Two-Photon Imaging of Cortical Surface
Microvessels Reveals a Robust Redistribution
in Blood Flow after Vascular Occlusion
Chris B. Schaffer1,2, Beth Friedman3,4, Nozomi Nishimura1,2, Lee F. Schroeder5, Philbert S. Tsai1,2, Ford F. Ebner6,
Patrick D. Lyden3,4,5, David Kleinfeld1,2,5*
1 Department of Physics, University of California San Diego, La Jolla, California, United States of America, 2 Center for Theoretical Biological Physics, University of California
San Diego, La Jolla, California, United States of America, 3 Department of Neurosciences, University of California San Diego, La Jolla, California, United States of America,
4 Department of Neurology, Veterans Affairs Medical Center, San Diego, California, United States of America, 5 Graduate Program in Neurosciences, University of California
San Diego, La Jolla, California, United States of America, 6 Department of Psychology, Vanderbilt University, Nashville, Tennessee, United States of America
A highly interconnected network of arterioles overlies mammalian cortex to route blood to the cortical mantle. Here we
test if this angioarchitecture can ensure that the supply of blood is redistributed after vascular occlusion. We use
rodent parietal cortex as a model system and image the flow of red blood cells in individual microvessels. Changes in
flow are quantified in response to photothrombotic occlusions to individual pial arterioles as well as to physical
occlusions of the middle cerebral artery (MCA), the primary source of blood to this network. We observe that perfusion
is rapidly reestablished at the first branch downstream from a photothrombotic occlusion through a reversal in flow in
one vessel. More distal downstream arterioles also show reversals in flow. Further, occlusion of the MCA leads to
reversals in flow through approximately half of the downstream but distant arterioles. Thus the cortical arteriolar
network supports collateral flow that may mitigate the effects of vessel obstruction, as may occur secondary to
Citation: Schaffer CB, Friedman B, Nishimura N, Schroeder LF, Tsai PS, et al. (2006) Two-photon imaging of cortical surface microvessels reveals a robust redistribution in
blood flow after vascular occlusion. PLoS Biol 4(2): e22.
Normal 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 [1,2]. 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 anasto-
moses . 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 [5–8].
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 [3,9,10], 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 neighbor-
ing branches in a network.
Here 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) [11,12] 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) [13–15]. 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 photo-
sensitizer 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 photo-
Received August 8, 2005; Accepted November 11, 2005; Published January 3, 2006
Copyright: ? 2006 Schaffer et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author
and source are credited.
Abbreviations: MAP2, microtubule associated protein 2; MCA, middle cerebral
artery; NA, numerical aperture; RBC, red blood cell; TPLSM, two-photon laser
Academic Editor: Maurizio Corbetta, Washington University School of Medicine,
United States of America
* To whom correspondence should be addressed. E-mail: email@example.com
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P PL Lo oS S BIOLOGY
thrombotic clot method [16–20] 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 .
Mapping and Perturbing Blood Flow in Communicating
Large-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 redun-
dancy 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) [14,15,23,24]. 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), con-
sistent with previous studies .
The 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
Figure 1. TPLSM of Fluorescently Labeled Cortical Vasculature In Vivo
(A) Low-magnification TPLSM image of fluorescently labeled brain
vasculature in rat parietal cortex. The axes indicate the rostral (R) and
medial (M) directions. In the inset is an image of latex-filled brain
vasculature taken from Scremin , with a box that indicates the
approximate size and location of a typical craniotomy and an arrow that
identifies the MCA.
(B) Tracing of the surface arterial vascular network from the image in (A).
Branches of the MCA are indicated, as are representative examples of the
communicating arterioles (CA) that form the surface network and diving
arterioles (DA) that supply cortex.
(C) Maximal projection of a TPLSM image stack through a cortical
arteriole. The dark line indicates the location where the line-scan data
were taken, and the arrow represents the direction of flow obtained from
(D) Line-scan data from the vessel in (C) to quantify the flow of RBCs.
Each scan is displayed below the previous one, forming a space–time
image with time increasing from top to bottom of the image. The dark
streaks running from upper right to lower left are formed by the motion
of the non-fluorescent RBCs. The RBC speed is given by the inverse of the
slope of these streaks; the direction of flow is discerned from the sign of
(E) RBC speed along the center of the arteriole shown in (C) and (D) as a
function of time. The periodic modulation of the RBC speed occurs at the
approximately 6-Hz heart rate. The dotted line represents the temporal
average of the speed.
