Two-Photon Imaging of Cortical Surface Microvessels Reveals a Robust Redistribution in Blood Flow after Vascular Occlusion

1Normal brain function requires adequate levels of blood flow to ensure the delivery of oxygen and nutrients to cells and to facilitate the removal of metabolites and heat. The vasculature that supplies and regulates this flow is comprised of a succession of feeder vessels and networks with extensive redundant connections. At the level of the supply to the brain, a pair of “communicating” arteries connects the carotid arteries to form the Circle of Willis. This loop is known to ensure a substantial level of fault tolerance to an occlusion of one of the member vessels. At the level of cerebral cortex, the branches of each cerebral artery form the artery's own network of communicating arterioles on the surface of its cortical territory. This network, in turn, gives rise to arterioles that plunge into the cortex and branch into the capillary bed. Each of the surface communicating networks is highly interconnected and, in addition, connects with the communicating networks of neighboring cerebral arteries through pial arteries called leptomeningeal anastomoses. In analogy with the large-scale redundancy afforded by the Circle of Willis, the communicating network has been hypothesized to provide a robust source of collateral flow in the event of an occlusion of a microvessel. Previous efforts to test this hypothesis, as well as the more general issue of the relationship between network topology and compensatory flow after an occlusion, have been hampered by a lack of methodology in which small vascular occlusions can be precisely targeted in time and space and in which flow can be quantified throughout multiple neighboring branches in a network.

2Here we address the issue of flow redistribution in a network of cortical communicating arterioles in rat parietal cortex that is supplied primarily by the middle cerebral artery (MCA). Our approach makes use of optical-based technologies. Two-photon laser scanning microscopy (TPLSM) in conjunction with labeling of the blood plasma with fluorescein-dextran is used to form maps of the angioarchitecture as well as quantify the transport of individual red blood cells (RBCs). Occlusions are performed in two complementary ways. In the first method, occlusions to individual targeted microvessels at the pial surface are achieved through the introduction of a photosensitizer to the blood stream and the subsequent irradiation of the microvessel with focused laser light. This technique requires, on the one hand, suprathreshold illumination of a surface vessel to form a clot and, on the other hand, subthreshold illumination of deep and lateral vessels to avoid undesired clots. Fulfillment of these conditions represents an extension of the conventional application of the photothrombotic clot method to allow for examination of dynamic flow changes in surface communicating arterioles after focal clot formation in a single surface arteriole. In the second method, an overall decrease in perfusion through the arterial source to parietal cortex is achieved by threading a filament along the carotid artery to block the base of the MCA. This decrease necessarily leads to a change in the overall pressure balance throughout parietal cortex.

3Large-scale maps of the fluorescent brain vasculature in the field of a craniotomy (Materials and Methods) (Figure S1) reveal distal branches of the MCA as well as communicating and diving arterioles (Figure 1A). A high degree of redundancy in the surface arteriole network is apparent, with anastomoses formed between vessels from both the same and different MCA branches (Figure 1B). Candidate vessels for photothrombotic clotting are identified from the maps (Figure 1A) and traced back to a readily identifiable artery or vein; we considered only target vessels that are arterioles. High-resolution planar images (Figure 1C) are used to determine the baseline diameter of the target as well as of the vessels that lie upstream and downstream from the target. Line scans, in which the laser focus is repetitively scanned along the axis of each vessel (Figure 1C), are used to form a space–time image in which the non-fluorescent RBCs are represented as dark streaks (Figure 1D). The sign and magnitude of the slope of the streaks reflect the direction and speed, respectively, of RBC motion (Figure 1D, Figure S2, and Protocol S1). The analysis of this data yields a time series of the RBC speed in an arteriole (Figure 1E). We average over time, typically 40-s epochs, to suppress the underlying cardiac contributions (Figure 1E). We observe that the average speed increases with increasing vessel diameter (Figure S3), consistent with previous studies.

