich cytoplasm. The soma of the neuron was enclosed by glial element resembling astrocyte. The glial processes terminated on the blood vessel where they were "tunnelled" by a variable number of nerve fibres some of which gained a direct access to the external basal lamina of the perivascular space. Synapses in the neuropil predominantly of the axodendritic variety were observed. Axon terminals containing round agranular vesicles were seen to make synaptic contacts with the neuronal soma. No structural changes were observed in the area postrema following bilateral cervical vagotomy. However, degenerating axon terminals were observed in the subpostremal zone 7, 14 and 21 days after vagotomy suggesting a direct afferent projection into this region.

Transneuronal changes in the myenteric ganglia of the monkey following vagotomy

This study describes the ultrastructural changes in the myenteric ganglia at the gastro-oesophageal junction of the monkey (Macaca fascicularis) after unilateral cervical vagotomy. Degenerating vagal fibres occurred within the ganglia and they represented only a small fraction of the endings present. The most striking finding at days 1-3 after vagotomy, was that some dendrites of myenteric neurons had a distinctly increased electron density which ranged from a moderate darkening of the cytoplasm to an almost complete blackening of the profile. This regressive change was widespread; however, apparently, it did not affect more than one dendrite in any neuron. Darkened dendrites were no longer found at 5-7 days after vagotomy, and they were not present in control ganglia. It is concluded that the degeneration of vagal endings is accompanied by transneuronal regressive changes that are very rapid in onset, are short lasting and possibly fully reversible, and affect only one dendrite of the neuron. It is suggested that the process observed represents a novel form of transneuronal influence.

Ultrastructural changes in the dorsal motor nucleus of monkey following bilateral cervical vagotomy

The neurons of the dorsal motor nucleus (DMN) of the monkey (Macaca fascicularis) were of two main types: small (13 X 8 micron) and medium-sized (20 X 13 micron). The latter, which were the predominant form, contained a pale oval nucleus surrounded by organelle-rich cytoplasm. Between one and three long principal dendrites per section profile arose from each of the somata. Both axosomatic and axodendritic synapses were seen on these cells although the latter were more common. No structural changes were noted in the DMN 1-3 days after bilateral cervical vagotomy. Some of the dendrites of the medium-sized axotomized vagal neurons appeared darkened 5-10 days after the operation. With longer surviving intervals, i.e. 21 and 28 days after operation, darkened dendrites were more commonly seen and the cytoplasmic density of these dendrites was dramatically enhanced. Their mitochondria were pale and some of them also showed vesiculation. Both normal and degenerating axon terminals were seen to form synaptic contacts with the darkened dendrites. The degenerating axon terminals were characterized by the clumping of their round agranular vesicles. Both darkened dendrites and degenerating axon terminals were phagocytosed by hypertrophied astrocytes and activated microglial cells. Blood elements infiltrating into the DMN were a possible source for some of the neural macrophages. It was concluded from the present study that the dendrites of the vagal neurons were the first structures to degenerate in axotomy and these were subsequently removed by glial elements. Degenerating axon terminals on the darkened dendrites could represent endings of the central processes of peripheral vagal ganglion cells that had undergone transganglionic degeneration after damage to their peripheral processes.

Labelling of amoeboid microglial cells in rats of various ages following an intravenous injection of horseradish peroxidase

The macrophagic amoeboid microglial cells in the corpus callosum of postnatal rats were labelled following an intravenous injection of horseradish peroxidase (HRP). The earliest time when these cells were labelled was 3 h after the injection of HRP in postnatal (1-10 days) rats. Similar cells around the mesencephalic aqueduct and the fourth ventricle were also labelled. These cells, however, were weakly labelled in developing (11-20 days) and unlabelled in weaning (21-30 days) rats. The results suggest that in the postnatal rats, the HRP passed through the endothelial lining of the blood vessels and was then ingested by the amoeboid microglial cells. In the developing and older rats, the wall of blood vessels had developed fully thereby preventing the free passage of HRP into the brain tissues.

Epiplexus cells in the embryos of the turtle, Trionyx sinensis

The epiplexus cells in the embryos of the turtle, Trionyx sinensis were examined in scanning and transmission electron microscopy. Although the cells showed diverse morphological forms, the majority of the cells however had a stellate cell body each with 3-5 radiating long processes with secondary processes terminating on the ventricular surface of the choroid epithelial cells. The cell body was relatively smooth. Occasional epiplexus cells displayed blebs on their cell bodies and processes. In the transmission electron microscope, the nucleus contained coarse chromatin clumps. The cytoplasm was endowed with large lysosomal bodies. Based on these observations, it is suggested the epiplexus cells in the turtle embryos are phagocytic even though most of them probably are at the resting stage.