(F) RBC speed in an arteriole, averaged over 40 s, as a function of the
transverse position in the vessel along horizontal (y) and vertical (z)
directions. The parabolic curve represents the laminar flow profile that
most closely matches the data, i.e., s¼A?(1?r/R)2where s is the speed of
the RBCs, r is a radius from the origin and corresponds to either the y or z
direction, R is the measured vessel radius of 26 lm, and A is a free
parameter (A ¼ 10 mm/s).
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Flow in Cortical Surface Arterioles
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.
Once 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.
Figure 2. Photothrombotic Clotting of Individual Targeted Surface Cortical Blood Vessels in Anesthetized Rat
(A) Maximal projection of TPLSM image stack showing several surface arterioles (red A) and venules (blue V). The green circle indicates the region of the
targeted arteriole that will be irradiated with green laser light. The white box indicates the region and orientation of the images in (C).
(B) Schematic illustration of the targeted photothrombotic occlusion of a vessel and experiment timeline. After baseline imaging and blood flow
measurements, rose bengal is intravenously injected into the animal. Green laser light is focused onto the wall of the target vessel, which excites the
rose bengal and ultimately triggers the natural clotting cascade. Surface vessels adjacent to the target vessel are not occluded because they are not
exposed to the 532-nm irradiation.
(C) Planar TPLSM images of photothrombotic clotting of a surface arteriole. The frame on the left is taken at baseline. The green circle indicates the
region of the targeted arteriole that will be irradiated, whereas the white arrows indicate the blood flow direction, as determined from line-scan
measurements in the targeted vessel and in the vessels downstream from the target. The numbers over the downstream vessels correspond to the
numbered line-scan data shown in (D). The streaked appearance of the vessels is due to the motion of RBCs during the acquisition of the image. The
center frame is taken after an intravenous injection of rose bengal and 2-min irradiation with 0.5 mW of 532-nm laser light. The vessel is partially
occluded (indicated by green double arrow). The right frame is taken after one more minute of irradiation. The target vessel is completely clotted
(indicated by red X) whereas surrounding vessels are unaffected. Stalled blood flow is indicated by the dark mass of clotted cells in the target region
and the brightly fluorescent region of blood plasma upstream from the target region. Note that blood flow is maintained in the branches downstream
from the target vessel by a reversal in the direction of blood flow in the center branch, as determined from the line-scan data in (D).
(D) Baseline and post-clot line-scan data for the numbered vessels downstream from the target vessel shown in (C). The average RBC speed determined
from the line-scan data is indicated for each case, with a positive speed taken to be along the baseline direction of flow.
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Flow in Cortical Surface Arterioles
Localization of Photothrombotic Effects
A combination of in vivo flow measurements and post-
mortem histology was used to determine the effects of focal
irradiation of the target surface vesselon 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 lm
beneath the target vessel and within a lateral distance of 100
lm 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% 6 8% (mean 6
standard error of the mean) of the baseline value and was
highly variable (standard deviation ¼ 64%). This reduction
appeared to result from an increase in the normal occurrence
of capillary stalls [14,27], from a baseline level of 8% to a post-
clot level of 30%. Flow in control vessels was 84% 6 5% of the
baseline value, consistent with slow rundown over the course
of the experiment (Figure S4), and was less variable (standard
deviation ¼ 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.
We assessed acute ischemic effects of photothrombosis
across cell types with the hypoxia marker pimonidazole
hydrochloride (Hypoxyprobe; Chemicon International, Te-
mecula, 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 fluo-
rescein-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 fluo-
rescein retention overlap (Figure 3A3). Thus there was no
indication of wide-spread tissue hypoxia asa result of the focal
clot formation in the surface arteriole.
To 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
Figure 3. Assay of Oxidative Stress and Vascular Disruption
Pimonidazole hydrochloride was introduced into the bloodstream at 1 h
after targeted photothrombosis. At the end of this period, the animal was
euthanized and transcardially perfused for brain tissue fixation. Labeling
is illustrated for a closely spaced series of sections (within 150 lm).
(A1–A4) The section, immunolabeled with an antibody against pimoni-
dazole adducts (Hypoxyprobe), shows localization of adducts at uptake
sites that are largely restricted to zones beneath the clot. Lateral and
medial directions are labeled by M and L, respectively. The intermediate-
and high-magnification views show immunolabeling across neural
compartments, including parenchyma and blood vessels (A2 and A4).
