Tracing intrinsic fiber connections in postmortem human brain with WGA-HRP

The fractal brain: scale-invariance in structure and dynamics

The past 40 years have witnessed extensive research on fractal structure and scale-free dynamics in the brain. Although considerable progress has been made, a comprehensive picture has yet to emerge, and needs further linking to a mechanistic account of brain function. Here, we review these concepts, connecting observations across different levels of organization, from both a structural and functional perspective. We argue that, paradoxically, the level of cortical circuits is the least understood from a structural point of view and perhaps the best studied from a dynamical one. We further link observations about scale-freeness and fractality with evidence that the environment provides constraints that may explain the usefulness of fractal structure and scale-free dynamics in the brain. Moreover, we discuss evidence that behavior exhibits scale-free properties, likely emerging from similarly organized brain dynamics, enabling an organism to thrive in an environment that shares the same organizational principles. Finally, we review the sparse evidence for and try to speculate on the functional consequences of fractality and scale-freeness for brain computation. These properties may endow the brain with computational capabilities that transcend current models of neural computation and could hold the key to unraveling how the brain constructs percepts and generates behavior.

Insights from the IronTract challenge: Optimal methods for mapping brain pathways from multi-shell diffusion MRI

Limitations in the accuracy of brain pathways reconstructed by diffusion MRI (dMRI) tractography have received considerable attention. While the technical advances spearheaded by the Human Connectome Project (HCP) led to significant improvements in dMRI data quality, it remains unclear how these data should be analyzed to maximize tractography accuracy. Over a period of two years, we have engaged the dMRI community in the IronTract Challenge, which aims to answer this question by leveraging a unique dataset. Macaque brains that have received both tracer injections and ex vivo dMRI at high spatial and angular resolution allow a comprehensive, quantitative assessment of tractography accuracy on state-of-the-art dMRI acquisition schemes. We find that, when analysis methods are carefully optimized, the HCP scheme can achieve similar accuracy as a more time-consuming, Cartesian-grid scheme. Importantly, we show that simple pre- and post-processing strategies can improve the accuracy and robustness of many tractography methods. Finally, we find that fiber configurations that go beyond crossing (e.g., fanning, branching) are the most challenging for tractography. The IronTract Challenge remains open and we hope that it can serve as a valuable validation tool for both users and developers of dMRI analysis methods.

Testing the suitability of neuroanatomical tracing method in human fetuses with long years of postmortem delay

PURPOSE

Human tissues in gross anatomical archives with long years of postmortem delays are considered suboptimal relative to recently fixed materials for neuroanatomical tracing studies, yet efficacy of neuroanatomical tracing on archival fetal tissues largely unexplored. We aimed to explore the suitability of human archival tissue in neuroanatomical tracing with lipophilic carbocyanine dyes.

METHODS

We used crystal and paste forms 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) and analogues for neuroanatomical tracing on different peripheral nerves in 15-18-year archival old formalin-fixed human fetuses. We employed bright-field, fluorescent and confocal microscopy to visualize the peripheric nerve traces, spinal cord and vibratome cut sections. Fluorescent signal of the dyes on epineurium and on axonal membranes were visualized under fluorescence and confocal microscopes and performance of the dye diffusion was assessed by semi-quantitative image analysis.

RESULTS

We followed up seven lipophilic dye embeddings in 16-28 gestational week-old human fetuses (n = 4) with 16.75 ± 1.29-year postmortem delay. The mean distance of distally moved carbocyanine dye diffusion measured on epineurium was detected as 25.11 ± 9.1 mm.

CONCLUSION

Based on the results of 13 distinct studies performed neuroanatomical tracing with human tissues in the immediate postmortem hours or days, average traced distance was 16.32 ± 15.95 mm, and a 95% confidence interval lower limit of 4.9 mm and upper limit of 27.73 mm. The tracing distances we observed in our current study fall entirely within this confidence interval. To our awareness, this is the first report to demonstrate that specific neuroanatomical tracing presented in axonal membrane level on peripheral nerves is possible on gross anatomical repositories.

The Cortico-Basal Ganglia-Cerebellar Network: Past, Present and Future Perspectives

Much of our present understanding of the function and operation of the basal ganglia rests on models of anatomical connectivity derived from tract-tracing approaches in rodents and primates. However, the last years have been characterized by promising step forwards in the in vivo investigation and comprehension of brain connectivity in humans. The aim of this review is to revise the current knowledge on basal ganglia circuits, highlighting similarities and differences across species, in order to widen the current perspective on the intricate model of the basal ganglia system. This will allow us to explore the implications of additional direct pathways running from cortex to basal ganglia and between basal ganglia and cerebellum recently described in animals and humans.

