Clot Fragments Formed from Original Thrombus Obstruct Downstream Arteries in the Ischemic Injured Brain

Microembolus clearance through angiophagy is an auxiliary mechanism preserving tissue perfusion in the rat brain

Considering its intolerance to ischemia, it is of critical importance for the brain to efficiently process microvascular occlusions and maintain tissue perfusion. In addition to collateral microvascular flow and enzymatic degradation of emboli, the endothelium has the potential to engulf microparticles and thereby recanalize the vessel, through a process called angiophagy. Here, we set out to study the dynamics of angiophagy in relation to cytoskeletal remodeling in vitro and reperfusion in vivo. We show that polystyrene microspheres and fibrin clots are actively taken up by (brain) endothelial cells in vitro, and chart the dynamics of the actin cytoskeleton during this process using live cell imaging. Whereas microspheres were taken up through the formation of a cup structure by the apical endothelial membrane, fibrin clots were completely engulfed by the cells, marked by dense F-actin accumulation surrounding the clot. Both microspheres and fibrin clots were retained in the endothelial cells. Notably, fibrin clots were not degraded intracellularly. Using an in vivo microembolization rat model, in which microparticles are injected into the common carotid artery, we found that microspheres are transported by the endothelium from the microvasculature into the brain parenchyma. Microembolization with microspheres caused temporal opening of the blood–brain barrier and vascular nonperfusion, followed by microsphere extravasation and restoration of vessel perfusion over time. Taken together, angiophagy is accompanied by active cytoskeletal remodeling of the endothelium, and is an effective mechanism to restore perfusion of the occluded microvasculature in vivo.

A concept of thrombolysis as a corrosion-erosion process verified by optical microscopy

OBJECTIVE

Outcome of the thrombolytic treatment is dependent on biochemical reactions of the fibrinolytic system as well as on hemodynamic conditions. However, understanding of the interaction between these two processes is still deficient.

METHODS

Model blood clot thrombolysis in an artificial perfusion system was studied by stroboscopic optical microscopy. The obtained images were analyzed by particle-tracking software for clot size distributions of removed clot fragments, and for non-lysed blood clot areas as function of time. Based on the experimental results, a probabilistic phenomenological model of blood clot dissolution was developed, in which mechanical forces of streaming plasma are in balance with binding forces of blood cells to the remaining clot.

RESULTS

The clot dissolution rate and maximum size of removed clot fragments were increased with greater flow rate. A 3.3-fold flow rate increase resulted in a two-fold clot dissolution rate increase, while sizes of the removed fragments were in the range of single blood cells, up to thousand-cell clusters. Our phenomenological microscale model of clot dissolution suggests that thrombolysis is a corrosion-erosion-like process.

CONCLUSIONS

The findings of this study provide a possible explanation for the origin of clot fragment formation in the blood clot dissolution process.

Hyperthermia worsens ischaemic brain injury through destruction of microvessels in an embolic model in rats

PURPOSE

Basal lamina is a major part of the microvascular wall and plays a critical role in the integrity of microvasculature. The aim of this study is to determine whether hyperthermia worsens the destruction of microvascular integrity in the ischaemic injured brain.

MATERIALS AND METHODS

Focal cerebral ischaemia was induced by embolising a pre-formed clot into the middle cerebral artery (MCA). Rats received either normothermic or hyperthermic treatment. Neurological score and infarct size were evaluated at 24 h after the MCA occlusion. Microvascular collagen type IV and laminin were measured with fluorescence microscopy. The activities of matrix metalloproteinases (MMP-2 and MMP-9) and plasminogen activators (tPA and uPA) were determined by zymography.

RESULTS

Treatment with hyperthermia significantly increased infarct volume (p<0.01), cortex swelling (p<0.01), striatum swelling (p<0.05) and neurologic score (p<0.01) at 24 h after the MCA occlusion. Compared to the normothermic groups, hyperthermia significantly worsened the losses of microvascular basal lamina structure proteins, collagen type IV and laminin, at 6 h (p<0.001) and 24 h (p<0.01) after MCA occlusion. Hyperthermia increased the MMP-9 activity at 6 and 24 h after MCA occlusion compared with normothermia (p<0.05), whereas increased the MMP-2 activity at 6 h only (p<0.05). Hyperthermia also elevated uPA activity significantly at 6 and 24 h after MCA occlusion compared to normothermia (p<0.05).

CONCLUSIONS

These results demonstrate that hyperthermia exacerbates the destruction of microvascular integrity possibly by increasing the activities of MMP-2, MMP-9 and uPA in the ischaemic cerebral tissues.

Critical role of microvasculature basal lamina in ischemic brain injury

Cerebral vascular system can be divided into two categories: the macrovessels and microvessels. The microvessels consist of arterioles, capillaries and venules. There are three basic components in the microvasculature: endothelial cells, basal lamina and end-feet of astrocytes. The basal lamina is situated between the endothelial cells and the end-feet of astrocytes, and connects these two layers together. Damage to the basal lamina causes the dismantlement of microvascular wall structures, which in turn results in increase of microvascular permeability, hemorrhagic transformation, brain edema and compromise of the microcirculation. The present article reviews microvascular changes during ischemic brain injury, with emphasis on basal lamina damage.