A mixed brightfield-fluorescent image shows that the fluorescein-
dextran retained in the vessels and extravasated into the parenchyma
overlaps the pimonidazole labeling (A3).
(B1–B3) Immunolabeling of the platelet marker CD41 indicates sparse
damage. A mixed image of fluorescein-dextran vascular retention and
the CD41 immunoreactivity indicates that the damage is confined to just
below the clot (B2).
(C1–C3) Immunolabeling of tissue with the reactive astrocyte marker
vimentin increased only marginally the labeling of vessels just below the
clot (indicated by an asterisk). A mixed image of fluorescein-dextran
retention and the vimentin immunoreactivity indicates that the damage
is co-localized (C2).
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Flow in Cortical Surface Arterioles
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).
To 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 neuro-
pathology 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.
Qualitative Aspects of Flow Rearrangement after a Single
The 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 lm in diameter), flow
reversal occurred in one vessel at the first downstream branch
after 100% of the clots. In a subset of targeted vessels (n¼7),
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.
Three 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
Figure 4. Neuronal Integrity near Photothrombotic Clot
The animal was sacrificed 1 h after the disruption, and the tissue was
harvested from below the target vessel shown in Figure 5C.
(A) The section, immunolabeled with an antibody against MAP2, shows
widespread staining of neurons.
(B) Fluorescent imaging of propidium iodide counterstain. The stained
endothelial cells demarcate vessels. Note the numerous nuclei with
segmented lobes characteristic of aggregates of leucocytes within the
(C) High-magnification view shows subtle neuropathology in some cells,
i.e., corkscrew dendrites (single arrows) and shrunken neurons with
eccentric nuclei (double arrow).
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Flow in Cortical Surface Arterioles
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
Quantitative Aspects of Flow Rearrangement after a
Single Arteriole Occlusion
We 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.
In 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
Figure 5. Examples of Flow Changes that Result from Localized Occlusion of a Cortical Surface Arteriole
(A–C) On the left and right are TPLSM images taken at baseline and after photothrombotic clotting of an individual vessel, respectively. Left center and
right center are diagrams of the surface vasculature with RBC speeds (in mm/s) and directions indicated. The red X indicates the location of the clot, and
vessels whose flow direction has reversed are indicated with red arrows and labels. In the examples of panels (A) and (B) we show maximal projections
of image stacks whereas the example in panel (C) shows single TPLSM planar images; the streaks evident in the vessels in these latter frames are due to
RBC motion, and the dashed box in the diagrams represents the area shown in the images.
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Flow in Cortical Surface Arterioles
average, the diameter of all classes of vessels remained
unchanged by an occlusion (see Table 1).
Our measurements of RBC speed, denoted s, and vessel
diameter, denoted d, may be combined to calculate the
volume flux of blood, F, where F ¼
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
Figure 6. Compendium of Flow Changes following Localized Photothrombotic Clotting of Communicating Surface Arterioles
(A) Illustration of the four different classes of vessels considered, each delineated by their connectivity to the target vessel (indicated by an X).
(B) Plots of post-clot RBC speed as a function of baseline RBC speed for each vessel class. The post-clot and baseline speeds were significantly correlated
for the upstream and parallel vessels, but uncorrelated for the downstream vessels (Table 1).
(C) Plots of post-clot vessel diameter as a function of baseline diameter. The pre- and post-clot diameters were correlated for all cases (Table 1).
(D) Plots of post-clot volume blood flux as a function of the baseline value. The diagonal lines represent post-clot flux levels of 10% and 100% of
baseline. As for the diameters, the pre- and post-clot fluxes were correlated for all cases (Table 1).
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Flow in Cortical Surface Arterioles
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).
