The Pericyte Microenvironment During Vascular Development

Received: 31 August 2018 | Revised: 29 April 2019 | Accepted: 3 May 2019 DOI: 10.1111/micc.12554

INVITED REVIEW

The pericyte microenvironment during vascular development

1Center for Heart and Reparative Medicine, Fralin Biomedical Research Abstract Institute, Roanoke, Virginia Vascular pericytes provide critical contributions to the formation and integrity of the 2 Department of Biomedical Engineering blood vessel wall within the microcirculation. Pericytes maintain vascular stability and Mechanics, Virginia Polytechnic State Institute and State University, Blacksburg, and homeostasis by promoting endothelial cell junctions and depositing extracellular Virginia matrix (ECM) components within the vascular basement membrane, among other 3Graduate Program in Translational Biology, Medicine, and Health, Virginia Polytechnic vital functions. As their importance in sustaining microvessel health within various Institute and State University, Blacksburg, tissues and organs continues to emerge, so does their role in a number of pathological Virginia conditions including cancer, diabetic retinopathy, and neurological disorders. Here, 4Department of Biological Sciences, College of Science, Virginia Polytechnic State we review vascular pericyte contributions to the development and remodeling of the Institute and State University, Blacksburg, microcirculation, with a focus on the local microenvironment during these processes. Virginia We discuss observations of their earliest involvement in vascular development and 5Department of Basic Science Education, Virginia Tech Carilion School of essential cues for their recruitment to the remodeling endothelium. Pericyte involve- Medicine, Roanoke, Virginia ment in the angiogenic sprouting context is also considered with specific attention to Correspondence crosstalk with endothelial cells such as through signaling regulation and ECM deposi- John C. Chappell, Fralin Biomedical Research tion. We also address specific aspects of the collective cell migration and dynamic Institute, 2 Riverside Circle, Roanoke, VA 24016. interactions between pericytes and endothelial cells during angiogenic sprouting. Email: [email protected] Lastly, we discuss pericyte contributions to mechanisms underlying the transition Funding information from active vessel remodeling to the maturation and quiescence phase of vascular This work was supported by NIH grants R00HL105779 and R56HL133826 and NSF development. grant 1752339 (to JCC). KEYWORDS endothelial cells, pericytes, vascular morphogenesis

1 | INTRODUCTION described these cells in the 1870s as perivascular cells that differed in morphology from vascular smooth muscle cells (vSMCs); peri- Vascular pericytes are essential components of the microcirculation. cytes appeared to be elongated along microvessels in perivascular These specialized cells wrap around and ensheath microvessels, pro- locations, while vSMCs were concentrically wrapped around the 1-3 moting endothelial cell junction stability and depositing extracellular endothelium. Despite being nearly ubiquitous in the microcircu- matrix (ECM), among many key functions. Eberth and Rouget first lation, research into vascular pericytes lagged behind their endothelial counterparts following their initial identification. More recently, however, pericytes have attracted significant attention across an Abbreviations: CNS, central nervous system; Col‐IV, Type IV collagen; ECM, extracellular matrix; EGFR, epidermal growth factor receptor; HB‐EGF, heparin‐binding EGF; HSPGs, array of biological disciplines. Potential new functions are being re- heparan sulfate proteoglycans; NG2, neural‐glial antigen‐2; PDGF‐BB, Platelet‐derived ported for these cells in vascular development and tissue homeo- growth factor‐BB; PDGFR , platelet‐derived growth factor receptor‐ ; vBM, vascular β β stasis, as well as in a broad range of disease states.4,5 Contributions basement membrane; VEGF‐A, vascular endothelial growth factor‐A; vSMC, vascular smooth muscle cell; αSMA, α‐smooth muscle actin. to microvessel stability, and to overall vascular barrier function, are

Laura B. Payne and Huaning Zhao equally contributed to this work.

