Extracellular matrix-cytoskeletal interactions in vascular cells

The stereotypical molecular cascade in neovascular age-related macular degeneration: the role of dynamic reciprocity

This review summarises our current understanding of the molecular basis of subretinal neovascularisation (SRNV) in age-related macular degeneration (AMD). The term neovascular AMD (NVAMD) is derived from the dominant early clinical features of haemorrhage, fluid, and lipid in the subretinal space (SRS) and the historical role of fluorescein angiography in detecting the presence of NV tissue. However, at the cellular level, SRNV resembles an aberrant but stereotypical tissue repair response that incorporates both an early inflammatory phase and a late fibrotic phase in addition to the neovascular (NV) component that dominates the early clinical presentation. This review will seek not only to highlight the important molecules involved in each of these components but to demonstrate that the development of SRNV has its origins in the earliest events in non-NV AMD pathogenesis. Current evidence suggests that this early-stage pathogenesis is characterised by complement-mediated immune dysregulation, leading to a state of chronic inflammation in the retinal pigment epithelium/Bruch's membrane/choriocapillaris complex. These initial events can be seamlessly and inextricably linked to late-stage development of SRNV in AMD by the process of dynamic reciprocity (DyR), the ongoing bidirectional communication between cells, and their surrounding matrix. Moreover, this correlation between disease onset and eventual outcome is reflected in the temporal and spatial correlation between chronic inflammation, NV, and fibrosis within the reparative microenvironment of the SRS. In summary, the downstream consequences of the earliest dysfunctional molecular events in AMD can result in the late-stage entity we recognize clinically as SRNV and is characterized by a spectrum of predictable, related, and stereotypical processes referred to as DyR.

Dynamic reciprocity in the wound microenvironment

Here, we define dynamic reciprocity (DR) as an ongoing, bidirectional interaction among cells and their surrounding microenvironment. In this review, we posit that DR is especially meaningful during wound healing as the DR-driven biochemical, biophysical, and cellular responses to injury play pivotal roles in regulating tissue regenerative responses. Such cell-extracellular matrix interactions not only guide and regulate cellular morphology, but also cellular differentiation, migration, proliferation, and survival during tissue development, including, e.g., embryogenesis, angiogenesis, as well as during pathologic processes including cancer, diabetes, hypertension, and chronic wound healing. Herein, we examine DR within the wound microenvironment while considering specific examples across acute and chronic wound healing. This review also considers how a number of hypotheses that attempt to explain chronic wound pathophysiology may be understood within the DR framework. The implications of applying the principles of DR to optimize wound care practice and future development of innovative wound healing therapeutics are also briefly considered.

Debridement: controlling the necrotic/cellular burden

PURPOSE

To provide physicians and nurses with an overview of the options for debriding a chronic wound to improve wound healing.

TARGET AUDIENCE

This continuing education activity is intended for physicians and nurses with an interest in learning about methods for debriding chronic wounds to promote wound healing.

LEARNING OBJECTIVES

After reading the article and taking the test, the participant will be able to: 1. Describe the 4 types of debridement most commonly used in clinical practice. 2. Describe when to debride a wound and how to determine which method to use.

Cyclic strain induces reorganization of integrin alpha 5 beta 1 and alpha 2 beta 1 in human umbilical vein endothelial cells

Cyclic strain has been shown to modulate endothelial cell (EC) morphology, proliferation, and function. We have recently reported that the focal adhesion proteins focal adhesion kinase (pp125FAK) and paxillin, are tyrosine phosphorylated in EC exposed to strain and these events regulate the morphological change and migration induced by cyclic strain. Integrins are also localized on focal adhesion sites and have been reported to induce by tyrosine phosphorylation of pp125FAK under a variety of stimuli. To study the involvement of different integrins in signaling induced by cyclic strain, we first observed the redistribution of alpha and beta integrins in EC subjects to 4 h cyclic strain. Human umbilical vein endothelial cells (HUVEC) seeded on either fibronectin or collagen surfaces were subjected to 10% average strain at a frequency 60 cycles/min. Confocal microscopy revealed that beta 1 integrin reorganized in a linear pattern parallel with the long axis of the elongated cells creating a fusion of focal adhesion plaques in EC plated on either fibronectin (a ligand for alpha 5 beta 1) or collagen (a ligand for alpha 2 beta 1) coated after 4 h exposure to cyclic strain. beta 3 integrin, which is a vitronectin receptor, did not redistribute in EC exposed to cyclic strain. Cyclic strain also led to a reorganization of alpha 5 and alpha 2 integrins in a linear pattern in HUVEC seeded on fibronectin or collagen, respectively. The expression of integrins alpha 5, alpha 2, and beta 1 did not change even after 24 h exposure to strain when assessed by immunoprecipitation of these integrins. Cyclic strain-induced tyrosine phosphorylation of pp125FAK occurred concomitant with the reorganization of beta 1 integrin. We concluded that alpha 5 beta 1 and alpha 2 beta 1 integrins play an important role in transducing mechanical stimuli into intracellular signals.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Regulation of vascular tone