4The line-scan method is accurate for large vessels only if the speed varies slowly across the diameter of the vessel. We thus measured the spatial profile of the average speed across both the lateral (y) and axial (z) directions and observe a profile that peaks in the middle of the vessel and smoothly decreases toward the vessel wall (Figure 1F). The observed profile is flatter than that for laminar flow of a Newtonian fluid in a stiff pipe (Figure 1F), as expected for particle flow in a soft vessel. The smooth nature of the spatial variation in speed implies that the extraction of speed from the line-scan measurements is relatively insensitive to misalignment between the scan and vessel axes so long as the measurements are performed close to the center of the vessel.

5Once the baseline measurements of vessel diameter and RBC speed were completed, the photosensitizer rose bengal was added to the blood stream (Methods). Green laser light, tightly focused on the target vessel (Figure 2A), was used to excite the dye (Figure 2B). We monitored the clot formation in real time, as illustrated for the example of flow into a trifurcating vessel in Figure 2 (Figure 2C; Video S1), and adjusted the laser power to near-threshold levels for clot formation. Once the clot was completed, the diameters of neighboring vessels and the speed of flow in these vessels was again measured (Figure 2D). Further, in control experiments the RBC speed decreased only slightly (5% per h) over 3 h (Figure S4), which far exceeds the duration of a set of measurements after a clot.

6A combination of in vivo flow measurements and post-mortem histology was used to determine the effects of focal irradiation of the target surface vessel on non-targeted surface vessels and the underlying cortical parenchyma. Surface vessels located lateral to the target vessel were never observed to clot, consistent with their exposure to insufficient laser power to trigger photothrombosis (Figure 2C). To reduce parenchymal damage deep to the irradiation site, the incident laser power on subsurface vessels was minimized by three physical mechanisms: (1) the absorption of incident light by the target vessel; (2) the divergence of the beam beyond the target vessel as a result of the strong focus; and (3) the scattering of light by brain tissue. Flow measurements were assessed in parenchymal capillaries situated less than 150 μm beneath the target vessel and within a lateral distance of 100 μm from the target vessel (55 capillaries, across six clots in four rats). In addition, flow was measured in control capillaries located at the same depth but 2 mm or more from the target vessel (18 capillaries). Flow was measured at baseline and after occlusion of the target vessel. The average flow speed across all capillaries was reduced to 57% ± 8% (mean ± standard error of the mean) of the baseline value and was highly variable (standard deviation = 64%). This reduction appeared to result from an increase in the normal occurrence of capillary stalls, from a baseline level of 8% to a post-clot level of 30%. Flow in control vessels was 84% ± 5% of the baseline value, consistent with slow rundown over the course of the experiment (Figure S4), and was less variable (standard deviation = 20%). In toto, these control data show that neighboring surface arterioles and venules remain unclotted after a target vessel is occluded. Further, clots in sub-surface capillaries are limited to vessels that lie directly beneath the target vessel, typically far from the downstream region perfused by the surface vessel.

7We assessed acute ischemic effects of photothrombosis across cell types with the hypoxia marker pimonidazole hydrochloride (Hypoxyprobe; Chemicon International, Temecula, California, United States). This probe forms an immobile adduct in viable but hypoxic tissue, that is visualized post hoc by antibody staining. This marker was injected intravenously after clot formation and allowed to circulate for 1 h prior to sacrifice. As illustrated by the example of Figure 3A, the cortical distribution of Hypoxyprobe is highly sequestered in vessels and cells immediately below the photothrombotic clot (Figure 3A2 and 3A4) within a volume of ~500 pl (four clots across four animals). Additional analysis revealed an increased tendency for the retention of fluorescein-dextran in the extracellular space that borders vessels that lie directly beneath a photothrombotic clot (Figure 3A3). In control experiments, we observed that irradiation of the brain surface in a region between surface vessels also led to fluorescein-dextran retention in subsurface capillaries (Figure S5). These data imply that the pathology seen in capillaries beneath the target vessel is the result of photochemical damage, consistent with previous studies and with our findings of increased capillary stalls beneath the target vessel. Critically, the territory of labeled Hypoxyprobe and fluorescein retention overlap (Figure 3A3). Thus there was no indication of wide-spread tissue hypoxia as a result of the focal clot formation in the surface arteriole.