Microcirculation. 2019;00:1–11. wileyonlinelibrary.com/journal/micc © 2019 John Wiley & Sons Ltd | 1 2 | PAYNE et al. well ‐accepted roles for pericytes. Intriguing nuances within these proximity to the endothelium as another criteria for positive identifi- more “canonical” roles are still being discovered in health6-8 and in cation of vascular pericytes.4 French et al70 utilized this approach in 9-12 diseases such as proliferative diabetic retinopathy, cancer and the developing mouse yolk sac and found PDGFRβ‐positive pericytes metastatic progression,13,14 and Alzheimer's disease.15 Pericyte adjacent to distinct Tie2‐expressing endothelial cells as early as E8.5. contractility, or the modulation of microvessel diameter, is an area Recently, Jung et al coupled a powerful double‐labeled pericyte of ongoing investigation, particularly in the central nervous system transgenic mouse line (Pdgfrβ‐EGFP and Cspg4‐DsRed) with endo- (CNS).16-23 In addition, the tissue regeneration capacity of pericytes, thelial immunostaining. The authors visualized vascular‐associated acting as a pool of perivascular mesenchymal stem cells,24-26 has also pericytes in the brain at E10.5, with increasing abundance through- been described as a potential role for these cells.27-29 This particu- out the remainder of embryonic and early postnatal development.71 lar role may be context‐ and/or model‐dependent however.27,30-39 Previous studies have suggested that the endothelium acquires These established and emerging functions for pericytes, among pericyte associations primarily, if not exclusively, after endothelial others, are still being elucidated and have been the subject of many sprouting events begin to establish basic vascular networks, perhaps insightful reviews.4,40-46 We acknowledge the wide breadth of in- allowing greater plasticity in endothelial remodeling.66,72-74 We, and triguing studies that have addressed pericyte biology thus far. Here, others,75-78 have found that mouse embryonic stem cells give rise we narrow the focus of this review on pericytes in their engagement not only to primitive endothelial cell networks, but also to presump- with the vascular endothelium during blood vessel development and tive pericytes (or pericyte precursors) during the earliest stages of remodeling. We give a specific consideration to the elements within cardiovascular development.64 Pericytes seem to emerge at approx- the pericyte microenvironment that are critical for their contribu- imately the same time as, or even prior to, endothelial cell differenti- tions to these processes. ation, homing to endothelium engaged in both vasculogenic (Payne, L.B. and Chappell, J.C., Unpublished observations) and angiogenic processes.64,75-78 Nevertheless, whether pericytes, or their precur- 2 | PERICYTE ORIGINS AND sors, directly engage with the developing endothelium to actively co- IDENTIFICATION ordinate the formation of primitive blood vessels remains an ongoing area of investigation. Insight from these studies could shed light on During development, pericytes arise from a wide range of embry- the involvement of pericytes in pathological conditions associated onic and extra‐embryonic regions. Neural crest and primordial mes- with vessel dysmorphogenesis and mis‐patterning such as arterio‐ enchyme are the most commonly described origins, though specific venous malformations and cerebral cavernous malformations.58,79,80 tissues and organs likely derive vascular pericytes from unique cellular niches.23,43,47,48 Because of their physical proximity and proposed functional overlap, pericyte lineage has frequently been inferred 3 | PERICYTE RECRUITMENT AND from tracing the differentiation of vSMCs.4 While pericytes and CONTRIBUTION TO EARLY VESSEL vSMCs may share a common derivation to a certain point, these cells FORMATION ultimately occupy unique regions of the vasculature and are mor- phologically and functionally distinct.49 Additional validation studies As endothelial cells form primitive tubes and more complex vascular will therefore be necessary, especially as pericyte‐specific tools and networks, they simultaneously release molecular cues that facilitate markers continue to evolve alongside the advancement of single‐ pericyte recruitment to the developing vessel wall.66,81,82 Platelet‐ cell analysis and next‐generation sequencing techniques.50-55 Volz derived growth factor‐BB (PDGF‐BB) is one of the most potent regu- et al56 for example recently observed vascular pericytes present in lators of pericyte recruitment, investment, and