Flow Rearrangement after Occlusion of the MCA
As 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
Table 1. Summary Statistics for Changes in Vascular Parameters
Branch Designation Post-Clot to Pre-Clot Ratio (Mean 6 Standard Error) Post-Clot to Pre-Clot Correlation Coefficient, r2
Speed, sDiameter, dFlux,p
0.41 6 0.08
1.17 6 0.08
0.60 6 0.08
0.99 6 0.17
0.51 6 0.08
1.07 6 0.05
1.00 6 0.03
1.03 6 0.02
1.01 6 0.01
1.06 6 0.03
0.43 6 0.08
1.09 6 0.09
0.59 6 0.07
1.00 6 0.16
0.55 6 0.08
D2 to D4
Downstream from filament
**Statistically significant (two-tailed Student t-test) at p , 0.003 or better.
Figure 7. Quantitative Measurements of Flow Changes in Cortical Arterioles after Filament Occlusion of the MCA
(A) Example of flow changes observed following MCA occlusion. Left: projection of a TPLSM image stack taken at baseline. Center: tracing of the surface
arteriole network from the image with the baseline blood flow speed and direction indicated in some vessels. Right: blood flow speed and direction
during MCA occlusion. Red arrows and speed labels indicate vessels whose direction has reversed. The axes indicate the rostral (R) and medial (M)
(B–D) RBC speed, vessel diameter, and volume blood flux, respectively, during MCA occlusion as a function of baseline values. The baseline and
occlusion values of the diameter and flux are significantly correlated (p , 0.005), with r2¼0.92 and 0.26, respectively; the baseline and occlusion values
of the speed are not significantly correlated.
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Flow in Cortical Surface Arterioles
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
As 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 direc-
tion during the MCA occlusion.
We 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 local-
ized 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).
Our observation that, on average, the diameters of vessels
remained constant after localized microvessel occlusion (see
Figure 6C and Table 1) is somewhat surprising. Complemen-
tary 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 anastomo-
ses that connect the anterior cerebral with the MCA , show
an approximately 50% increase in diameter 1 mo after MCA
occlusion [36,37]. 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 lm–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
Figure 8. Summary of Quantitative Measurements of Changes in Volume
Blood Flux in Response to Single Microvessel Occlusion and MCA
(A) Illustration showing topological relationship of vessels in (B) to (E)
relative to the clotted arteriole (indicated with an X).
(B–E) Histograms of the ratio of the post-clot flux to the baseline flux for
vessels with different topological relationships to a photothrombotically
clotted cortical surface arteriole: (B) upstream, (C) parallel, (D) first branch
downstream, and (E) second to fourth branch downstream.
(F) Histogram of the ratio of the flux measured during intra-luminal
filament occlusion of the MCA to the baseline flux for cortical arterioles.
In the interests of clarity, outliers with post-clot flux greater than twice
the baseline flux were excluded from these histograms: one parallel
vessel (ratio¼7), two D1 vessels (ratio¼2.5, 2.8), one D2 – 4 vessel (ratio
¼11), and one filament occlusion vessel (ratio¼2.1). The arrows point to
the mean values across all data points for each vessel class.
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Flow in Cortical Surface Arterioles
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 [38,39] or vary widely .
With 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 [41–43] 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.
The 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 immedi-
ately 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 [42,44].
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 compen-
sations 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 [4,46]. In contrast to the present results for
thesurface vasculature ofcortex, theapparent lackofasimilar
system of anastomoses in the basal ganglia may contribute to
its greater vulnerability after a blockage of the MCA .
The 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
formlocalizedocclusionsis idealforsurface vessels but cannot
selectively target vessels at depth because, as for all techniques
that rely on the absorption of a single photon, all vessels
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 down-
stream branches of the surface communicating arteriole
network (see Figures 6, 8D, and 8E).
In humans, damage to microvessels is a known pathological
condition [50–54]. 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 [56–58]. 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
Materials and Methods
Surgery. Our 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 lg per 100-g rat) at the
start of surgery and supplemented hourly at a reduced dose (1 lg 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 8C 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.
TPLSM. Images 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), 53-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, 403-magnification water-immersion objective
for high-resolution imaging, line-scan measurements, and photo-
thrombotic 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.
PLoS Biology | www.plosbiology.org February 2006 | Volume 4 | Issue 2 | e220267
Flow in Cortical Surface Arterioles
Photothrombotic clotting. A continuous wave green-light laser (k¼
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-lm 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
Vessels 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
[16,17]. 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).
Filament occlusion of MCA. A 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
Postmortem histology. The cortex of seven of the animals that
received localized photothrombotic clots, four with pimonidazole
hydrochloride (Hypoxyprobe-1 ; 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 lA
though a single tungsten electrode that translated at 2 mm/s. The
brain was cryoprotected with sucrose and then 50 lm–thick sections
were cut in a coronal plane on a freezing-sliding microtome.