8To probe ischemic cascades in hypoxic parenchymal vessels, we localized activated platelets by immunostaining with CD41 antibodies. Anti-CD41 immunoreactivity is also seen just below the clot (Figure 3B1 and 3B3) and overlaps with retention of fluorescein-dextran (Figure 3B2). To assay potential damage to other neural elements, we probed for reactive astrocytes by immunostaining for the filamentous astrocyte protein vimentin, as this is strongly up-regulated in reactive astrocytes that proliferate in response to injury. Vimentin immunostaining is relatively evenly distributed in the cortex, most likely in glial endfeet on vessels, but shows only negligibly increased staining (Figure 3C1 and 3C3) among those ischemic vessels that retain fluorescein (Figure 3C2). Together these data indicate that photothrombosis results in a restricted zones of marked vascular ischemia (five clots across five animals).

9To selectively assess neuronal integrity in the vicinity of photothrombosis, we stained tissue sections with antibodies that recognize microtubule associated protein 2 (MAP2); MAP2 staining intensity is a sensitive and early indicator of neuropathology associated with ischemia. As illustrated by the example in Figure 4, relevant tissue could be localized near a clotted vessel that remained intact (Figure 4A) and contained intra-luminal aggregates of nucleated blood cells (Figure 4B). The tissue near the clotted vessel exhibits near uniform staining for MAP2 (Figure 4A). Only subtle neuropathology is observed below the clot as indicated by a slight loss of staining for MAP2 in layer 1 (Figure 4A) and angulated neuronal somata with eccentric nucleus location and a corkscrew appearance in scattered dendrites (Figure 4C). In general, there was no indication of severe neuropathology in neurons that lay directly beneath the targeted vessel (seven clots across three rats) (Figure S6). Thus, we have achieved the desired goal of a localized clot to a single surface vessel with minimum collateral damage.

10The essential consequence of an occlusion to a surface arteriole is illustrated by the example in Figure 2. We observe that blood flow downstream from the occlusion is maintained through a reversal in the direction of flow in a vessel at the first downstream branch (middle branch, Figure 2C and 2D). The reversal occurs within 1 s, i.e., within the acquisition time of an image, after the target vessel was occluded. As a population across all experiments (47 clots in 34 rats with arterioles that ranged from 15 to 140 μm in diameter), flow reversal occurred in one vessel at the first downstream branch after 100% of the clots. In a subset of targeted vessels (n = 7), the clot broke free midway through the experiment and had to be reformed. In each of these cases, the pattern of downstream flow observed after reclotting was the same as that observed after the first clot.

11Three additional examples illustrate typical flow changes in vessels upstream as well as downstream from an occlusion of a communicating arteriole (Figure 5; RBC speeds are shown for baseline and post-clotting conditions). In the circular architecture of Figure 5A, the reversal of flow direction at the first branch downstream from the clot is accompanied by a decrease in the speed of the RBCs. An anastomosis that connects to the same MCA branch as the targeted vessel is clearly the origin of this reversed flow. In the tree-like structure of Figure 5B, the reversal of flow direction at the first branch downstream is again accompanied by a decrease in RBC speed. A complicated pattern of reversed and non-reversed flow is observed in vessels farther downstream from the clot. In addition, flow is slowed in the upstream vessel that originally was a source to the targeted vessel (** in Figure 5B), whereas flow is essentially unchanged in a parallel vessel that shares the same source as the targeted vessel (* in Figure 5B). For the final example (Figure 5C), the targeted vessel was originally fed by the confluence of flow from two branches of the MCA. A photothrombotic clot results in a reversal in downstream flow, as well as a reversal in flow in one of the vessels that was a source of blood to the target vessel. The confluence of separate sources was observed to move to another point along the anastomosis between the MCA branches.