retention. Genetic the mouse heart as early as embryonic day 11.5 (E11.5), and these loss or mutation of this ligand (eg, altering its ECM retention motif) cells provided a source for coronary artery vSMCs. Interestingly, and its primary signaling receptor tyrosine kinase, PDGFRβ, lead this elegant study found that Notch pathway signals, known to be to substantial loss of pericytes and subsequent vascular complica- important for vSMC differentiation and development,57-61 were not tions.8,66,83-87 In an in vitro model of vasculogenic tube formation, required for the differentiation, recruitment, or retention of micro- endothelial cells also secrete high levels of heparin‐binding EGF‐like vascular pericytes. These observations are consistent with CNS‐fo- growth factor (HB‐EGF), which engages epidermal growth fac- cused62,63 and developmental studies.64 However, these results may tor receptor (EGFR, or ErbB1) and ErbB4 on pericytes to promote be context‐dependent65 and could be confounded by the fact that, their recruitment.88 A pericyte recruitment factor that can be re- although platelet‐derived growth factor receptor‐β (PDGFRβ) and leased from non‐endothelial sources appears to be stromal derived 89 neural‐glial antigen‐2 (NG2, gene name: Cspg4) are useful markers factor‐1α (SDF‐1α), which binds to CXCR4 on pericytes. SDF1α for pericytes, they are also expressed by other cell types such as may in fact work synergistically with PDGF‐BB to elicit pericyte mi- vSMCs and brain glia,61,63,66 and oligodendrocyte precursor cells,67- gration to growing vessels.90 These factors, as well as others, can 69 respectively. facilitate pericyte recruitment to developing vessels, perhaps during To clarify the interpretation of these markers, vascular endo- the earliest stages of vessel formation. Comparable to observations thelial cells are often co‐labeled within a given specimen, providing from extra‐embryonic yolk sac vessels,70 our recent observations PAYNE et al. | 3 suggest that this recruitment may occur during vasculogenic forma- surrounding ECM, as recent studies suggest that ECM components tion of primitive vessels from nascent endothelial cells (Payne, L.B. secreted by adjacent cell types such as astrocytes may activate peri- and Chappell, J.C., Unpublished observations). Pericytes may also cytes and even promote their differentiation.102 While pericytes ulti- physically participate in endothelial cell coalescence75 and additional mately provide important regulation of the vBM, it is unclear to what processes that are critical for vasculogenesis, though more work re- extent pericytes deposit each of these vBM components during: (a) mains to further support these findings. early blood vessel formation, (b) the later stages of vascular develop- As vessels develop, specialized ECM known as the vascular basement and maturation when vessel remodeling nears completion, and ment membrane (vBM) is deposited around the cells within the blood (c) aging and in vascular‐related pathologies. Mis‐regulated synthesis vessel wall, conferring stability and providing a scaffold for signal- of ECM components may also implicate pericytes in fibrotic disease ing proteins, among other functions.91,92 Pericytes make important processes. contributions to this ECM deposition and vBM formation.93,94 In an in vitro co‐culture model with endothelial cells mimicking developmental vasculogenic tube formation, pericytes deposited ECM 4 | PERICYTE INVOLVEMENT IN components such as fibronectin, laminin isoforms, perlecan, and ANGIOGENIC SPROUTING nidogen‐1.95 These observations, along with ultra‐structural imaging techniques such as scanning electron microscopy,96,97 have revealed During angiogenic remodeling, endothelial cells sprout and migrate that their synthesis, remodeling of, and location within the vBM is an- outward from existing vessels such as those formed during vascu- other key characteristic of vascular pericytes.98 This feature is likely logenesis. Sprouting angiogenesis can also occur from quiescent an important biomarker for distinguishing pericytes from other cell microvasculature stimulated with endogenous, pro‐angiogenic cues types that may be positioned adjacent to blood vessels but may like vascular endothelial growth factor‐A (VEGF‐A) (Figure 1). In not be physically incorporated within the vessel wall proper that the adult brain (Figure 1) and early postnatal retina,103 for example, is encased within the vBM. Type IV collagen (col‐IV) for instance is pericytes are found directly