Imaging and reconstruction of the cytoarchitecture of axonal fibres: enabling biomedical engineering studies involving brain microstructure

Abstract

There is an increased need and focus to understand how local brain microstructure affects the transport of drug molecules directly administered to the brain tissue, for example in convection-enhanced delivery procedures. This study reports the first systematic attempt to characterize the cytoarchitecture of commissural, long association and projection fibers, namely: the corpus callosum, the fornix and the corona radiata. Ovine samples from three different subjects were stained with osmium tetroxide (to enhance contrast from cell organelles and the fibers), embedded in resin and then imaged using scanning electron microscope combined with focused ion beam milling to generate 3D volume reconstructions of the tissue at subcellular spatial resolution. Particular focus has been given to the characteristic cytological feature of the white matter: the axons and their alignment in the tissue. Via 2D images a homogeneous myelination has been estimated via detection of ~40% content of lipids in all the different fiber tracts. Additionally, for each tract, a 3D reconstruction of relatively large volumes (15μm x 15μm x 15μm – including a significant number of axons) has been performed. Namely, outer axonal ellipticity, outer axonal cross-sectional area and their relative perimeter have been measured. The study of well-resolved microstructural features provides useful insight into the fibrous organization of the tissue, whose micromechanical behaviour is that of a composite material presenting elliptical tortuous tubular fibers embedded in the extra-cellular matrix. Drug flow can be captured through microstructurally-based models, leading to a workflow to enable physically-accurate simulations of drug delivery to the targeted tissue.

On the microstructurally driven heterogeneous response of brain white matter to drug infusion pressure

“…Furthermore, the geometry of axon was also simplified. The mathematical model in this paper can be combined with realistic brain microstructure reconstructed from microscopic images (Bernardini et al 2021;Vidotto et al 2021) to further improve the model accuracy.…”

“…Axons' diameter is not uniform but in a range with the mean value of around 1 μm (Bernardini et al 2021). Therefore, in the present microscopic model, the diameter of the axon was assumed to be 1 μm .…”

Porosity-permeability tensor relationship of closely and randomly packed fibrous biomaterials and biological tissues: Application to the brain white matter

“…A key parameter to reconstruct the microstructure is the sizes of nerve fibres, which we have obtained experimentally by adopting the Focused Ion-Beam Scanning Electron Microscopy (FIB-SEM) technique (Fig. 1c) [28]. For the sake of data statistics and without loss of generality, the cross-sections of the nerve fibres were simplified to be circular (note ellipticity can also be captured and later introduced as a second order effect) and Fig.…”

Insights into Infusion-Based Targeted Drug Delivery in the Brain: Perspectives, Challenges and Opportunities

“…Furthermore, microstructural heterogeneities among different regions of CNS tissue can affect the flow behaviour and distribution, though little is known about the detailed microstructure of CNS tissue. Only a recent study on fixed tissue of sheep brain has revealed localised microstructural differences such as axon volume fractions and geometry in specific regions, i.e., the corpus callosum, corona radiata and fornix of CNS [ 58 ]. Appreciating this fact, Vidotto et al [ 19 ] used a computational approach and found a significant difference between hydraulic permeability in the corpus callosum and fornix.…”

“…While assuming localised homogeneity and isotropy agrees well with geology experiments [ 148 ], the same hypothesis may not applicable for brain tissues [ 18 ]. To achieve more accurate outcomes, it is worth moving to a microscopic scale in order to analyse the interaction between the hydraulic pressure and microstructures, which also raise the need for a reconstruction method with higher resolution imaging techniques [ 19 , 58 ]. It is also worth mentioning the mathematical models for backflow simulation here.…”