12We now consider the systematics of changes in RBC speed as well as potential changes in vessel diameter that resulted from occlusion of the target. Vessels in the neighborhood of the target vessel were classified according to their topological relationship to the target (Figure 6A): Upstream vessels (U) provided a source of blood to the target vessel; parallel vessels (P) shared the same source as the target; downstream vessels drained from the target vessel and are grouped as those immediately downstream (D1) and those two to four branches downstream (D2 to D4) from the target. We found that the RBC speed in upstream vessels slows substantially, yet the RBC speed in parallel vessels was increased slightly (Figure 6B, left). In two cases the target vessel was fed by a confluence of sources, and one of the upstream vessels reversed direction (as in Figure 5C). For vessels that lie at the first downstream branch from the target, the average RBC speed is reduced and half of the vessels reverse their flow direction (Figure 6B, middle). For vessels that lie farther downstream, the average speed is unchanged and the flow direction is reversed in approximately half the vessels (Figure 6B, right). Summary statistics are given in Table 1.

13In contrast to the substantial changes in the speed of RBC flow in vessels in the neighborhood of the target vessel, only small changes are induced in the diameter of individual vessels as a result of clotting the target vessel (Figure 6C). On average, the diameter of all classes of vessels remained unchanged by an occlusion (see Table 1).

14Our measurements of RBC speed, denoted s, and vessel diameter, denoted d, may be combined to calculate the volume flux of blood, F, where under the approximation of laminar flow. We find that the flux is reduced by ~55%, relative to the baseline value, for upstream vessels and slightly increased for parallel vessels (Figure 6D, left, and Table 1). For vessels downstream from the clot, the post-clot flux ranges between 5% and 200% of the baseline value, with the flux reduced by approximately 40% at the first downstream branch (Figure 6D, center, and Table 1) and unchanged for vessels lying farther downstream (Figure 6D, right and Table 1). We estimate that random errors in the measurement of speed and diameter contribute a 10% uncertainty to the estimate of the flux for each vessel and that drift in the flux contributes an additional 10% uncertainty (see Figure S4 and Protocol S1). The observed spread in the baseline and post-clot values of the flux (Figure 6D and Table 1) is thus dominated by biological variability. Lastly, the changes in flux were correlated with changes in RBC speed for all vessels (p < 0.001), but the changes in flux were correlated with changes in vessel diameter only for the case of parallel vessels (p < 0.01).

15As a means to probe the possible ubiquity of reversals in flow in the arteriole communicating network, we studied the effects of a filament occlusion of the MCA on the flow in individual surface arterioles (Figure 7A). This artery is the primary supply to the surface network. The arterioles measured in response to MCA occlusion were, on average, of larger diameter than those measured in response to photothrombotic occlusion. Similar to the case of the localized occlusion (see Figure 2), we measured the speed of RBC flow and the diameter of vessels before and after occlusion of the MCA. The example data of Figure 7A, in which flow velocities are indicated in six of the surface arterioles, illustrate the essential results. The speed of RBCs was substantially reduced in five of these arterioles, consistent with a drop in the source perfusion. Critically, the direction of flow in four of the six arterioles was reversed after the occlusion.

16As a population across all measurements (42 vessels across 11 rats), we observed an overall reduction in the magnitude of RBC speed and the presence of vessels with reversed as well as non-reversed flow in each experiment (Figure 7B). The diameter of the vessels was essentially unaffected by the occlusion (Figure 7C and Table 1), and there was no correlation between changes in the volume flux and vessel diameter for individual vessels. The volume flux was reduced by 45% after the occlusion (Figure 7D and Table 1). On average, approximately half the vessels reversed flow direction during the MCA occlusion.

17We examined the resilience of blood flow in a network of communicating arterioles that lies in the territory fed primarily by branches of the MCA (see Figure 1). Blood flow downstream from a targeted, localized occlusion does not stop, but rather is reestablished at the first downstream branch by a reversal in the direction of flow in one of the downstream branches (see Figures 2 and 5). Such flow reversals are common phenomena downstream from localized microvessel occlusions (see Figure 5). They are also a feature within this territory in response to occlusion of the MCA (Figure 7). The magnitude of the blood flow change following a localized occlusion in a surface arteriole depends on the topological relationship of a vessel to the clotted vessel (see Figure 6). The average flow at the upstream and the first downstream branch is reduced by less than half, whereas the average flow in vessels parallel to the clotted vessel and far downstream from the occlusion remain near baseline levels (Figure 6). In general terms, this study has revealed the persistent nature of perfusion at the level of cortical surface communicating arteriole networks (Figure 8).