Light and scanning electron microscopical study of the cavernous sinus of the monkey, Macaca fascicularis

The cavernous sinus of Macaca fascicularis is in many respects similar to the human sinus. It consists predominantly of one main venous channel that, together with the internal carotid artery, occupies a meningo-endocranial compartment lateral to the pituitary gland. Trabeculae are few and do not in any way cause the sinus to appear cavernous. They are mostly flattened in the direction of the main venous channel. Cranial nerves three, four, six and the ophthalmic division of five are all located in the lateral wall of the meningo-endocranial compartment with cranial nerve six located most medially adjacent to the internal carotid artery.

Transformation of amoeboid microglial cells into microglia in the corpus callosum of the postnatal rat brain. An electron microscopical study

An electron microscope study of the corpus callosum in postnatal rats of various ages was carried out to elucidate the fate of the amoeboid microglial cells. The cells present in the corpus callosum of younger rats (3-5 days) were round and showed an eccentric nucleus with marginal chromatin. They displayed numerous lysosomal granules and vacuoles in the cytoplasm. In older animals, i.e., from 7 days onwards some of the cells became oval so that by 15-20 days of age most of the cells were elongated and branched. In the latter, the cells showed a flattened or angular nucleus with dense chromatin clumps. The cytoplasm showed fewer lysosomal granules and vacuoles which were absent in cells of 20 day old animals. Quantitative measurements showed that there was a gradual diminution in the amount of cytoplasm at the cell body of amoeboid microglial cells with age, so that by the age of 20 days the cells were reduced to less than one-third of their original size as seen in 3 day old rats. The reduction of cytoplasm at the cell body is probably because it is channelled to the cytoplasmic processes which are evident in older rats. Some cytoplasm may have been extruded and phagocytosed by companion cell types.

An electron microscopical study of epiplexus and supraependymal cells in the prenatal rat brain following a maternal injection of 6-aminonicotinamide

Epiplexus and supraependymal cells in the lateral ventricles of both normal and experimental prenatal rats were studied with the scanning and transmission electron microscopes. In normal rats, the epiplexus cells, which were predominantly spindle shaped, displayed long filopodial processes inserted into the palisade of microvilli of the choroid epithelium; their cell bodies appeared relatively smooth with occasional blebs. In experimental animals in which the mother was given an injection of 6-aminonicotinamide (6-AN), the cell bodies of the epiplexus cells became extremely ruffled or blebbed. They were seen to be actively involved in the phagocytosis of leaked erythrocytes. The behaviour of supraependymal cells was rather similar. When stimulated by a 6-AN injection, the cells showed large prominent blebs and they too were engaged in phagocytic activity. It was concluded from this study that intraventricular macrophages bearing numerous filopodia are the less active cells whereas those bearing shorter processes, in particular, blebs or ruffles, represent the active form.

Transneuronal degeneration in the heart and gastro-oesophageal junction in the monkey (Macaca fascicularis) following unilateral vagotomy

This study describes, at the ultrastructural level, the transneuronal degeneration that occurs in the atrial and gastro-oesophageal ganglia of the monkey following unilateral vagotomy. One day after unilateral vagotomy, some postganglionic neurons showed an overall increase in electron density, with the dendrites darkening first. Three days after the operation, the dendrites appeared jet black and their intracytoplasmic organelles were hardly distinguishable except for some pale mitochondria. The perikarya of the affected neurons showed a slight increase in electron density. Five and seven days after the operation, satellite cells and macrophages with engulfed dendritic processes were commonly observed. The majority of the postganglionic neurons appeared normal, except for the presence of numerous macrophages in their vicinities. The results of the study suggest that transneuronal degeneration occurs very rapidly in the heart and gastro-oesophageal junction of the monkey, Macaca fascicularis, and that most of the affected neurons have recovered one week after the operation.

