Diffusion in the extracellular space in brain and tumors.

Area of Science:

• Neuroscience
• Biophysics
• Biomedical Engineering

Background:

• Diffusion in the brain's extracellular space (ECS) is crucial for intercellular communication, ionic buffering, and metabolite/drug transport.
• Understanding diffusion in tumor ECS is vital for effective anti-tumor drug delivery.
• The brain ECS, approximately 20% of parenchymal volume, features narrow cell-cell gaps (down to 50 nm).

Purpose of the Study:

• To develop and apply fluorescence methods for quantifying solute diffusion in the ECS, even deep within solid tissues.
• To investigate the factors contributing to diffusion slowing in the brain ECS, including geometry and extracellular matrix (ECM).
• To compare diffusion characteristics in healthy brain ECS versus tumor ECS, particularly at varying depths.

Main Methods:

• Development of fluorescence-based techniques for in situ diffusion measurements.
• Utilized microfiberoptics with micron tip size for deep tissue measurements.
• Employed mathematical modeling to simulate and validate diffusion in anatomically accurate ECS models.

Main Results:

• Diffusion in healthy brain ECS is slowed 3-5 fold compared to water, attributed equally to tortuous geometry and ECM viscosity.
• In superficial tumors, small macromolecule diffusion is only mildly slowed (<3 fold).
• Diffusion is significantly slowed (>10 fold) deeper within tumors (few millimeters) as tissue compactness increases, with ECM components like collagen contributing.

Conclusions:

• The ECS geometry and ECM significantly impede solute and macromolecule diffusion in the brain.
• Tumor compactness and ECM composition dramatically alter diffusion, posing challenges for drug delivery.
• Cellular crowding and tortuosity alone cause only minor diffusion slowing in the ECS, unlike within cells.