18Our observation that, on average, the diameters of vessels remained constant after localized microvessel occlusion (see Figure 6C and Table 1) is somewhat surprising. Complementary studies on the acute effects of mechanical occlusion on vessel diameter report that the cessation of flow in MCA tributaries, realized through multiple ligations to the surface branches, leads to an immediate, apparoximately 20% increase in vessel size. An even larger increase has been reported immediately following the occlusion of the common carotid arteries. In contrast, a decrement in diameter of the MCA is seen after its occlusion by photothrombotic formation of a large, approximately 1.5 mm–long clot. On the chronic timescale of 1 mo, multiple ligations to the surface branches of the MCA led to dilation to 200% of the initial vessel diameter. In addition, other work has shown that cerebral arterioles, especially leptomeningeal anastomoses that connect the anterior cerebral with the MCA, show an approximately 50% increase in diameter 1 mo after MCA occlusion. The dilation observed in previous acute and chronic studies has been attributed to active vascular regulation and/or to remodeling mechanisms. In contrast to these past results, we observe no systematic change in the diameter of downstream pial arterioles over a few hours in response to a small, approximately 100 μm–long clot formed in single surface arterioles by photothrombotic occlusion (Figure 6C and Table 1). One possible mechanism for this observed stability in vessel diameter may be that the new flow pattern is rapidly established, in about 1 s after the formation of an occluding clot (Video S1). Of interest, a classic means to demonstrate vascular auto-regulation in humans is based on modulation of the partial pressure of inspired CO2. However, recent work now shows that although cerebral blood flow undergoes predictable changes, the diameter of large cerebral arteries may be unchanged or vary widely.

19With regard to the magnitude of reperfusion in the vicinity of a localized clot, the observed flux was 60% of normal flow at the first downstream branch and 100% of normal flow at more distal downstream branches (Figures 6 and 8). The results of past studies suggest that flow rates must drop to 10% to 30% of baseline values before irreversible neuropathology occurs. Such damage is neither expected nor observed to occur under the conditions of this study (see Figures 3 and 4), consistent with a past study of distal MCA occlusions. In particular, we observed limited pathology directly below the clotted surface vessel, which was caused by photochemical damage to the capillaries, and no pathology in cortical regions that lie downstream from the site of clot formation (Figures 3 and 4). Thus the surface network of communicating arterioles appears to be well protected against single-point occlusions of surface arterioles by virtue of its architecture alone.

20The redundancy of brain vasculature changes as one proceeds from the level of the internal carotid arteries to that of the brain capillaries. At the level of the supply to the brain, the Circle of Willis provides sufficient redundancy so that all cephalic arteries are perfused in the event of an occlusion to one of the internal carotids. Blockages immediately downstream from the Circle of Willis, at the level of a cerebral artery, lead to a variety of neuropathologies. In particular, an occlusion to the base of the MCA typically results in infarction and widespread cell death in the basal ganglia whereas the same blockage produces less-severe deficits in the cerebral cortex or penumbral region. Compensation at the level of cortex has been attributed to flow through leptomeningeal anastomoses that connect the territories of the intact anterior cerebral artery and the MCA. The present work demonstrates that functional compensations can also occur at the scale of surface communicating arterioles, formed through anastomoses between branches of the MCA (Figure 1A and 1B), since perfusion is maintained in neighboring branches after photothrombosis of a single arteriole (see Figures 5, 6, and 8). This failsafe mechanism most likely results from the many short-range loops in this arterial network, which may also play a role in rerouting a fixed supply of blood among different cortical columns during shifts in cortical electrical activity. The highly redundant cortical surface vasculature discussed here for rats is also present in humans. In contrast to the present results for the surface vasculature of cortex, the apparent lack of a similar system of anastomoses in the basal ganglia may contribute to its greater vulnerability after a blockage of the MCA.