The intimal surface of the blood vessel in vivo is subject to shear stress resulting from blood flow, which in most of the circulation, at least at rest, is laminar. Turbulence can occur at bifurcations, especially those of the large arteries, and where vessels curve significantly. Shear stress is a frictional tangential force exerted at the fluid-intimal interface in the long axis of the vessel. It is now known that hemodynamic shear stress can influence a large variety of biological processes in endothelial cells, which vary from those with a short response time, just a few milliseconds, such as the opening of ion channels, to those that change over a period of minutes to several hours, for example, endocytosis and cytoskeleton rearrangement, and those features that alter much more slowly, such as cell shape and stiffness. In addition to these types of changes, there are suggestions that flow acting through shear stress may be responsible for several basic attributes of the vasculature, including the relative size and diameter of the components of a branching vascular system. In this symposium on the flow regulation of the blood vessel, the first presentation dealt with optimality principles that appear to govern the dimensions of the vasculature, in particular the geometry of the arterial branching and the role of shear stress. An optimally designed system is one that requires the least metabolic work to perform its function.(ABSTRACT TRUNCATED AT 250 WORDS)

Shear stress modulates endothelial cell morphology and F-actin organization through the regulation of focal adhesion-associated proteins

Flow-related shear stress has been shown to modulate endothelial cell structure and function including F-actin microfilament organization. Focal adhesion-associated proteins such as vinculin, talin, and specific integrins may play a role in the modulation of these cytoskeletal and morphological changes. Double-label immunofluorescence studies indicated that, in static culture, alpha 5 beta 1 fibronectin receptors (alpha 5 beta 1 FNRs) and alpha v beta 3 vitronectin receptors (alpha v beta 3 VNRs) were found predominantly in the peripheral regions of bovine aortic endothelial cells (BAECs) corresponding to the localization of vinculin, talin, and actin microfilament terminations. In response to shear stress, concomitant with cell elongation and the appearance of stress fibers aligned with the direction of flow, there was a prominent localization of vinculin and alpha v beta 3 VNRs as the "upstream" end of the cells. Stress fiber terminations were clearly evident at these concentrations of focal adhesion-associated proteins. These data suggest that the upstream concentration of these proteins may direct shear stress-induced stress fiber formation and may function in the alignment of the fibers in the direction of flow. Levels of surface alpha v beta 3 VNRs were found to decrease in response to flow, possibly reflecting the decrease in numbers of "downstream" receptors. Unlike the arrangement of vinculin and alpha v beta 3 VNRs observed following exposure to flow, talin and alpha 5 beta 1 FNRs, in addition to being localized at the upstream end of the cell, were also evenly distributed throughout the rest of the cell. Surface levels of alpha 5 beta 1 FNRs increased in response to shear stress, perhaps providing an increased adherence of BAECs to the extracellular matrix through these receptors. These data suggest that focal adhesion-associated proteins play specific roles in the response of BAECs to shear stress.