An ultrastructural study of the effects of right cervical sympathectomy on the sinuatrial and atrioventricular nodes in the heart of the monkey (Macaca fascicularis)

This study describes the ultrastructural changes in the sinuatrial and atrioventricular nodes of the heart of the monkey (Macaca fascicularis) after right cervical sympathectomy. Obvious changes in the nodal cells were seen one day after operation. Numerous glycogen particles grouped together to form electron-dense patches containing vacuoles in the cytoplasm. At three days after operation, intracellular organelles exhibited fragmentation and dissolution. By five and seven days after operation, the affected cells were vacuolated and some were swollen and appeared to have degenerated. Simultaneously, there was massive infiltration of macrophages were present nodal tissues. Axon profiles and terminals showing various degrees of degeneration were present in the vicinity of the nodal cells throughout the period of study. It is concluded that right cervical sympathectomy resulted in a rapid degeneration in some of the cells in the sinuatrial and atrioventricular nodes.

Cytochemical localisation of 5'-nucleotidase in amoeboid microglial cells in postnatal rats

The activity of the enzyme 5'-nucleotidase has been studied in postnatal rats. In 5 days old rats, under the light microscope, round amoeboid microglial cells were stained for the enzyme. The cytoplasm was stained dark brown, with a clear nucleus. In older rats, the reactive amoeboid microglial cells assumed an oval or flattened form. In the electron microscope, in 5 days old rat, reaction product was deposited at various sites in the cytoplasm of the amoeboid microglial cells: over lysosomes in subplasmalemmal vacuoles as well as in saccules of the Golgi apparatus. In older rats the cells showed reaction over the lysosomes. The cells became oval and elongated.

A scanning and transmission electron microscopic study of amoeboid microglial cells in the prenatal rat brain following a maternal injection of 6-aminonicotinamide

The amoeboid microglial cells in the cavum septum pellucidum of prenatal rats aged 22 days post-conception were studied by scanning and transmission electron microscopy. The cells in normal rats had numerous filopodia, while those in experimental animals whose mothers had received an intraperitoneal injection of 6-aminonicotinamide (6-AN) on the 14th day of gestation, showed an abundance of blebs or possessed smoother contours. Extravasated red blood cells and nerve fibres were seen to be engulfed by the amoeboid cells of the blebbed as well as the smooth type. Transmission electron microscopy confirmed the phagocytosis of red blood cells and nerve fibres in the cytoplasm of the amoeboid cells. It is postulated that the amoeboid microglial cells responded to 6-AN by increasing their phagocytic activity and by changing their surface morphology. The factors leading to such alterations are discussed.

An ultrastructural study of the non-neuronal cells in the cardiac ganglia of the monkey (Macaca fascicularis) following unilateral vagotomy

Non-neuronal cells in the intracardiac ganglia of the monkey (Macaca fascicularis) showed rapid responses to neuronal damage following unilateral vagotomy. Most of the satellite cells appeared normal at one day after operation. However, from three to seven days after operation they were transformed into phagocytic cells, thereby engulfing the transneuronally affected dendrites of the neurons. By seven days after operation, most of the affected dendrites from the neuronal soma were engulfed by the satellite cells. Few macrophages were seen amongst the neurons of the intracardiac ganglia at one day. In monkeys at three to seven days after operation there was a massive infiltration of macrophages into the vicinity of the affected neurons. Several of these macrophages were filled with numerous phagosomes containing partially digested debris. Macrophages also accumulated perivascularly and some intravascular monocytes lay in conformations suggesting that they were undergoing diapedesis. Therefore, perivascular macrophages are probably derived from the circulating monocytes, though the possibility of them being transformed fibroblasts should not be excluded. Schwann cells appeared to be the main scavengers in removing the degenerating myelinated and unmyelinated axons in the intracardiac ganglia after unilateral vagotomy.

Light and electron microscopic features of the structure and innervation of the gastro-oesophageal junction of the monkey (Macaca fascicularis)

This study describes the light and electron microscopical features of the gastrooesophageal junction of the monkey (Macaca fascicularis). In longitudinal histological sections, the circular muscle layer at the junction is greatly thickened. This thickening is further demonstrated in a reconstructed model of the whole circular muscle coat and is therefore referred to as a 'sphincter'. Quantitative ultrastructural studies on the circular muscle at the gastro-oesophageal junction or 'sphincter' show denser nerves when compared with the body of the stomach or the pyloro-duodenal junction. Two types of vesiculated axonal profiles are recognised in the circular muscle layer of the stomach in normal animals: round agranular vesicles which measure between 40 and 60 nm and large granular vesicles measuring 80-120 nm. Animals which received 5-hydroxydopamine treatment show, however, in addition, small dense-cored vesicles measuring 30-60 nm. The gastro-oesophageal and the pyloro-duodenal junctions contain more vesiculated axonal profiles than the body of the stomach. Moreover, both junctions show a predominance of axons with granular vesicles when compared with the body of the stomach.