Histological methods for ex vivo axon tracing: A systematic review

OBJECTIVES

Axon tracers provide crucial insight into the development, connectivity, and function of neural pathways. A tracer can be characterized as a substance that allows for the visualization of a neuronal pathway. Axon tracers have previously been used exclusively with in vivo studies; however, newer methods of axon tracing can be applied to ex vivo studies. Ex vivo studies involve the examination of cells or tissues retrieved from an organism. These post mortem methods of axon tracing offer several advantages, such as reaching inaccessible tissues and avoiding survival surgeries.

METHODS

In order to evaluate the quality of the ex vivo tracing methods, we performed a systematic review of various experimental and comparison studies to discern the optimal method of axon tracing.

RESULTS

The most prominent methods for ex vivo tracing involve enzymatic techniques or various dyes. It appears that there are a variety of techniques and conditions that tend to give better fluorescent character, clarity, and distance traveled in the neuronal pathway. We found direct comparison studies that looked at variables such as the type of tracer, time required, effect of temperature, and presence of calcium, however, there are other variables that have not been compared directly.

DISCUSSION

We conclude there are a variety of promising tracing methods available depending on the experimental goals of the researcher, however, more direct comparison studies are needed to affirm the optimal method.

The role of long-range connectivity for the characterization of the functional-anatomical organization of the cortex

This review focuses on the role of long-range connectivity as one element of brain structure that is of key importance for the functional–anatomical organization of the cortex. In this context, we discuss the putative guiding principles for mapping brain function and structure onto the cortical surface. Such mappings reveal a high degree of functional–anatomical segregation. Given that brain regions frequently maintain characteristic connectivity profiles and the functional repertoire of a cortical area is closely related to its anatomical connections, long-range connectivity may be used to define segregated cortical areas. This methodology is called connectivity-based parcellation. Within this framework, we investigate different techniques to estimate connectivity profiles with emphasis given to non-invasive methods based on diffusion magnetic resonance imaging (dMRI) and diffusion tractography. Cortical parcellation is then defined based on similarity between diffusion tractograms, and different clustering approaches are discussed. We conclude that the use of non-invasively acquired connectivity estimates to characterize the functional–anatomical organization of the brain is a valid, relevant, and necessary endeavor. Current and future developments in dMRI technology, tractography algorithms, and models of the similarity structure hold great potential for a substantial improvement and enrichment of the results of the technique.

Laminar specificity of intrinsic connections in Broca's area

Broca's area and its right hemisphere homologue comprise 2 cytoarchitectonic subdivisions, FDgamma and FCBm of von Economo C and Koskinas GN (1925, Die Cytoarchitektonik der Hirnrinde des erwachsenen Menschen. Vienna/Berlin [Germany]: Springer). We report here on intrinsic connections within these areas, as revealed with biotinylated dextran amine and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate tracing in postmortem human brains. Injections limited to supragranular layers revealed a complex intrinsic network of horizontal connections within layers II and III spreading over several millimeters and to a lesser extent within layers IV, V, and VI. Ninety percent of the retrogradely labeled neurons (n = 734) were in supragranular layers, 4% in layer IV, and 6% in infragranular layers; most were pyramids and tended to be grouped into clusters of approximately 500 microm in diameter. Injections involving layer IV revealed extended horizontal connections within layers I-IV (up to 3.7 mm) and to a lesser extent in layers V and VI. Injections limited to the infragranular layers revealed horizontal connections mainly limited to these layers. Thus, intrinsic connections within Broca's area display a strong laminar specificity. This pattern is very similar in areas FDgamma and FCBm and in the 2 hemispheres.

Commissural connections of human superior colliculus

The superior colliculus of higher mammals is a laminated structure of the midbrain that receives visual input in superficial layers, and visual, auditory and somatosensory input in deep layers. The superior colliculi on either side are interconnected via the intercollicular commissure, which has been proposed to play a role in visual transfer and gaze orienting. Intercollicular connections have been anatomically demonstrated in various species including macaque monkeys but not in man. Here we describe the organization of commissural connections of the superior colliculus in man. A single injection of the carbocyanine tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate was made into the superior colliculus in five post-mortem brains. Contralateral to the injection, labelled axons formed a dense bundle in the deep collicular layers and isolated axons were present in the superficial layers. Synaptic-like boutons were found in all collicular layers. Injections placed at different rostro-caudal levels revealed a roughly topographical organization; the bulk of the labelled axons were found opposite to the injection, with a progressive decrease in labelling at more rostral and caudal levels. Our results demonstrate that superficial and, to a larger extent, deep layers participate in intercollicular connections, and suggest that visual information crosses at the collicular level.