21The present work focused on the surface network of communicating arterioles (Figure 1B, CA), a two-dimensional network formed by anastomoses between branches of a cerebral artery. This network delivers blood to the cortical parenchyma through a series of diving arterioles (Figure 1B, DA). The topology and resilience of the three-dimensional, subcortical microvascular network remain an open issue. From a technical perspective, the present methodology to form localized occlusions is ideal for surface vessels but cannot selectively target vessels at depth because, as for all techniques that rely on the absorption of a single photon, all vessels superficial to the target will also be affected. Thus a technology that depends on the nonlinear absorption of light is required. Such a technique, which makes use of high-fluence ultrashort pulses of light that can precisely ablate tissue, has been demonstrated to work with vessels in superficial cortical layers. This method cannot, in turn, be used to target clots in the surface communicating arteriole network, as the lack of containment by parenchymal tissue causes irradiated vessels to burst. Preliminary data suggest that clots to subsurface microvessels leads to perfusion failure in the immediate downstream vessels. This is in marked contrast to the redistribution of flow that preserves perfusion in all downstream branches of the surface communicating arteriole network (see Figures 6, 8D, and 8E).

22In humans, damage to microvessels is a known pathological condition. In particular, occlusion of small-scale arterioles is a likely cause of clinically silent lacunar infarcts that are correlated with an increased risk of dementia and cognitive decline. It is thus interesting that the Rotterdam Scan study, which identified clinically silent lacunar infarcts through magnetic resonant imaging, found that few cortical infarcts were located near the surface and thus where the vasculature appears to be most redundant. Our results for surface blood flow dynamics (see Figures 6, 7, and 8) suggest an emerging relation between vascular topology and susceptibility to stroke in different regions of the brain.

23Our subjects were 51 Sprague-Dawley rats. In 34 animals of both sexes, 150 to 350 g in mass, one to five individual microvessels were occluded photothrombotically. In 11 male animals, 290 to 310 g in mass, the MCA was occluded by the filament method. The remaining six rats were used for control experiments. All animals were anesthetized by interperitoneal injection of urethane (150 mg per 100-g rat), supplemented as necessary; urethane is reported to maintain autoregulation of cerebral blood flow. Atropine sulfate was delivered by subcutaneous injection (5 μg per 100-g rat) at the start of surgery and supplemented hourly at a reduced dose (1 μg per 100-g rat) to reduce secretions. We further supplemented by subcutaneous injection of 5% (w/v) glucose in phosphate buffered saline (PBS) (0.5 ml per 100-g rat) every hour. Body temperature was maintained at 37.5 °C and heart rate was continuously monitored. A 4-mm by 6-mm craniotomy was prepared over parietal cortex, with the center at medial-lateral equal to 4.5 mm and anterior-posterior equal to −4.5 mm relative to Bregma. The dura was removed, a chamber consisting of a metal frame and a removable coverglass (no. 1) lid was glued to the skull, and the space between the exposed brain surface and the coverglass was filled with 1.5% (w/v) low–melting point agarose in an artificial cerebro-spinal fluid. A 0.3-ml bolus of a 5% (w/v) solution of 2 MDa fluorescein-conjugated dextran in PBS was injected into the tail vein to label the blood plasma. The care and experimental manipulation of our animals have been reviewed and approved by the Institutional Animal Care and Use Committee at the University of California, San Diego.

24Images were obtained with a two-photon laser scanning microscope of local design that was modified to include a path for a green laser beam (see Figure S1 and Protocol S1). A 0.12 numerical aperture (NA), 5×-magnification air objective was used to obtain images of the surface vasculature across the entire cranial window to aid in navigating around the cortical vasculature. We changed to a 0.8-NA, 40×-magnification water-immersion objective for high-resolution imaging, line-scan measurements, and photothrombotic clotting with the green laser. The line-scan rate was 1.3 kHz for measuring RBC speed, and we typically acquired line-scans for 40 s and report the average speed over this period.

