Cell number changes in Alzheimer's disease relate to dementia, not to plaques and tangles.

Alzheimer's disease is the commonest cause of dementia in the elderly, but its pathological determinants are still debated. Amyloid-ß plaques and neurofibrillary tangles have been implicated either directly as disruptors of neural function, or indirectly by precipitating neuronal death and thus causing a reduction in neuronal number. Alternatively, the initial cognitive decline has been attributed to subtle intracellular events caused by amyloid-ß oligomers, resulting in dementia after massive synaptic dysfunction followed by neuronal degeneration and death. To investigate whether Alzheimer's disease is associated with changes in the absolute cell numbers of ageing brains, we used the isotropic fractionator, a novel technique designed to determine the absolute cellular composition of brain regions. We investigated whether plaques and tangles are associated with neuronal loss, or whether it is dementia that relates to changes of absolute cell composition, by comparing cell numbers in brains of patients severely demented with those of asymptomatic individuals-both groups histopathologically diagnosed as Alzheimer's-and normal subjects with no pathological signs of the disease. We found a great reduction of neuronal numbers in the hippocampus and cerebral cortex of demented patients with Alzheimer's disease, but not in asymptomatic subjects with Alzheimer's disease. We concluded that neuronal loss is associated with dementia and not the presence of plaques and tangles, which may explain why subjects with histopathological features of Alzheimer's disease can be asymptomatic; and exclude amyloid-ß deposits as causes for the reduction of neuronal numbers in the brain. We found an increase of non-neuronal cell numbers in the cerebral cortex and subcortical white matter of demented patients with Alzheimer's disease when compared with asymptomatic subjects with Alzheimer's disease and control subjects, suggesting a reactive glial cell response in the former that may be related to the symptoms they present.

Resilience of Neural Cellularity to the Influence of Low Educational Level.

BACKGROUND: Education is believed to contribute positively to brain structure and function, as well as to cognitive reserve. One of the brain regions most impacted by education is the medial temporal lobe (MTL), a region that houses the hippocampus, which has an important role in learning processes and in consolidation of memories, and is also known to undergo neurogenesis in adulthood. We aimed to investigate the influence of education on the absolute cell numbers of the MTL (comprised by the hippocampal formation, amygdala, and parahippocampal gyrus) of men without cognitive impairment. METHODS: The Isotropic Fractionator technique was used to allow the anisotropic brain tissue to be transformed into an isotropic suspension of nuclei, and therefore assess the absolute cell composition of the MTL. We dissected twenty-six brains from men aged 47 to 64 years, with either low or high education. RESULTS: A significant difference between groups was observed in brain mass, but not in MTL mass. No significant difference was found between groups in the number of total cells, number of neurons, and number of non-neuronal cells. Regression analysis showed that the total number of cells, number of neurons, and number of non-neuronal cells in MTL were not affected by education. CONCLUSIONS: The results indicate a resilience of the absolute cellular composition of the MTL of typical men to low schooling, suggesting that the cellularity of brain regions is not affected by formal education.

Do age and sex impact on the absolute cell numbers of human brain regions?

What is the influence of sex and age on the quantitative cell composition of the human brain? By using the isotropic fractionator to estimate absolute cell numbers in selected brain regions, we looked for sex- and age-related differences in 32 medial temporal lobes (comprised basically by the hippocampal formation, amygdala and parahippocampal gyrus), sixteen male (29-92 years) and sixteen female (25-82); and 31 cerebella, seventeen male (29-92 years) and fourteen female (25-82). These regions were dissected from the brain, fixed and homogenized, and then labeled with a DNA-marker (to count all nuclei) and with a neuron-specific nuclear marker (to estimate neuron number). Total number of cells in the medial temporal lobe was found to be 1.91 billion in men, and 1.47 billion in women, a difference of 23 %. This region showed 34 % more neurons in men than in women: 525.1 million against 347.4 million. In contrast, no sex differences were found in the cerebellum. Regarding the influence of age, a quadratic correlation was found between neuronal numbers and age in the female medial temporal lobe, suggesting an early increase followed by slight decline after age 50. The cerebellum showed numerical stability along aging for both neurons and non-neuronal cells. In sum, results indicate a sex-related regional difference in total and neuronal cell numbers in the medial temporal lobe, but not in the cerebellum. On the other hand, aging was found to impact on cell numbers in the medial temporal lobe, while the cerebellum proved resilient to neuronal losses in the course of life.

Sexual dimorphism in the human olfactory bulb: females have more neurons and glial cells than males.

Sex differences in the human olfactory function reportedly exist for olfactory sensitivity, odorant identification and memory, and tasks in which odors are rated based on psychological features such as familiarity, intensity, pleasantness, and others. Which might be the neural bases for these behavioral differences? The number of cells in olfactory regions, and especially the number of neurons, may represent a more accurate indicator of the neural machinery than volume or weight, but besides gross volume measures of the human olfactory bulb, no systematic study of sex differences in the absolute number of cells has yet been undertaken. In this work, we investigate a possible sexual dimorphism in the olfactory bulb, by quantifying postmortem material from 7 men and 11 women (ages 55-94 years) with the isotropic fractionator, an unbiased and accurate method to estimate absolute cell numbers in brain regions. Female bulbs weighed 0.132 g in average, while male bulbs weighed 0.137 g, a non-significant difference; however, the total number of cells was 16.2 million in females, and 9.2 million in males, a significant difference of 43.2%. The number of neurons in females reached 6.9 million, being no more than 3.5 million in males, a difference of 49.3%. The number of non-neuronal cells also proved higher in women than in men: 9.3 million and 5.7 million, respectively, a significant difference of 38.7%. The same differences remained when corrected for mass. Results demonstrate a sex-related difference in the absolute number of total, neuronal and non-neuronal cells, favoring women by 40-50%. It is conceivable that these differences in quantitative cellularity may have functional impact, albeit difficult to infer how exactly this would be, without knowing the specific circuits cells make. However, the reported advantage of women as compared to men may stimulate future work on sex dimorphism of synaptic microcircuitry in the olfactory bulb.

Automatic isotropic fractionation for large-scale quantitative cell analysis of nervous tissue.

Isotropic fractionation is a quantitative technique that allows reliable estimates of absolute numbers of neuronal and non-neuronal brain cells. However, being fast for single small brains, it requires a long time for processing large brains or many small ones, if done manually. To solve this problem, we developed a machine to automate the method, and tested its efficiency, consistency, and reliability as compared with manual processing. The machine consists of a set of electronically controlled rotation and translation motors coupled to tissue grinders, which automatically transform fixed tissue into homogeneous nuclei suspensions. Speed and torque of the motors can be independently regulated by electronic circuits, according to the volume of tissue being processed and its mechanical resistance to fractionation. To test the machine, twelve paraformaldehyde-fixed rat brains and eight human cerebella were separated into two groups, respectively: one processed automatically and the other, manually. Both pairs of groups (rat and human tissue) followed the same, published protocol of the method. We compared the groups according to nuclei morphology, degree of clustering and number of cells. The machine proved superior for yielding faster results due to simultaneous processing in multiple grinders. Quantitative analysis of machine-processed tissue resulted in similar average numbers of total brain cells, neurons, and non-neuronal cells, statistically similar to the manually processed tissue and equivalent to previously published data. We concluded that the machine is more efficient because it utilizes many homogenizers simultaneously, equally consistent in producing high quality material for counting, and quantitatively reliable as compared to manual processing.