Quantitative estimation of 3-D fiber course in gross histological sections of the human brain using polarized light

Series of polarized light images can be used to achieve quantitative estimates of the angles of inclination (z-direction) and direction (in xy-plane) of central nervous fibers in histological sections of the human brain. (1) The corpus callosum of a formalin-fixed human brain was sectioned at different angles of inclination of nerve fibers and at different thicknesses of the samples. The minimum, and maximum intensities, and their differences revealed a linear relationship to the angle of inclination of fibers. It was demonstrated that sections with a thickness of 80--120 microm are best suited for estimating the angle of inclination. (2) Afterwards the optic tracts of eight formalin-fixed human brains were sliced at different angles of fiber inclination at 100 microm. Measurements of intensity in 30 pixels in each section were used to calculate a linear function of calibration. The maximum intensities and the differences between maximum and minimum values measured with two polars only were best suited for estimation of fiber inclination. (3) Gross histological brain slices of formalin-fixed human brains were digitized under azimuths from 0 to 80 degrees using two polars only. These sequences were used to estimate the inclination of fibers (in z-direction). The same slices were digitized under azimuths from 0 to 160 degrees in steps of 20 degrees using a quarter wave plate additionally. These sequences were used to estimate the direction of the fibers in xy-direction. The method can be used to produce maps of fiber orientation in gross histological sections of the human brain similar to the fiber orientation maps derived by diffusion weighted magnetic resonance imaging.

Visualization of nerve fiber orientation in gross histological sections of the human brain

Diffusion weighted magnetic resonance imaging (DWMRI) allows visualization of the orientation of the nervous fibers in the living brain. For comparison, a method was developed to examine the orientation of fibers in histological sections of the human brain. Serial sections through the entire human brain were analyzed regarding fiber orientation using polarized light. Direction of fibers in the cutting plane was obtained by measuring the azimuth with the lowest intensity value at each point, and inclination of fibers in the section was evaluated using fuzzy logic approximations. Direction and inclination of fibers revealing their three-dimensional orientation were visualized by colored arrows mapped into the images. Using this procedure, various fiber tracts were identified (pyramidal tract, radiatio optica, radiatio acustica, arcuate fascicle, and 11 more). Intermingled fibers could be separated from each other. The orientation of the fiber tracts derived from polarized light microscopy was validated by confocal laser scanning microscopy in a defined volume of the internal capsule, where the fiber orientation was studied in four human brains. The polarization method visualizes the high degree of intermingled fiber bundles in the brain, so that distinct fiber pathways cannot be understood as solid, compact tracts: Neighbouring bundles of fibers can belong to different systems of fibers distinguishable by their orientation.

Neural tract tracing using Di-I: a review and a new method to make fast Di-I faster in human brain

The use of Di-I in tract-tracing is briefly reviewed and a novel delayed-fixation approach to neural tract-tracing in the postmortem human adult brain is reported. Using the new approach, fast Di-I, a highly lipophilic fluorescent dye was injected into a particular region or nucleus and labelled tracts were followed for distances of some 20-40 mm. The procedure required approximately 36 h, yielding dye penetration rates of 1.0 mm/h or more. This contrasts with previous Di-I, silver impregnation, and horseradish peroxidase protocols, where the tracer penetration rate is typically 0.003 mm/h or less, and the distance traversed amounts to only a few mm even after months of incubation. The new method hinges on the simple consideration that aldehyde fixation, which is normally employed prior to administration of the marker, crosslinks membrane proteins and impedes dye diffusion. The short postmortem samples used in our protocol permit delaying fixation until after the dye has had time to penetrate, dramatically increasing the length and scope of neural circuits that can be traced. Using these methods, for example, we have confirmed the presence of an ipsilateral olivocerebellar climbing fiber projection in the human.