Simultaneous oscillations in the membrane potential of pig coronary artery endothelial and smooth muscle cells

1. The effects of tetrabutylammonium (TBA) on the mechanical tension and on the electrical behaviour of endothelial and smooth muscle cells were studied in intact porcine coronary artery strips. 2. Superfusion of strips with TBA (2-20 mM) induced mechanical oscillations, leading to an increase in tonic isometric tension. 3. TBA-induced mechanical oscillations were correlated with fluctuations of the membrane potential of endothelial cells, which were identified by iontophoretic injection of Lucifer Yellow. 4. The endothelial cell membrane potential fluctuations appeared as action potentials or smaller amplitude slow waves, and were synchronized with electrical membrane potential fluctuations of the underlying coronary smooth muscle cells. 5. Oscillations induced by TBA in smooth muscle cells were not affected by removal of the endothelium, and depended on the presence of calcium in the external medium. 6. To our knowledge, this is the first description of action potential-like fluctuations in the endothelium. It is concluded that the oscillations were generated in the smooth muscle and that they propagate to the endothelium. The question of the mode of propagation of the signal is discussed.

Pericyte growth and contractile phenotype: modulation by endothelial-synthesized matrix and comparison with aortic smooth muscle

We compared the effects of endothelial-synthesized matrix and purified matrix molecules on pericyte (PC) and aortic smooth muscle cell (SMC) growth, heparin sensitivity, and contractile phenotype in vitro. When PC are plated on endothelial-synthesized (EC) matrix, cell number is, on average, 3.1-fold higher than identical populations grown on plastic. Under the same conditions, SMC proliferation is stimulated 1.6-fold. Purified matrix molecules, such as collagen type IV (COLL) or fibronectin (FN), both major components of the EC matrix, stimulate PC/SMC growth 1.2-1.7-fold. Heparin (100 micrograms/ml), which inhibits the growth of early passage SMC by 60%, inhibits PC growth approximately 50%, when cells were plated on plastic. However, PC plated on EC matrix in the presence of heparin (100 micrograms/ml) grow as well as parallel cultures grown on plastic (in the absence of heparin). Concomitant with matrix-stimulated proliferation, we observed a marked reduction in PC containing alpha vascular smooth muscle actin (alpha VSMA), as seen by immunofluorescence using affinity-purified antibodies (173/615 positive pericytes on DOC matrix (28%) vs. 221/285 (77%) positive on glass). SMC respond similarly. Whereas alpha VSMA protein is markedly altered when PC and SMC are cultured on EC matrix, similar reductions in mRNA are not observed. However, Northern blotting does reveal that PC contain 17-30 times the steady-state levels of alpha VSMA mRNA compared to SMC. When SMC and PC cultures on plastic are treated with heparin, the steady-state levels of vascular smooth muscle actin mRNA increase 5 and 1.5 fold, respectively. Similarly, heparin treatment of PC grown on plastic induces a 1.8 fold increase in nonmuscle actin mRNA. These heparin-induced alterations in isoactin mRNA levels are not seen when PC are cultured on EC matrix. We also observed reductions in alpha VSMA and beta actin mRNA levels when PC are plated on FN, where they maintain a ratio of 13:1 (alpha:beta). Similar ratios are found in SMC present in rat and bovine aortae in vivo. These steady-state isoactin mRNA ratios are slightly different from those seen in cultured PC (8-10:1; alpha:beta). These results suggest that selective synthesis and remodelling of the endothelial basal lamina may signal alterations in pericyte growth and contractile phenotype during normal vascular morphogenesis, angiogenesis, or during the microvascular remodelling that accompanies hypertensive onset.

Changes in the microstructure of cultured porcine aortic endothelial cells in the early stage after applying a fluid-imposed shear stress

Time course changes in the cell shape and in the patterns of microfilament distribution were analyzed quantitatively using cultured porcine aortic endothelial cell monolayers before and after a shear flow exposure. Geometrical parameters of the cell and of the microfilament were measured on fluorescent photomicrographs of the cells stained with rhodamine-phalloidin. After the shear flow exposure (20 dyn cm-2, 0-24 h), the endothelial cells on glass were elongated and oriented to the direction of the flow. Under the no-flow condition, F-actin filaments were mainly localized at the periphery of the cell, although some filaments were seen in the more central portion. The angles of the filaments were randomly distributed. After 3 h, the stress fiber-like structure of an F-actin bundle was formed in the central part of the cells, and these filaments were oriented to the direction of the flow. The degree of orientation increased as the time of exposure to shear stress became longer. This change in F-actin preceded cell elongation and orientation; these changes were statistically significant only after 6 h. After 24 h, peripheral filaments were again observed, and the fluorescence intensity of rhodamine-phalloidin-stained cells was enhanced. These findings suggest that the redistribution of F-actin filaments is one of the early cellular responses to the onset of shear stress and that it is one of the most important factors controlling cell elongation and orientation to the direction of the flow.