adjacent to endothelial “tip” cells that an important component of the vBM that pericytes and endothe- are leading the elongation of nascent vessel branches. This scenario lial cells synthesize or degrade depending on the phase of vascular has also been described previously in a variety of vascular formation remodeling, pruning, or stabilization.88,92,98-100 In the developing models and assays.76,96,97,104-106 This close spatial proximity between and adult mouse CNS, pericytes have also been found to secrete sprouting endothelial cells and neighboring pericytes suggests a vitronectin into the vBM.50 Pericyte‐derived vitronectin may in fact crosstalk, and potentially a reciprocal signaling relationship, be- exacerbate pathologies such as cerebral autosomal dominant arteri- tween these two cell types during angiogenic remodeling. Hellstrom opathy with sub‐cortical infarcts and leukoencephalopathy by con- et al107 provided support for this idea from observations of VEGF‐A‐ tributing to white matter lesion severity in aged mice.101 There may stimulated endothelial tip cells synthesizing PDGF‐BB as they mi- also be unique signaling relationships between pericytes and their grated outward from a parent vessel. Because PDGF‐BB possesses a

(A) (B) (C)(D)

FIGURE 1 Confocal image of a sprouting endothelial cell labeled for platelet‐endothelial cell adhesion molecule‐1 (PECAM‐1; A and green in D, indicated by light green arrows) with an adjacent pericyte labeled for PDGFRβ (B and red in D, indicated by pink arrows) extending from a microvessel in the adult mouse brain. Cell nuclei are labeled with DAPI (C, and blue in D). The pericyte appears to engage the sprouting endothelial tip cell from the base of the filopodial extensions, along the length of the sprout and the parent vessel. Scale bar, 10 microns 4 | PAYNE et al.

FIGURE 2 Time‐lapse images of a sprouting endothelial cell visualized by eGFP expression from the Flk‐1 promoter (left column, and green in right column) and an associated pericyte expressing the fluorescent reporter DsRed from the Ng2/ Cspg4 promoter (middle column, and red in the right column) in an ex vivo model of vessel formation in the developing back skin from embryonic day 14.5 mice. Light green arrowheads indicate the apparent front of the endothelial tip cell, and pink arrowheads indicate the apparent leading edge of the associated pericyte. Time in the upper left corner of the left column indicates hours and minutes as hh:mm. Scale bar, 10 microns. Non‐consecutive images were taken from the time‐lapse sequence provided in Movie S1. See Darden et al64 Angiogenesis 2019 for full experimental details and animal use certifications

critical ECM retention motif,84 this ligand is thought to localize to the in which endothelial‐secreted cues such as PDGF‐BB are critical for endothelial cell surface and/or become anchored within the ECM synchronizing pericyte migration along growing sprouts. As endo- alongside endothelial cells. This deposition also appears to require thelial cells extend and retract in a dynamic competition for the tip appropriate configuration of heparan sulfate proteoglycans (HSPGs) cell position,108,109 pericytes likely engage recruitment ligands via such as perlecan (ie, Hspg2).83 receptor binding and dynamically reduce or deplete local concentra- We recently visualized NG2‐DsRed‐positive pericytes migrating tions of these chemo‐attractants at the leading front. along sprouting endothelial cells in an ex vivo model of mouse embry- Our observations (Figure 2 and Movie S1) suggest that, if these onic (E14.5) back skin (Figure 2 and Movie S1).64 Throughout remod- migration cues are not rapidly replenished to maintain a concen- eling vascular networks, pericyte migration appeared to be tightly tration gradient,84 pericytes may retract and pause their migration entrained to the endothelial sprout. These observations were consis- until their endothelial counterparts advance and deposit additional tent with the notion that endothelial‐derived PDGF‐BB, and perhaps cues such as PDGF‐BB. The spatial distribution of these cues may other recruitment and retention factors, guide and focus pericyte also be shaped by the pericytes themselves, as pericytes are ca- migration to the abluminal microvessel surface.84 Interestingly, we pable of generating soluble isoforms of critical receptors such as 64,110-114 also noted a relatively frequent occurrence in the dynamic position- PDGFRβ. This hypothetical model also suggests that a base- ing of endothelial tip cells and