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In neurological diseases, the actions of microglia, the resident myeloid cells of the CNS parenchyma, may diverge from, or intersect with, those of recruited monocytes to drive immune-mediated pathology. However, defining the precise roles of each cell type has historically been impeded by the lack of discriminating markers and experimental systems capable of accurately identifying them. Our ability to distinguish microglia from monocytes in neuroinflammation has advanced with single-cell technologies, new markers and drugs that identify and deplete them, respectively. Nevertheless, the focus of individual studies on particular cell types, diseases or experimental approaches has limited our ability to connect phenotype and function more widely and across diverse CNS pathologies. Here, we critically review, tabulate and integrate the disease-specific functions and immune profiles of microglia and monocytes to provide a comprehensive atlas of myeloid responses in viral encephalitis, demyelination, neurodegeneration and ischemic injury. In emphasizing the differential roles of microglia and monocytes in the severe neuroinflammatory disease of viral encephalitis, we connect inflammatory pathways common to equally incapacitating diseases with less severe inflammation. We examine these findings in the context of human studies and highlight the benefits and inherent limitations of animal models that may impede or facilitate clinical translation. This enables us to highlight common and contrasting, non-redundant and often opposing roles of microglia and monocytes in disease that could be targeted therapeutically., (© 2021. The Author(s).)

The aim of this study was to assess the variation between neuropathologists in the diagnosis of common dementia syndromes when multiple published protocols are applied. Fourteen out of 18 Australian neuropathologists participated in diagnosing 20 cases (16 cases of dementia, 4 age-matched controls) using consensus diagnostic methods. Diagnostic criteria, clinical synopses and slides from multiple brain regions were sent to participants who were asked for case diagnoses. Diagnostic sensitivity, specificity, predictive value, accuracy and variability were determined using percentage agreement and kappa statistics. Using CERAD criteria, there was a high inter-rater agreement for cases with probable and definite Alzheimer's disease but low agreement for cases with possible Alzheimer's disease. Braak staging and the application of criteria for dementia with Lewy bodies also resulted in high inter-rater agreement. There was poor agreement for the diagnosis of frontotemporal dementia and for identifying small vessel disease. Participants rarely diagnosed more than one disease in any case. To improve efficiency when applying multiple diagnostic criteria, several simplifications were proposed and tested on 5 of the original 20 cases. Inter-rater reliability for the diagnosis of Alzheimer's disease and dementia with Lewy bodies significantly improved. Further development of simple and accurate methods to identify small vessel lesions and diagnose frontotemporal dementia is warranted.

Inorganic mercury has been proposed as a neurotoxin that could cause sporadic motor neuron disease (SMND). We were therefore interested to see if mercury could be detected in the upper and lower motor neurons of SMND patients, and if mercury accumulated within motor neurons during life. Paraffin sections of formalin-fixed spinal cord (22 control adults, 20 SMND adults, 25 infants) and frontal primary motor cortex (9 control adults, 18 SMND adults, 20 infants) were stained with silver nitrate autometallography to detect ionic mercury. Mercury was found in the spinal motor neurons of 36% of adult control cases and 45% of adult SMND cases, with no significant difference between groups. No mercury was seen in infant spinal motor neurons, or in any adult or infant corticomotoneurons. In conclusion, many humans appear to accumulate mercury in their spinal motor neurons by the time they are adults, but mercury does not appear to play a major role in the loss of upper or lower motor neurons in SMND.

A 24-year-old man injected himself intravenously with metallic mercury in a suicide attempt, and died 5 months later after cutting his wrists. The brain was removed at postmortem and 7-micron paraffin sections were cut from representative blocks. Dense deposits of mercury were found on autometallography in large cortical motor neurons, but in no other cerebral neurons. Smaller mercury deposits were found in the brain stem (in the mesencephalic trigeminal nucleus, noradrenergic neurons, and in neurons for extraocular muscles), the cerebellum (in the dentate nucleus) and in lateral motor neurons in the C2/3 spinal cord. Mercury deposits were found in glial cells in all regions. The finding that elemental mercury enters human cortical motor neurons in preference to other cerebral neurons raises the possibility that this neurotoxin may play a part in the pathogenesis of some human motor neuron diseases.

It has been suggested that delayed development in either the peripheral or central nervous system could underlie the sudden infant death syndrome (SIDS). We studied the phrenic nerve in an attempt to find if maturation in this nerve was delayed in SIDS, and to see if fiber size differences could explain the paucity of type I muscle fibers described in SIDS diaphragms. Samples of phrenic nerves were taken at post mortem from 11 SIDS and 10 control infants. Myelinated fibers were quantitated in 1-micron silver-stained resin sections using a semi-automatic image analysis system. The numbers and sizes of all myelinated fibers in the right phrenic nerve were compared between SIDS and control groups. The proportion of small (less than 6-microns diameter) phrenic nerve fibers was similar in SIDS (53.1%, SD 12.8%) and control (53.6%, SD 14.0%) cases. The proportion of small fibers decreased with increasing age at an equivalent rate in both SIDS and control groups. Total numbers of myelinated fibers were similar in the right phrenic nerves of SIDS (3429, SD 440) and control (3457, SD 507) infants, but varied widely between cases and were not correlated with age. Maturation of the phrenic nerve, as judged by increasing size of myelinated fibers, is therefore similar in SIDS and control infants. This argues against a widespread development delay in SIDS. No changes in phrenic nerve fiber sizes were found to account for the finding of fewer type I fibers in SIDS diaphragms.