Type V collagen represses the attachment, spread, and growth of porcine vascular smooth muscle cells in vitro

We attempted to clarify the effects of various purified extracellular components, including types I, III, IV, and V collagen and fibronectin on attachment, spread, growth, and DNA synthesis of porcine aortic smooth muscle cells (SMCs) in vitro. The number, area and shape index (SI = 4 pi S/L2) of cells attached to different substrates were determined at various intervals of incubation. The cell number and [3H]thymidine incorporation into DNA were measured on the 1st and 6th days of culture. SMCs showed the largest number of attached cells on fibronectin, but the smallest number of attached cells on type V collagen. There was no evidence of effects of the serum in media on the attachment of SMCs to the substrates. The areas of attached SMCs were the largest on fibronectin and the smallest on type V collagen. The shape index of SMCs on fibronectin decreased relative to those on other substrates. On the 6th day in culture, the number and population doubling of SMCs on type V collagen were significantly fewer than those on other substrates. Both the incorporation rate of [3H]thymidine into DNA and the percentage of nuclei labeled with [3H]thymidine were significantly less in the SMCs on type V collagen on the 1st day than those on other substrates. SMCs on types I, III, and IV collagen showed intermediate levels of cell attachment, spread, and growth. These results suggest that attachment, spread, and growth of SMCs are affected mainly by solid phase purified extracellular components and are most strongly suppressed by type V collagen. When DNA synthesis of growth-arrested SMCs was reinitiated by the addition of serum, type V collagen most intensively inhibited the rate and amount of [3H]thymidine incorporation. Flow cytometric analysis demonstrated an increased in the proportion of cells in G0/G1 phase on type V collagen in comparison with that on other substrates. Thus, the antiproliferative effect of type V collagen may relate to inhibition of transition of SMCs from the G0/G1 into the S phase.

Mechanisms of endothelial growth control

It is well documented that vascular endothelial cells constitute an unusually quiescent epithelial cell population. These slowly dividing cells are known to undergo more rapid division in association with wound healing as well as a variety of pathologic conditions such as tumor vascularization and diabetic retinopathy. Although the precise mechanisms responsible for vascular quiescence have not been elucidated, some insights into the regulators of vascular growth control are beginning to be gained. Nearly all of the investigations have utilized cultured vascular cells. Regulation of vascular endothelial growth by polypeptide growth regulators, extracellular matrix molecules, cell-cell interactions, and mechanical forces has been demonstrated in these tissue culture systems.

Density-dependent expression of hyaluronic acid binding to vascular cells in vitro

Alterations in glycosaminoglycans and their receptors have been associated with changes in cell proliferation during development, wound healing, regeneration, and remodeling. We have previously found a differential effect of hyaluronic acid on the attachment of vascular cells in vitro; endothelial cell (EC) attachment was improved on hyaluronic acid-coated substrates, whereas that of smooth muscle cells (SMC) was reduced (Orlidge and D'Amore, 1986). To determine if hyaluronic acid binding sites are involved in these different substrate preferences, we have studied specific hyaluronic acid binding to cultured bovine aortic EC and SMC. Since very large numbers of cells are required for these binding assays (3 x 10(6)/data point), and since the level of hyaluronate binding to fixed and native SMC and EC was similar, fixed cells were used throughout this study. The effect of cell density on hyaluronic acid binding was investigated. No significant difference was observed between hyaluronic acid binding to sparse and high density SMC. On the other hand, a more than threefold elevation in specific hyaluronic acid binding was observed on low density EC when compared to binding on high density EC. Hyaluronic acid binding was found to be specific; excesses of heparan sulfate and chondroitin sulfate had no effect on the levels of specific binding. Finally, the effect of cell passage on SMC binding of hyaluronic acid was measured. Specific binding was measured from 1st to 12th passage cells and was found to increase with passage number so that by passage 12, hyaluronic acid binding was fourfold that of 1st passage cells. These data support the concept that SMC may become less differentiated upon continuous culture. Our results indicate quantitative changes in the level of hyaluronic acid binding to vascular cells as a function of their growth state. Further, these data correlate well with in vivo observations which suggest a role for hyaluronic acid in vascular development.

