Researchers from Keio University (Yokohama, Japan) and Harvard Medical School (Boston, USA) have constructed 3D neurovascular tissues through combining neural and vascular models within a microfluidic system.
The researchers built a microfluidic chip with compartments for neural cells and vascular cells, separated by a central portion filled with a specialised 3D biomaterial, called a hydrogel.
The hydrogel selected was a fibrin-Matrigel-hyaluronan mix, which the group found to crucially promote the growth of both neural and vascular cells, effectively replicating the 3D environment of the brain. This is advantageous over using plastic flasks, which fail to replicate the architecture of physical properties of the brain.
The specific dimensions of the microfluidic device which the researchers created allowed for the interface between these cells to develop, forming neurovascular unit tissue. This system can be studied to discover the underlying mechanisms of neurovascular unit dysfunction for a range of diseases, or even for drug screening.
Tissue Type Interface
Researchers investigated whether neural (neural stem cells) and vascular cells (brain microvasculature endothelial cells and perivascular-like cells – cells which surround the endothelial cells in living blood vessels) could branch across the central hydrogel filled region of the microfluidic device to interact. Through fluorescent staining the groups found that the cells do indeed grow and migrate into the central 3D portion of the device, forming an interface between the two tissue types.
The interaction between the brain’s blood vessels and the archetypal brain cells (neurons and glia) is crucial to maintaining brain health. This unit of neurovascular tissue is termed as the neurovascular unit.
When the neurovascular unit is damaged – often after stroke or in neurodegenerative diseases, like Alzheimer’s disease – the brain struggles to take in sufficient nutrients from the blood supply and fails to remove damaging proteins and compounds from the brain, resulting in further damage to occur.
Schematic illustrations of microfluidic device. (A) A PDMS device copied by an SU-8 mold was plasma-bonded with a coverglass to form microfluidic channels. Hydrogel pre-polymer was then injected into the central channel from two gel inlets (arrows). (B) There are two parallel microchannels separated by the gel region. BMECs and NSCs were injected into one of the microchannels (arrows). Credit: Hiroyuki Uwamori, et al. CC-BY
The vascular component of the neurovascular unit is composed of cells which form the blood-brain barrier. This barrier separates the blood flow from the hyper-sensitive central nervous system, stopping any foreign entities from crossing and causing damage whilst also removing any damaging compounds.
The interplay between neural cells and this vasculature is crucial to brain health, and the development of a model which replicates this and can be tested within the lab – outside of an animal model – is a necessity to discovering the mechanisms which lead to such neurovascular unit damage and contribute to neurodegeneration.
The most common method of modelling the neurovascular unit is through animal models, which have provided invaluable data referring to the effect of vasculature in the brain. The method, however, has limited translation to human benefit due to inter-species variation – what happens in animals doesn’t necessarily happen in humans. It is also hard to monitor outcomes in a living animal.
Optimization of the concentration of fibrin-Matrigel mixed gel for 3D migration of neurons. (A–D) Immunofluorescence projection images and corresponding 3D views of neurons in 2–2 mg/ml and 2–8 mg/ml fibrin-Matrigel mixed gel, respectively. Cells were fixed on day 21 and stained for neurons (Tuj1, green) and nuclei (DAPI, blue). Scale bar, 100 µm. (E–F) Quantitative analysis of the percentage of migrating cells in a 3D manner. The number of NSCs migrating into the 3D gel region. Data are shown as the mean ± s.e.m. (N = 3, n ≥ 12). *p < 0.05 vs. 2–2 mg/ml. Credit: Hiroyuki Uwamori, et al. CC-BY
In vitro models on the other hand are easier to model the outcome of any study, but fail to replicate the in vivo environment of the brain; commonly consisting of one cell type and stiff plastic materials.
The model developed in this study offers the 3D architecture and multicellular functionality of a living system where the crosstalk between cells improves the outcome of the tissue. The in vitro microfluidic system also allows for easy imaging and monitoring of neurovascular tissue development.
Hiroyuki Uwamori, Takuya Higuchi, Ken Arai & Ryo Sudo Integration of neurogenesis and angiogenesis models for constructing a neurovascular tissue Scientific Reports. 11 December 2017. doi:10.1038/s41598-017-17411-0
Author: Geoffrey Potjewyd; Regenerative Medicine & Neuroscience PhD student at the University of Manchester.
Top Image: The process of neurogenesis in 21-day culture. Credit: Hiroyuki Uwamori, et al. CC-BY