25A continuous wave green-light laser (λ = 532 nm) (TIM-622; Transverse Industries, Taipei Hsien, Taiwan) was directed onto the beam axis of the microscope with a dichroic mirror (see Figure S1). The green-light beam underfilled the back aperture of the objective and was aligned so it focused at the center of the same plane as the near-infrared pulsed laser beam and formed an approximately 5-μm diameter spot. As the green-light overlapped with the fluorescent spectrum of fluorescein, the green-light was delivered in pulses of 1-s duration that were interspersed with TPLSM imaging.

26Vessels targeted for clotting were centered in the imaging field (see Figure 2A). The rat was then given a 0.3-ml intravenous injection of 1% (w/v) rose bengal (Na salt) in PBS, and the wall of the target vessel was irradiated with 0.1 to 5 mW of green laser light for a total of 30 to 600 s. Irradiation of a photosensitizer leads to the production of singlet oxygen, which damages the wall of the vessel and subsequently triggers a clotting cascade that leads to an occlusion. We slowly increased the green laser power while monitoring clot formation in near real time and used the minimum power required to trigger clot formation in the target vessel. This procedure led to the formation of a single, localized clot (see Figure 2C and Video S1). In control experiments, green laser irradiation at a typical power led to no visible effects on the target vessel in the absence of rose bengal (Figure S7) and no observable retention of fluorescein-dextran in the tissue below the target vessel (see Figure S5).

27A filament is advanced inside the internal carotid artery until it blocks the origin of the MCA, as described. Briefly, an incision is made in the neck, exposing the left common carotid artery. The external carotid and pterygopalatine arteries are ligated with 4–0 silk suture. An incision is made in the wall of the common carotid artery and a 4–0 nylon suture, whose tip has been blunted by heating on a microforge, is advanced 17.5 mm from the bifurcation point of the external and internal carotid arteries, thereby blocking the ostium of the MCA. Flow velocities are measured at baseline and during the MCA occlusion. A large-scale TPLSM image (see Figure 7A) is used to ensure the same vessels are measured at baseline and during the occlusion. The occlusion measurements are performed within two hours of the insertion of the filament.

28The cortex of seven of the animals that received localized photothrombotic clots, four with pimonidazole hydrochloride (Hypoxyprobe-1 [90201]; Chemicon) injected 1 h before sacrifice, were perfused transcardially with 100 ml of PBS, followed by 100 ml of 4% (w/v) paraformaldehyde in PBS. Fiducial marks were made in the corners of the craniotomy by passing −20 μA though a single tungsten electrode that translated at 2 mm/s. The brain was cryoprotected with sucrose and then 50 μm–thick sections were cut in a coronal plane on a freezing-sliding microtome.

29Sections near the location of the clots were selected based on the location of the targeted vessels relative to the fiducial marks, as determined from widefield images of the craniotomy taken before and after the perfusion and the retention of fluorescein-dextran in capillaries beneath the target vessel (see Figure 3A3). To confirm the completeness of a clot in the surface vessels, we looked for intra-luminal aggregates of blood-borne white cells by staining sections with propidium iodide (1 μg/ml) mixed into aqueous mountant (see Figure 4B).

30Sections lying beneath photothrombotic occlusions were mounted onto Fisher Superfrost Plus slides (Pittsburgh, Pennsylvania, United States) and underwent antigen retrieval in 10 mM citrate buffer (pH 6.0) heated to boiling in a microwave oven (300 s at full power). The slides were incubated overnight in one of four monoclonal antibodies in diluent with PBS and 0.2% (v/v) Triton X–100 followed by incubation with a biotinylated anti-mouse secondary antibody: (1) Hypoxyprobe antibody (90204; Chemicon); (2) MAP2 antibody (M1406; Sigma, St. Louis, Missouri, United States); (3) CD41 antibody (MAB1207; Chemicon); and (4) vimentin antibody (MAB3400; Chemicon). In all cases, bound antibody was visualized with the Vector ABC Kit (Vector Laboratories, Burlingame, California, United States) using diaminobenzadine as the chromagen. The sections were cover-slipped with Prolong mountant (Molecular Probes, Eugene, Oregon, United States).