adjacent pericytes. Specifically, as a line level of retention/guidance factors must be present to main- tip cell retracted, the corresponding pericyte remained extended tain pericyte position alongside the sprouting endothelial cell. This well beyond the endothelial cell soma (Figure 2 and Movie S1). Over proposed mechanism may also keep pericytes from detaching and time, the pericyte pulled back to match the apparent leading front of migrating into the interstitium away from a growing vessel. While the endothelial cell. The tip cell would eventually resume its outward more studies will be necessary to better understand the existence of migration, and the pericyte would continue to keep pace with ves- these and potentially other pericyte‐retention mechanisms, pericyte sel sprouting (Figure 2 and Movie S1). Considering these anecdotal loss by detachment and interstitial migration has been described observations alongside current published data64,74 suggests a model for numerous disease conditions.24,25,94,115-123 It will therefore be PAYNE et al. | 5 important to further elucidate this phenomenon in healthy and by both endothelial cells and pericytes throughout sprouting angio- pathological conditions. genesis,109 it is critical therefore that the activities of these two cell This dynamic positioning of pericytes alongside endothelial cells types are well integrated such that pericyte coverage and vBM/ECM during vessel formation raises several questions about the peri- deposition keep pace with endothelial cell sprouting and new vessel cyte‐endothelial cell interface as angiogenic sprouting unfolds. For branch formation. instance, as the pericyte remains extended (Figure 2 and Movie S1), it must adhere to and migrate along a permissive substrate, which might be the endothelial cell surface itself. Alternatively, pericytes 5 | PERICYTES IN THE TRANSITION FROM may engage an intermediate form of the vBM that is being jointly ANGIOGENIC SPROUTING TO VESSEL produced by the endothelium and associated pericytes. Pericytes MATURATION appear to prefer only “touch‐point” contacts with the endothelium (and with other pericytes124), as seen in “peg‐and‐socket” connec- Along with vBM production, pericytes have been described as tions between the pericyte and endothelial cell compartments in playing additional roles in endothelial cell sprouting and facilitating more mature vessels.29,125-127 It is therefore intriguing to speculate the transition of new blood vessels from immature and highly plas- that pericytes migrate primarily along a unique configuration of the tic toward stable and quiescent conduits. Angiopoietin‐1 (Ang‐1), vBM that must be composed of specific ECM components, depos- for instance, is produced by pericytes, along with other mesenchy- ited at appropriate concentrations. Type IV collagen for example is mal cells. Angiopoietin‐1 stabilizes microvascular endothelial cells known to be a major constituent of the mature vBM and may me- via Tie2 signaling,137 limits vessel permeability,138 and promotes diate adequate pericyte adhesion. Type IV collagen levels must be quiescence139,140 within a vascular network.4 In addition, pericytes maintained within a narrow physiological range, however, as sub- may influence endothelial tip cell competition and Notch pathway strates dominated by Col‐IV have been shown to be non‐permissive signaling dynamics by physically and spatially restricting the distri- for cell adhesion in a number of in vitro systems.128,129 Laminins, bution of these signals.103 Pericytes may also influence endothelial which can integrate within the Col‐IV scaffold,92,130,131 are also Jagged1 (Jag1) availability through the binding of Notch receptors important elements of the vBM.91,95,132 In addition, both pericytes on the pericyte surface. These cell‐cell interactions may in turn and endothelial cells can secrete distinct isoforms of these ECM impact delta‐like 4 (Dll4)/Jag1 binding of Notch1 receptors on ad- proteins.50,94,120 Pericytes must also utilize a corresponding set of jacent endothelial cells.141 Pericytes have also been described as integrins such as the α2, αV, β1, and β3 subunits to facilitate their regulators of VEGF‐A signaling through the production of VEGF interaction with this pro‐migration vBM.50,133-135 As vessel forma- receptor‐1 (VEGFR‐1/Flt‐1),142,143 though pericyte expression of tion progresses, pericytes synthesize and deposit an additional, and VEGF‐A receptors may be context‐dependent. Several reports likely molecularly distinct, “outer layer” of the vBM that contributes from a variety of experimental models support the notion that to the eventual stabilization of nascent vessels (Figure 3). This out- pericytes generally lack VEGF receptor expression.50,51,53-55,144-147 ermost layer of the vBM may limit non‐vascular cell types (eg, fibro- In the context of pro‐angiogenic cues stimulating the remodeling blasts) from penetrating the vBM and may therefore restrict their of established vessels, pericytes likely contribute to the transient inclusion within the vessel wall.136 With the vBM being established degradation of the vBM to permit sprouting endothelial cells to

