3D ultrastructural study of synapses in the human entorhinal cortex

The entorhinal cortex (EC) is a brain region that has been shown to be essential for memory functions and spatial navigation. However, detailed 3D synaptic morphology analysis and identification of postsynaptic targets at the ultrastructural level have not been performed before in the human EC. In the present study, we used Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) to perform a three-dimensional analysis of the synapses in the neuropil of medial EC in layers II and III from human brain autopsies. Specifically, we studied synaptic structural parameters of 3561 synapses, which were fully reconstructed in 3D. We analyzed the synaptic density, 3D spatial distribution, and type (excitatory and inhibitory), as well as the shape and size of each synaptic junction. Moreover, the postsynaptic targets of synapses could be clearly determined. The present work constitutes a detailed description of the synaptic organization of the human EC, which is a necessary step to better understand the functional organization of this region in both health and disease.Significance StatementThe present study represents the first attempt to unveil the detailed synaptic organization of the neuropil of the human entorhinal cortex — a brain region that is essential for memory function and spatial navigation. Using 3D electron microscopy, we have characterized the synaptic morphology and identified the postsynaptic targets of thousands of synapses. The results provide a new, large, quantitative ultrastructure dataset of the synaptic organization of the human entorhinal cortex. These data provide critical information to better understand synaptic functionality in the human brain.HighlightEstimation of the number of synapses, as well as determination of their type, shapes, sizes and postsynaptic targets, provides critical data to better understand synaptic functionality. This study provides a new, large, quantitative ultrastructure dataset of the synaptic organization of the human entorhinal cortex using 3D electron microscopy.

Mitochondrial biogenesis during cellular differentiation

Mitochondrial biogenesis was studied during differentiation of two immortalized cell lines (C2C12, 3T3) with enzyme measurements, Northern blots, and quantitative ultrastructure. Citrate synthase, isocitrate dehydrogenase, and 3-hydroxyacyl-CoA dehydrogenase (nuclear encoded, mitochondrial matrix location) showed linear, four- to sixfold increases in enzymatic activity in C2C12 cells but increased exponentially in 3T3 cells. Cytochrome oxidase and NADH dehydrogenase (nuclear and mitochondrial encoded, cristae location) increased to a lesser extent and with a pattern dissimilar to the first group. Northern blots and activity of succinate dehydrogenase (cristae location but entirely nuclear encoded) suggested the groupings were based on location of the genes rather than the mature enzyme. However, quantitative electron microscopy and comparisons with adult tissue suggested that mitochondrial ultrastructure can influence the change in cristae enzymes. Cristae surface area per unit mitochondrial volume and per unit cell volume increased much less than did cristae enzymes. Available space on the inner membrane may become limiting and account for some aspects of the pattern of change in electron transport enzymes during differentiation.

Quantitative ultrastructure of gill epithelial cells in the larval lamprey Petromyzon marinus

Cells and organelles in the respiratory epithelium of the sea lamprey (Petromyzon marinus) gill were measured in electron micrographs of plastic-embedded sections to provide baseline data for future quantitative studies of how toxicants affect gill morphology. Stereology was used, based on the [Formula: see text] method for determining cell volumes. The method was checked against unbiased methods and was shown to be accurate within 12%. We found that cells in the basal layer of the epithelium, basal cells, are smaller (averaging 338 μm3) and contain smaller volumes of most organelles, than cells in the superficial layer. These features suggest that basal cells are either undifferentiated or differentiating cells. The superficial layer contains larger, more specialized cells: presumed ion-uptake cells (817 μm3) and mucous-pavement cells (548 μm3). Ion-uptake cells contain the largest volume of mitochondria (270 μm3), consistent with the high energy demands of ion transport. Mucous-pavement cells contain the largest volume of secretory vesicles (55 μm3), presumably for coating the lamellae with protective mucus. A new cell type was discovered, with features intermediate between basal cells and mucous-pavement cells, suggesting that the former cells transform into the latter. Volumes of gill epithelial cells are independent of animal size.