Beta actin and its mRNA are localized at the plasma membrane and the regions of moving cytoplasm during the cellular response to injury

Previous work in our laboratory has shown that microvascular pericytes sort muscle and nonmuscle actin isoforms into discrete cytoplasmic domains (Herman, I. M., and P. A. D'Amore. 1985. J. Cell Biol. 101:43-52; DeNofrio, D.T.C. Hoock, and I. M. Herman. J. Cell. Biol. 109:191-202). Specifically, muscle (alpha-smooth) actin is present on the stress fibers while nonmuscle actins (beta and gamma) are located on stress fibers and in regions of moving cytoplasm (e.g., ruffles, lamellae). To determine the form and function of beta actin in microvascular pericytes and endothelial cells recovering from injury, we prepared isoform-specific antibodies and cDNA probes for immunolocalization, Western and Northern blotting, as well as in situ hybridization. Anti-beta actin IgG was prepared by adsorption and release of beta actin-specific IgG from electrophoretically purified pericyte beta actin bound to nitrocellulose paper. Anti-beta actin IgGs prepared by this affinity selection procedure showed exclusive binding to beta actin present in crude cell lysates containing all three actin isoforms. For controls, we localized beta actin as a bright rim of staining beneath the erythrocyte plasma membrane. Anti-beta actin IgG, absorbed with beta actin bound to nitrocellulose, failed to stain erythrocytes. Simultaneous localization of beta actin with the entire F-actin pool was performed on microvascular pericytes or endothelial cells and 3T3 fibroblasts recovering from injury using anti-beta actin IgG in combination with fluorescent phalloidin. Results of these experiments revealed that pericyte beta actin is localized beneath the plasma membrane in association with filopods, pseudopods, and fan lamellae. Additionally, we observed bright focal fluorescence within fan lamellae and in association with the ends of stress fibers that are preferentially associated with the ventral plasmalemma. Whereas fluorescent phalloidin staining along the stress fibers is continuous, anti-beta actin IgG localization is discontinuous. When injured endothelial and 3T3 cells were stained through wound closure, we localized beta actin only in motile cytoplasm at the wound edge. Staining disappeared as cells became quiescent upon monolayer restoration. Appearance of beta actin at the wound edge correlated with a two- to threefold increase in steady-state levels of beta actin mRNA, which rose within 15-60 min after injury and returned to noninjury levels during monolayer restoration. In situ hybridization revealed that transcripts encoding beta actin were localized at the wound edge in association with the repositioned protein. Results of these experiments indicate that beta actin and its encoded mRNA are polarized at the membrane-cytoskeletal interface within regions of moving cytoplasm.

Effects of collagen matrix on proliferation and differentiation of vascular smooth muscle cells in vitro

In an attempt to better define the relationship between collagen matrices and vascular smooth muscle cells in vitro, proliferation of smooth muscle cells was observed in the early stages of culture. Cells spread on collagen gels had a longer doubling time and less incorporation of [3H]thymidine into DNA on the first day of culture than did cells grown on a plastic substrate. Cells on collagen gels were more elongated than were those on the plastic substrate and showed a "hills and valleys" arrangement from the first day in culture on the collagen type III gel. All cells were identified as smooth muscle having definite microfilaments, dense bodies, and pinocytotic vesicles. They had a distinct actin filament running from end to end when labeled with nitrobenzoxadiazole-phallacidin. Cells on the collagen gels had a larger number of actin filaments traveling parallel to the direction of the major axis of their cytoplasm than did those on the glass substrate. Therefore, cultured smooth muscle cells in the more physiological environment for cells in vitro, i.e., on collagen gels, show a suppression of cellular proliferation and an enhancement of differentiation in the early stages of culture. The effects of collagens on the differentiation of cells vary with the collagen phenotype.