Sections 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 lg/ml) mixed into aqueous mountant (see
Immunohistochemistry. Sections lying beneath photothrombotic
occlusions were mounted onto Fisher Superfrost Plus slides (Pitts-
burgh, 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, Cal-
ifornia, United States) using diaminobenzadine as the chromagen.
The sections were cover-slipped with Prolong mountant (Molecular
Probes, Eugene, Oregon, United States).
Figure S1. Schematic of the Two-Photon Laser Scanning Microscope
Our realization is based on an 800-nm fs (femtosecond) laser with an
integrated continuous-wave (CW) 532-nm laser for photothrombotic
clotting. The CW laser, attenuated using neutral density (ND) filters,
is directed onto the beam axis of the TPLSM with a dichroic mirror
(600 nm–long pass, dichroic 1). An approximately 2-mm hole was
etched in the coating of the TPLSM dichroic (dichroic 2) to allow
transmission of the green laser beam. The 532-nm laser was aligned so
it focused in the same plane as and at the center of the TPLSM image.
The rat is bolted, via a metal head frame affixed to the skull, onto a
two-dimensional translation stage that allows precise positioning of
the rat relative to the TPLSM field of view and the CW laser focus.
HWP, half-wave plate; PMT, photomultiplier tube.
Found at DOI: 10.1371/journal.pbio.0040022.sg001 (141 KB PDF).
Figure S2. Illustration of Automated Algorithm for Finding Slope of
Streaks Formed by Moving RBCs in TPLSM Line-Scan Data
The data are for the same vessel shown in Figure 1C, 1D, and 1E,
although not from the same time point as Figure 1D.
(A) Line-scan data from an epoch in time are transformed to a square
matrix with normalized axes. In the left image, an abrupt change in
the slope of the streaks due to a heartbeat is indicated.
(B) The central region of the square matrix is rotated, and we search
for the angle that yields horizontal streaks, as in the middle panel.
(C) Separability of line-scan data as a function of rotation angle;
separability is maximal for vertical or horizontal streaks (Protocol
S1). The rotation angle corresponding to horizontal streaks is chosen,
yielding the RBC speed and direction, in this case: 11.9 mm/s and a
flow direction of right to left.
Found at DOI: 10.1371/journal.pbio.0040022.sg002 (262 KB PDF).
Figure S3. Baseline Measurements of the Time-Averaged RBC Speed
at the Center of a Vessel as a Function of the Diameter of the Vessel
The data include all arterioles in this study. Arterioles measured as
part of the photothrombotic (rose bengal) and MCA (filament)
occlusion studies are indicated separately. The line represents a best-
fit linear regression to the data, and shows a statistically significant
correlation, valid for diameters between 10 and 130 lm, between
speed, S, and diameter, D, given by S ¼ (4.9 mm/s) þ (124/s) 3 D (p ,
Found at DOI: 10.1371/journal.pbio.0040022.sg003 (765 KB PDF).
Figure S4. Normalized Volume Blood Flux in Five Arterioles as a
Function of Time
For each vessel, each measurement of the flux was normalized to the
average over all measurements for that vessel. The five vessels varied
substantially in their average volume flux: 0.17 (circles), 0.013
(diamonds), 0.0048 (squares), and 0.0021 ll/s (triangles). The inset
shows the histogram of the normalized flux for all vessels, after the
long-term decrease in flux (;5% per h) is removed.
Found at DOI: 10.1371/journal.pbio.0040022.sg004 (710 KB PDF).
Figure S5. Epi-Fluorescence Images Overlaid on Wide-Field Images
Showing Vascular Retention of Circulating Fluorescein-Dextran
(A) The brain surface was irradiated with 1 mW of 532-nm irradiation
for 1 min after an intravenous injection of rose bengal. The laser
focus was deliberately located in a region where there were no surface
vessels; therefore no surface vessel was clotted. The vascular retention
and parenchymal extravasagation of the fluorescein-conjugated
dextran used for in vivo imaging is somewhat more extensive than
that observed in Figure 3. This is likely because a surface target vessel
was not present to absorb and scatter the incident laser light, leading
to a higher fluence incident on the sub-surface capillaries, and
increasing the extent of the photochemical damage.
(B) A surface vessel was irradiated at 1 mW for 1 min without any rose
bengal present. No retention of the fluorescein-dextran is evident in
the sub-surface capillaries, indicating no photochemical damage.
Found at DOI: 10.1371/journal.pbio.0040022.sg005 (2.4 MB PDF).