A 66-year-old woman presented with an alien limb syndrome without dementia. The course of her illness was unremitting and at autopsy 6 years later her diagnosis was confirmed as corticobasal degeneration without Alzheimer-type pathology. Although the presence of ballooned achromatic cortical neurons and cell loss from the substantia nigra distinguishes such patients, the site and density of achromatic neurons has not previously been quantified. We show that immunohistochemistry for the cell stress protein ubiquitin selectively stains these achromatic neurons, whereas they do not stain for abnormally phosphorylated tau protein. Phosphorylated neurofilament antibodies recognise both ballooned and non-ballooned neurons. In this case, high densities of ubiquitin-positive ballooned neurons were found in frontal cortical regions with the highest densities in layers V and VI of the anterior cingulate cortex. In addition, high densities of ubiquitin-positive ballooned neurons were found in the insular cortex, claustrum and amygdala. These results confirm past reports of frontal pathology, but show that there is also considerable pathology in insular and parahippocampal cortical regions and some subcortical regions. Our findings suggest that the distribution and staining characteristics of ballooned neurons in corticobasal degeneration may help to differentiate these cases pathologically, while the absence of dementia appears to be an important clinical criterion.

After lead (Pb) is injected into striated muscle it binds to the sarcolemma of the neuromuscular junction (NMJ) and crosses into the terminal axons of motor neurons. To find out whether this intra-axonal accumulation of Pb is due to active transport or to diffusion down a concentration gradient, Pb uptake into motor axons of mice was studied at active and inactive NMJs. Twenty-four hours after sciatic nerve crush, 0.1 ml of 5% lead nitrate was injected into the tibialis anterior muscle and 30 min later the location of Pb was sought with electron microscopy and X-ray elemental analysis. A greatly reduced amount of Pb entered the axons after nerve crush compared to non-nerve crush animals, indicating that an active NMJ is required for intra-axonal Pb accumulation. To test if Pb could be entering the axon via recycling vesicles, botulinum toxin (BoTx) was injected into the muscle 24 h before Pb injection. There was no difference in intra-axonal Pb uptake in control and BoTx-injected animals, indicating that Pb is unlikely to use recycled vesicles to enter the axon.

The location and number of brain stem serotonin-synthesizing neurons were analyzed in 11 patients with Alzheimer's disease (AD) and 5 age-matched controls using immunohistochemical techniques. In addition, the number of neuritic plaques and neurofibrillary tangles in the cortex and brain stem raphe was evaluated, as was the number of Nissl-stained raphe neurons. AD patients could be classified into two groups based on their raphe pathology; patients with such pathology (AD+) and those without (AD-). The number of large raphe neurons correlated significantly with the number of serotonin-synthesizing neurons in control material, indicating that all large neurons were serotonergic. This relationship was not apparent in AD+ patients, in whom the number of serotonin-synthesizing neurons correlated with the number of neurofibrillary tangles in the raphe of these patients. This indicates that in AD+ patients the serotonin-synthesizing neurons were selectively affected. There was no correlation between raphe and cortical pathology or raphe pathology and patient sex, age, mini-mental score or depression score, even when such scores were weighted for the interval between testing and death. There was a trend for the raphe pathology to correlate with the age of onset and duration of dementia and the Blessed dementia score in AD+ patients. Most AD+ patients with severe raphe lesions had clinical dementia only, while AD- patients had additional clinical features. The raphe lesions were more dramatic in AD+ patients with a rapid progression of symptoms.

A 64-year-old woman developed septicemia and a generalized peripheral neuropathy while being ventilated postoperatively. No cause for the neuropathy could be found during life. At autopsy she was found to have infective endocarditis and multifocal inflammatory lesions in the central and peripheral nervous systems, consistent with damage due to septic emboli. Infective endocarditis may be a cause of a generalized polyradiculoneuropathy and could be responsible for a proportion of cases of "critical illness polyneuropathy".

A brother and sister developed a slowly progressive neurological disorder with cerebellar and pyramidal signs and mild dementia. The brother developed symptoms at 6 months and died aged 11 years; the sister developed symptoms at 3 years and died aged 18 years. At post-mortem both had severe widespread central nervous system demyelination with islands of preserved myelin, and small amounts of sudanophilic lipid products. Metachromatic material, globoid cells, and adrenal abnormalities were not seen. The features were those of Pelizaeus-Merzbacher disease (PMD). It has been proposed, on the basis of only a few family studies, that PMD is an X-linked recessive disorder. These cases suggest that autosomal recessive inheritance may occur.