The size of mesenchymal stem cells is a significant cause of vascular obstructions and stroke.

BACKGROUND AND PURPOSE: Intravascular injection of mesenchymal stem cells (MSCs) has been found to cause considerable vascular obstructions which may lead to serious outcomes, particularly after intra-arterial injection. However, the underlying mechanisms have been poorly understood.

METHODS: In this study, we fractionated MSCs that had been cultured in monolayer for six passages into small (average diameter = 17.9 μm) and large (average diameter 30.4 μm) populations according to their sizes, and examined their vascular obstructions after intra-internal carotid artery injection in rats and mice in comparison with MSCs derived from 3D spheroids which were uniformly smaller in size (average diameter 12.6 μm).

RESULTS: We found that 3D MSCs did not cause detectable infarct in the brain as evidenced by MRI scan and TTC stain, 2D MSCs in small size caused a microinfarct in one of five animals, which was co-localized to the area of entrapped MSCs (labeled with DiI), while 2D MSCs in large size caused much larger infarcts in all five animals, and substantial amounts of DiI-positive MSCs were found in the infarct. Meanwhile, corresponding neurological defects were observed in the animals with stroke. In consistence, injection of 2D MSCs (average diameter 26.5) caused a marked loss of cortical neurons and their axons in Thy1-GFP transgenic mice and the activation of microglia in CX3CR1-GFP transgenic mice in the area with MSC entrapment.

CONCLUSIONS: Our results suggest that the size of MSCs is a significant cause of MSC caused vascular obstructions and stroke.