The human nucleus of the solitary tract: visceral pathways revealed with an "in vitro" postmortem tracing method

Visceral relay neurons in the nucleus of the solitary tract (NTS) regulate behavior and autonomic reflex functions. NTS projections have been extensively characterized in animal studies but not in humans. For the first time, NTS fiber trajectories in the human medulla oblongata were revealed with an "in vitro" postmortem tracing method. Local intramedullary pathways were labeled by direct pressure injections of free horseradish peroxidase centered on the medial subnucleus at a level adjacent to true obex. Labeled elements were resolved by peroxidase histochemistry as a dark brown intracellular reaction product. A prominent transtegmental system of axons emerged from the NTS injection sites and entered the intermediate reticular zone, a region corresponding to an autonomic reflex center in other mammals. A medial system of axons arched across the dorsomedial reticular formation toward the dorsal medullary raphe and projected ventrally toward the nucleus gigantocellularis. A small lateral fiber trajectory coursed towards the dorsomedial zone of spinal trigeminal nucleus caudalis. Presumptive terminals appeared as dustings of fine punctate processes within the NTS, dorsomotor nucleus and reticular formation. NTS projections in humans resemble those identified in other mammals including primates. Axonal tracing studies predict that visceral impulses in humans may transmit over evolutionarily conserved pathways involved in autonomic feedback control and stress adaptation.

Quantitative analysis of a vulnerable subset of pyramidal neurons in Alzheimer's disease: I. Superior frontal and inferior temporal cortex

Various cytoskeletal proteins have been implicated in the cellular pathology of Alzheimer's disease. A monoclonal antibody (SMI32) that recognizes nonphosphorylated epitopes on the medium (168 kDa) and heavy (200 kDa) subunits of neurofilament proteins has been used to label and analyze a specific subpopulation of pyramidal neurons in the prefrontal and inferior temporal cortices of normal and Alzheimer's disease brains. In Alzheimer's disease, the distribution of neuropathological markers predominates in layers III and V in these association areas. In these neocortical regions, SMI32 primarily labels the perikarya and dendrites of large pyramidal neurons, predominantly located within layers III and V. In Alzheimer's disease, a dramatic loss of SMI32-immunoreactive (ir) cells was observed, affecting particularly the largest cells (i.e., cells with a cross-sectional perikaryal area larger than 350 microns 2). The staining intensity of the largest SMI32-ir neurons was significantly reduced in Alzheimer's disease cases, suggesting that an inappropriate phosphorylation of these cytoskeletal proteins may take place in the course of the pathological process. In addition, the SMI32-ir neuron loss and total neuron loss were highly correlated with neurofibrillary tangle counts, whereas such a correlation was not observed with neuritic plaque counts. These quantitative data suggest that SMI32-ir neurons represent a small subset of pyramidal cells that share certain anatomical and molecular characteristics and are highly vulnerable in Alzheimer's disease. Other studies have suggested that SMI32-ir neurons are likely to furnish long corticocortical projections. Thus, their loss would substantially diminish the effectiveness of the distributed processing capacity of the neocortex, resulting in a neocortical isolation syndrome as reflected by the clinical symptomatology observed in these patients. Such correlations between the expression of a selective cellular pathology and specific elements of cortical circuitry will increase our understanding of the molecular and cellular characteristics underlying a given neuronal subclass vulnerability in Alzheimer's disease or other neurodegenerative disorders.

Tracing neuronal connections in postmortem human hippocampal complex with the carbocyanine dye DiI

This report describes the ability of the carbocyanine dye DiI to trace hippocampal complex connections in a paraformaldehyde immersion-fixed human postmortem brain. Six months after the placement of DiI crystals into the hilus of the dentate gyrus, the CA1 hippocampal subfield and the lateral entorhinal cortex, 50-microns thick, vibratome cut sections were examined using an epifluorescence microscope with a rhodamine filter. In association with DiI-labeled granule, pyramidal and multipolar type neurons, we observed dendrites containing dendritic spines and axons. DiI-labeled fibers were observed coursing within classically described hippocampal pathways for at least 8 mm distal to the injection site. Photoconversion of diaminobenzidine (DAB)-treated DiI sections produced a stable record of labeled profiles. These findings indicate that DiI is a useful method for investigating intrinsic local circuit connections in normal aldehyde-fixed postmortem human brain and suggests that DiI could be a powerful tool to examine altered neural connectivity in humans with neurological disease.

An in vitro study of the projections of enteric vasoactive intestinal polypeptide-immunoreactive neurons in the human colon

The anatomical basis of the peptidergic neural control of the human colon is largely unknown. In this study, in vitro retrograde tracing methods have been used on fresh human colon to determine the projection pathways of the enteric nerves and, in particular, those containing vasoactive intestinal polypeptide, one of the most abundant and potent of the gut neuropeptides. Two components of the submucous plexus were identified, the inner one projecting to the lamina propria, and the outer to the circular muscle. The lengths of projections within the submucous plexus were up to 5-14 mm in all directions. Myenteric ganglion cells projected to both longitudinal and circular muscles, for distances of up to only 5 mm. The subpopulation of nerves containing vasoactive intestinal polypeptide arose mainly from the submucous plexus and projected up to 6.5 mm anally, 5 mm orally, and 14 mm within the submucous layer to the mucosa or circular muscle. These findings provide entirely new data on the neuroanatomy of the human colon and may help in the understanding of the neural control of colonic secretion and motility.

