Visualizing three-dimensional microscopic structures in the human brain histology samples

Visualizing three-dimensional microscopic structures in the human brain histology samples

Researchers have made a breakthrough in the visualization of human brain tissue at the microscopic level. The findings are now published in Nature Communications.

To understand how the brain works, scientists need to map how nerve cells (neurons) are wired to form circuitries in both healthy and disease states. Traditionally this was accomplished by serial cutting of brain tissue into thin slices and tracing the cut nerve fibres over many sections. However, this approach is difficult and labor-intensive as the neuronal circuitries span across great distances in three dimensions (3D) and are tightly entangled microscopically. To avoid the sectioning of tissues, tissue clearing techniques - methods that turn opaque tissue transparent - have been developed, enabling high-resolution and deep imaging of neuronal circuitries. Although such techniques have been very effective on rodent brain tissue, only limited studies have found success with human brain tissue. The difficulties and challenges may be attributed to fundamental differences between the human and the mice brain.

To overcome these barriers, the team developed a new tissue clearing solution, OPTIClear. OPTIClear selectively adjusts the optical properties of tissue without damaging or changing their structural components. Authors use of cresyl violet for visualisation of neurons in cleared tissue, with the potential for 3D quantification in regions of interest. Furthermore, they use lipophilic tracers for tracing of neuronal processes in post-mortem tissue, enabling the study of the morphology of human dendritic spines in 3D. 

Nerve cells, glial cells, and blood vessels were visualized in exquisite detail, with their 3D relationship determined. For example, the team performed 3D morphological analysis on human brainstem dopaminergic neurons in the millimetre scale, and imaged more than 3,000 large neurons in the human basal forebrain in merely five days, which normally is extremely laborious and takes at least three weeks. These neurons have been implicated in neurological and psychiatric diseases such as dementia and depression; the promising results suggest that this novel method is applicable to future research on these conditions. More remarkably, OPTIClear can also be applied in both archived (>30 years) and clinical specimens.

The team hopes that this simple method can catalyse further scientific development. By allowing scientists to study human tissue quicker and better, OPTIClear could potentially speed up the elucidation of circuitry mechanisms in a multitude of brain diseases. Co-supervisor of the study, commented, "We hope that a better understanding of the connections and circuitries of the brain will help uncover the pathologies that underlie the common degenerative diseases of the brain, such as Alzheimer's and Parkinson's disease.