SPARC, a secreted protein associated with cellular proliferation, inhibits cell spreading in vitro and exhibits Ca+2-dependent binding to the extracellular matrix

SPARC (Secreted Protein Acidic and Rich in Cysteine) is a Ca+2-binding glycoprotein that is differentially associated with morphogenesis, remodeling, cellular migration, and proliferation. We show here that exogenous SPARC, added to cells in culture, was associated with profound changes in cell shape, caused rapid, partial detachment of a confluent monolayer, and inhibited spreading of newly plated cells. Bovine aortic endothelial cells, exposed to 2-40 micrograms SPARC/ml per 2 x 10(6) cells, exhibited a rounded morphology in a dose-dependent manner but remained attached to plastic or collagen-coated surfaces. These round cells synthesized protein, uniformly excluded trypan blue, and grew in aggregates after replating in media without SPARC. SPARC caused rounding of bovine endothelial cells, fibroblasts, and smooth muscle cells; however, the cell lines F9, PYS-2, and 3T3 were not affected. The activity of native SPARC was inhibited by heat denaturation and prior incubation with anti-SPARC IgG. The effect of SPARC on endothelial cells appeared to be independent of the rounding phenomenon produced by the peptide GRGDSP. Immunofluorescence localization of SPARC on endothelial cells showed preferential distribution at the leading edges of membranous extensions. SPARC bound Ca+2 in both amino- and carboxyl-terminal (EF-hand) domains and required this cation for maintenance of native structure. Solid-phase binding assays indicated a preferential affinity of native SPARC for several proteins comprising the extracellular matrix, including types III and V collagen, and thrombospondin. This binding was saturable, Ca+2 dependent, and inhibited by anti-SPARC IgG. Endothelial cells also failed to spread on a substrate of native type III collagen complexed with SPARC. We propose that SPARC is an extracellular modulator of Ca+2 and cation-sensitive proteins or proteinases, which facilitates changes in cellular shape and disengagement of cells from the extracellular matrix.

Matrix control of tumor angiogenesis

Endothelial cell migration is a key feature of angiogenesis. Epidermal Growth Factor (EGF) or Tumor Angiogenesis Factor (TAF) induce cell migration and angiogenesis. When the matrix components, collagen or fibronectin, were used as a substratum in the phagokinesis assays, EGF- or TAF-induced cell migration was inhibited. It has been proposed that TAF activates cellular protease causing the matrix degradation that is evident during neovascularization in vitro. If such degradation leads to cell migration and angiogenesis, then other agents that interfere with the synthesis or assembly of matrix components should stimulate cell migration and angiogenesis. The proline analogues cis hydroxyproline, azetidine and dehydroproline are known modulators of cellular collagen synthesis. At optimal concentration (10(-5)M) these analogues caused 3-fold increases in endothelial cell migration rates in vivo as tested by a subcutaneous implant assay. We conclude from these studies that: (i) matrix components control cellular migration rates; high concentration of collagen or fibronectin inhibit angiogenically active inducers of endothelial cell migration. (ii) Intracellular modulation of synthesis of collagens leads to angiogenesis by stimulating cell migration. These findings relate to tumor angiogenesis and that TAF might trigger angiogenesis either by activation of latent proteases or by some modification of matrix assembly during synthesis that affects cell adhesion and migration.

In situ analysis of microvascular pericytes in hypertensive rat brains

We used immunofluorescence microscopy and isoactin-specific antibodies to characterize the pattern and prevalence of pericytes within the brain microcirculation. Blood pressures of normotensive, Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats were measured prior to sacrifice and pressure-perfusion fixation. WKY and SHR brains were subdivided into ten major regions prior to ultracryomicrotomy. Sections 0.3-0.5 micron wide were treated with 10-40 micrograms/ml affinity-purified antibodies to the muscle and non-muscle actin isoforms. These localization studies show that there are four times the number of pericyte-rich capillaries in the SHR motor cortex compared to WKY counterparts (59.9 vs. 15.3%). In contrast, the sensory cortex of both rat strains is deficient in muscle actin staining surrounding the capillaries. The most striking difference in pericyte presence and muscle actin antibody staining between the SHR and WKY was observed in the tegmentum of the brainstem. There is nearly a one-to-one coincidence observed in pericyte and capillary profiles present within thin, frozen sections of the SHR midbrain. SHR pons capillaries were also pericyte-enriched. WKY analyses of plastic embedded thin sections confirmed the presence of pericytes and their filament-enriched processes encircling the capillaries of the hypertensive brains. These results suggest that pericytes may play important roles in hypertension and cerebrovascular disease processes.