Figure S6. MAP2 Immunohistology of Coronal Brain Slices beneath a
Photothrombotic Clot of a Surface Arteriole (Ischemic Side) and
from the Corresponding Location on the Contralateral Side
(A, B, E, and F) Clotted side. (C, D, G, and H) Contralateral side. The
boxes in panels (A), (C), (E), and (G) indicate the locations of the
images in panels (B), (D), (F), and (H), respectively. The arrows in (B)
and (F) indicate cells showing minor neuropathology (single arrow
indicates cork-screw dendrites; double arrow indicates shrunken cells
with eccentric nuclei. Most cells in panels (B) and (F) exhibit no
pathology. The example on the left is the same as that shown, in part,
in Figure 3. In the example on the right, some trapped RBCs are
visible in capillaries beneath the photothrombotically clotted vessel
PLoS Biology | www.plosbiology.org February 2006 | Volume 4 | Issue 2 | e220268
Flow in Cortical Surface Arterioles
(E), although the capillaries were still flowing after clot formation,
based on in vivo TPLSM.
Found at DOI: 10.1371/journal.pbio.0040022.sg006 (6.8 MB PDF).
Figure S7. Control Experiment Showing that Photothrombotic Clot
Formation Requires Both Rose Bengal and Green Laser Irradiation
(A) Baseline TPLSM image of same vessel shown in Figure 2. The
green circle indicates the region that will be irradiated with 532-nm
(B) After 2 min irradiation with 0.5 mW of 532-nm laser light before
intravenous injection of rose bengal. No clot formation is evident.
(C) After an additional 2-min irradiation after intravenous injection
of rose bengal. Forming clot indicated by green arrow.
Found at DOI: 10.1371/journal.pbio.0040022.sg007 (386 KB PDF).
Protocol S1. TPLSM for Photothrombotic Clotting and Character-
ization of RBC Flux
A locally designed two-photon laser scanning microscope was
modified to allow real-time photothrombotic clotting of individual
cortical surface blood vessels. TPLSM line-scans were used to collect
space–time data of RBC motion in individual cortical blood vessels,
and an algorithm based on singular value decomposition was used to
find the slope of the streaks formed by moving RBCs in this space–
time data, thereby determining the RBC speed. Control experiments
established that systematic flux changes in vessels due to intravenous
injections or to shedding of clot material during the formation of a
localized photothrombotic clot are less than 10%.
Found at DOI: 10.1371/journal.pbio.0040022.sd001 (419 KB PDF).
Video S1. Two-Photon Fluorescence Image Sequence Showing the
Formation of a Localized Photothrombotic Clot in a Surface
This is the same example shown in Figure 2. Images are displayed at a
rate that is sped up by a factor of 15. The fluctuating, streaked
appearance of the vessels is due to the motion of RBCs, and indicates
flow. The initial direction of flow in the targeted arteriole (vertically
centered in image) is right to left, branching into three vessels on the
left of the frame. A saturated white strip at the top of the frame
indicates irradiation with 523-nm light. Clot material is formed just
downstream from the irradiated region of the vessel, and some sheds
off and is carried away down one of the downstream branches. Note
that after a complete occlusion is formed, the streaked appearance is
maintained in the downstream branches, indicating they are still
Found at DOI: 10.1371/journal.pbio.0040022.sv001 (7.3 MB MOV).
We thank Pablo Blinder, Eshel Ben-Jacob, Herbert Levine, and
Stephen Segal for useful discussions, Earl Dolnick for assistance with
the electronics, Donald Pizzo and Leon Thal for use of their
photomicroscope, and Coherent, Inc., for the loan of equipment.
This work was funded by the David and Lucile Packard Foundation
(DK), the Veteran’s Affairs Medical Research Department (PDL), the
National Institute of Health grants NS/041096 (DK), EB/003832 (DK),
NS/025907 (FFE), and NS/043300 (PDL), a La Jolla Interfaces in
Science Postdoctoral Fellowship (CBS), and the National Science
Foundation Graduate Fellowship Program (NN).
Competing interests. The authors have declared that no competing
Author contributions. CBS, BF, NN, LFS, PDL, and DK conceived
and designed the experiments. CBS, BF, NN, LFS, PST, and FFE
performed the experiments. CBS, BF, NN, PDL, and DK analyzed the
data. PDL and DK contributed reagents/materials/analysis tools. CBS,
BF, and DK wrote the paper.
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