Gut endocrine and neural peptides

The once exponential growth in the number of new gut endocrine peptides being discovered has become slightly slower in recent years, and expansion of the field of gut hormones has involved mainly the application of new investigative methods. Some new peptides have been described and major inroads have been made into establishing the ontogeny of gut endocrine cells, the origins and pathways of the enteric innervation, and the involvement of the diffuse neuroendocrine system as a whole in disease states. Further insight is being gained into the functional activity of the peptide cell system by studying the control, sites and rates of peptide gene expression, and the localization and characterization of peptide binding sites on target cells.

Local intra- and interlaminar connections in mouse barrel cortex

Focal injections of horseradish peroxidase (HRP) in dimethylsulfoxide (DMSO) were targeted into mouse somatosensory cortex, in vitro, with a template. Injections were made at different depths and in different locations in the whisker-barrel-defined somatosensory map in order to determine quantitative connectivity patterns within and between barrel-defined cortical columns. Cortices were sectioned in a plane parallel to the pia at 75 microns. Data were collected directly from microscope slides by computer. Data are presented as: 1) Plots of computer-mapped HRP reaction product density in neurons and cell locations for each section in relation to barrel boundaries; 2) histograms of label in cortical layers related to individual barrel-defined columns; 3) polar plots of relative amounts of label within individual barrel columns in sections through each barrel column; 4) vectors which represent HRP reaction product density as a function of direction and distance from the injection site; 5) statistical analysis of the shape of the label distribution pattern in the plane of the cortex as a function of injection site depth; and 6) probability of labeling of any other barrel column given a labeled barrel column. The principal findings are: 1) The pattern of label distribution, after an injection directly above or directly below an individual barrel, is hour-glass shaped with the waist of the hour-glass in layer IV. 2) Connections within barrel cortex are asymmetrical. Barrel-related columns within a row are more strongly interconnected than those in different rows. 3) Connections of the small barrels associated with whiskers on the upper lip are strongest with other small barrels, but strong connections also exist between these small barrels and the larger barrels. 4) The pattern of intracortical connections in SII is not asymmetrical; interlaminar connections in SII are fundamentally different from those in barrel cortex. 5) Quantitative intracortical projection patterns are highly consistent with functional data on intracortical processing of whisker information. As such, the quantitative data clearly indicate the spatial extent and relative magnitude of populations of neurons involved in intracortical processing of sensory information. The spatial arrangements of these intracortical connections, in conjunction with known developmental events, make it highly likely that the distribution of intracortical axons in mouse barrel cortex is sculpted in part by experience.

Post-fixation horseradish peroxidase tracing in rat fetus and pups

Horseradish peroxidase (HRP) has been shown to be applicable to fixed tissue in tracing projections in adult animals. This technique has several advantages in studying short projections in difficult attainable brain areas, especially with fetuses in utero. We have used this method to trace developing fibers (e.g., vestibulo-cerebellar connections) in rat fetuses and pups. It has been possible to make discrete and precise injections in any location. The quality of labeling by this method is distinctly related to the grade of fixation. Fixation grade determines how well HRP is anchored inside fibers and cell bodies. In optimally fixed preparations fibers and cell bodies are intensively stained with HRP up to over 3 mm from the center of an injection site. HRP is probably only taken up by damaged fibers so that the amount of fibers labeled is determined by the extent of the lesion caused by the injection pipette.

Organization of corticocortical connections in human visual cortex

Clinical and psychophysical observations indicate that the visual cortex is critical for the perception of color, form, depth, and movement. Little, however, is known about the cortical circuitry that underlies these functions in humans. In an attempt to learn more about these connections, we have traced projections of primary (V1) and secondary (V2) visual cortex in the postmortem, fixed human brain, using the fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate as an axonal marker. The results show that V1 makes a forward projection to layers 3 and 4 of V2, and V2 projects back to layers 1, 2, 3, 5, and 6 of V1. Some V2 injections also show an input to layer 4B of V1. Projections to 4B probably originate from cytochrome oxidase (CO)-reactive stripes that we have identified in V2. Differential connections between CO-rich (blobs) and CO-poor regions (interblobs) also exist within V1; blobs are connected to blobs and interblobs are connected to interblobs. The results show that the connections in human visual cortex are similar to those of nonhuman primates and that their organization is consistent with the concept of multiple processing streams in the visual system.