FIGURE 3 Schematic illustrating the interactions between a sprouting endothelial cell (EC, green) and an associated pericyte (PC, orange) during angiogenic remodeling. A, Pericyte recruitment and retention factors (yellow) are secreted by endothelial tip cells to maintain pericyte coverage during vessel formation, while endothelial cell‐derived ECM proteins are also deposited (purple). B, As pericytes establish coverage along endothelial sprouts, they secrete additional ECM components (brown) adjacent to the vessel wall to form the vBM supporting cell migration as well as vessel stability 6 | PAYNE et al. migrate outward from existing vessels148,149 (as well as in certain however, cautions that advances in experimental approaches are inflammation states150). This process must be tightly regulated, needed to more fully resolve this question, specifically in the CNS. as widespread or elevated levels of matrix‐degrading enzymes One concern raised in this study is that microvascular pericytes ap- secreted by pericytes would lead to pathological conditions. For pear to lack the morphological and biochemical features necessary instance, pericyte‐mediated vessel destabilization and rupture151 to constrict or dilate the vessel lumen. In particular, it is not entirely has been implicated in age‐related retinopathy152 and in neona- clear whether pericytes sufficiently “wrap around” microvessels (ie, 98,153 173 tal intraventricular hemorrhage. Therefore, the crosstalk be- concentrically) and/or express α‐smooth muscle actin (αSMA) or tween pericytes and endothelial cells at multiple levels ensures other contractile machinery proteins necessary to elicit vessel diam- that their coordinated activities yield blood vessels that are (a) eter changes.18 Advances in imaging modalities, single‐cell transcrip- patterned correctly and (b) capable of transitioning quickly and tional profiling, and tissue‐processing techniques23 will certainly efficiently into stable conduits for blood flow. shed more light on this potential pericyte function. Furthermore, As with angiogenic sprouting, pericytes may contribute to other resolving this question may inform clinical approaches to managing phases of vessel formation in ways that have previously been under- serious pathological conditions such as coronary no‐reflow,169,174 appreciated. While endothelial cells are likely capable of completing ischemic stroke,16 and spinal cord injury.121 In each of these situa- several of these stages unilaterally, pericytes may be more involved tions, maintaining adequate tissue perfusion following the primary than initially thought. Pericyte‐focused tools and models are con- insult remains a challenge and could involve targeting pericytes. We tinuing to emerge, and it is therefore worth revisiting many of these recently found that disrupting oxygen‐sensing mechanisms via in- processes with an eye toward understanding unique pericyte con- duced genetic Vhl mutations accelerated vessel maturation and spe- tributions.154 For example, the mechanisms underlying vessel anas- cifically caused pericytes within developing retinal vasculature to 155-162 175 tomosis and stabilization may entail a component of pericyte ectopically express αSMA. Taken together with previous studies, regulation. Specifically, pericytes may influence precisely where en- it is intriguing to speculate that the hypoxic conditions within sev- dothelial cells may connect and form branch points, similar to mecha- eral of the aforementioned disease states might not only antagonize nisms described for macrophages.158 Pericytes have been described pericyte constriction mechanisms16,170,176 but also cause an aberrant 163,164 as important regulators of vessel pruning and regression, thus increase in the expression of αSMA and other contractile machin- contributing to the refinement of an initial microvascular network ery proteins. This “hyper‐muscularization” of pericytes has also been toward its final configuration. Mechanisms underlying blood vessel reported in other contexts.177 This working model requires further lumen formation may also be uniquely influenced by the presence studies to determine whether such relationships exist. In summary, of pericytes. Apical‐basal polarity of microvascular endothelial cells in addition to their potential role in modulating capillary tone, peri- may be enhanced or reinforced through contact with pericytes and/ cytes within the microcirculation contribute to a number of import- or pericyte‐derived vBM proteins on their abluminal surface. In par- ant processes that promote the transition of microvessels from ticular, pericytes form unique junctions with endothelial cells. For sprouting and remodeling to mature, quiescent networks. example, gap junctions between pericytes and endothelial cells are likely composed of hemi‐channels containing Connexin43 (Cx43) and Connexin45 (Cx45),165,166 though other connexins may also be 6 | CURRENT AND FORWARD‐LOOKING involved. Pericytes also form adherens junctions such as through PERSPECTIVES ON PERICYTES N‐cadherin78,127,167 that may orient the polarity of endothelial cells as well as pericytes themselves. These intercellular junctions may The recent surge in studies exploring the potential roles for peri- enhance pericyte‐specific processes, including differentiation into cytes in developmental and disease processes allows us to revise vascular‐specific mural cells.165,166 They may also provide important and extend existing models for pericyte function within the mi- luminal‐abluminal polarity cues to the developing endothelium. As crocirculation. New tools and models will provide a more detailed nascent microvessels acquire perfusion, the biochemical and me- understanding of the well‐established roles of pericytes in vessel chanical inputs from blood flow reinforce these processes and likely stabilization during angiogenic sprouting and barrier function6-8,15 provide additional vessel maturation cues to the endothelium as well as well as their contributions within specific tissues and organs and as to the pericyte compartment. potential influence on diverse cell populations.178 Though certain As blood vessels mature into a stable and hierarchical network, challenges remain in studying pericytes in a variety of biological they acquire contractile cells capable of shifting blood flow to regions contexts such as during angiogenesis, it will be important to over- of high metabolic activity though coordinated vasoconstriction and come these hurdles and further clarify the roles that pericytes play vasodilation. Vascular SMCs on arteries and arterioles provide this in the formation and homeostasis of the microcirculation. Pericytes regulation of perfusion to downstream tissues. In certain instances, are implicated in the onset and progression of numerous patho- such as for the coronary arteries of the developing mouse heart, logical conditions such as tumor vascularization and dysfunctional pericytes provide a source for these vSMCs.56 Within the microcir- angiogenesis in diabetes‐ and mutation‐associated (eg, VHL) retin- culation, regulation of blood vessel diameter has been ascribed in opathies. Their connection to these and other disease states war- part to vascular pericytes.17,20,22,29,168-172 Recent work by Hill et al,18 rants extensive investigation into their behaviors and the underlying PAYNE et al. | 7 signaling mechanisms. Such insight will inspire new ideas for target- CONFLICT OF INTEREST ing pericytes in the treatment and management of these and other The authors have declared that no conflict of interest exists. vascular‐related conditions such as tissue fibrosis, blood‐brain barrier dysfunction, and metastatic disease. Clinical strategies are con- tinuing to emerge wherein pericytes are being considered as viable ORCID therapeutic targets, and this trend is likely to persist well into the Laura B. Payne https://orcid.org/0000-0001-6439-5329 future. Our knowledge of the pericyte microenvironment during vascu- John C. Chappell https://orcid.org/0000-0002-0427-5170 lar development will also continue to build upon the solid foundation of the studies discussed herein, as well as many others. Nevertheless, REFERENCES fundamental challenges and questions within this line of research remain to be fully addressed, making it an emerging topic that is gaining . 1 Sims DE. The pericyte–a review. 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