Scanning electron microscopic studies of the vascular smooth muscle cells and pericytes in the rat heart

The fate and role of the pericytes in myocardial diseases

The adult mammalian heart contains a large population of pericytes that play important roles in homeostasis and disease. In the normal heart, pericytes regulate microvascular permeability and flow. Myocardial diseases are associated with marked alterations in pericyte phenotype and function. This review manuscript discusses the role of pericytes in cardiac homeostasis and disease. Following myocardial infarction (MI), cardiac pericytes participate in all phases of cardiac repair. During the inflammatory phase, pericytes may secrete cytokines and chemokines and may regulate leukocyte trafficking, through formation of intercellular gaps that serve as exit points for inflammatory cells. Moreover, pericyte contraction induces microvascular constriction, contributing to the pathogenesis of 'no-reflow' in ischemia and reperfusion. During the proliferative phase, pericytes are activated by growth factors, such as transforming growth factor (TGF)-β and contribute to fibrosis, predominantly through secretion of fibrogenic mediators. A fraction of pericytes acquires fibroblast identity but contributes only to a small percentage of infarct fibroblasts and myofibroblasts. As the scar matures, pericytes form a coat around infarct neovessels, promoting stabilization of the vasculature. Pericytes may also be involved in the pathogenesis of chronic heart failure, by regulating inflammation, fibrosis, angiogenesis and myocardial perfusion. Pericytes are also important targets of viral infections (such as SARS-CoV2) and may be implicated in the pathogenesis of cardiac complications of COVID19. Considering their role in myocardial inflammation, fibrosis and angiogenesis, pericytes may be promising therapeutic targets in myocardial disease.

Pericytes and the Control of Blood Flow in Brain and Heart

Pericytes, attached to the surface of capillaries, play an important role in regulating local blood flow. Using optogenetic tools and genetically encoded reporters in conjunction with confocal and multiphoton imaging techniques, the 3D structure, anatomical organization, and physiology of pericytes have recently been the subject of detailed examination. This work has revealed novel functions of pericytes and morphological features such as tunneling nanotubes in brain and tunneling microtubes in heart. Here, we discuss the state of our current understanding of the roles of pericytes in blood flow control in brain and heart, where functions may differ due to the distinct spatiotemporal metabolic requirements of these tissues. We also outline the novel concept of electro-metabolic signaling, a universal mechanistic framework that links tissue metabolic state with blood flow regulation by pericytes and vascular smooth muscle cells, with capillary K_ATP and Kir2.1 channels as primary sensors. Finally, we present major unresolved questions and outline how they can be addressed.

Mechanisms of the "No-Reflow" Phenomenon After Acute Myocardial Infarction: Potential Role of Pericytes

Pericytes contract during myocardial ischemia resulting in capillary constriction and no reflow. Reversing pericyte contraction pharmacologically reduces no reflow and infarct size. These findings open up an entire new venue of research aimed at altering pericyte function in myocardial ischemia and infarction.

Persistent Coronary Vasomotor Tone During Myocardial Ischemia Occurs at the Capillary Level and May Involve Pericytes

Background

There is persistent coronary vasomotor tone during myocardial ischemia, despite ongoing coronary arteriolar dilatation. The mechanism underlying this vasodilatory tone, which can be unmasked by coronary vasodilators, is unclear. We hypothesized that persistent microvascular resistance during myocardial ischemia occurs at the level of capillaries and may be caused by pericytes.

Methods

We studied nine instrumented dogs where coronary blood flow and coronary driving pressure were reduced to half by placement of stenoses. Myocardial blood flow and myocardial blood volume were measured with myocardial contrast echocardiography before and during adenosine administration. In three animals, the heart was perfusion-fixed under these conditions for electron microscopic assessment of capillary and pericyte size.

Results

During ischemia, myocardial blood volume decreased and myocardial vascular resistance remained unchanged. Adenosine administration reversed the decline in myocardial blood volume and decreased myocardial vascular resistance. Electron microscopy showed larger capillaries in ischemic beds receiving adenosine than ischemic beds not receiving adenosine. Pericytes in beds receiving adenosine also tended to be larger.

Conclusion

Capillaries are the site of persistent vasomotor tone during myocardial ischemia; any other site of vascular regulation (arterioles or venules) cannot explain these myocardial contrast echocardiography findings, which are confirmed on post-mortem electron microscopic examination. The decrease in capillary size is likely caused by pericyte contraction in an attempt to maintain a constant capillary hydrostatic pressure. Adenosine relaxes pericytes, restores myocardial blood volume, reduces myocardial vascular resistance, and improves regional function during ischemia. These findings could have important therapeutic implications.

Role of angiotensin-converting enzyme 2 and pericytes in cardiac complications of COVID-19 infection

The prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) quickly reached pandemic proportions, and knowledge about this virus and coronavirus disease 2019 (COVID-19) has expanded rapidly. This review focuses primarily on mechanisms that contribute to acute cardiac injury and dysfunction, which are common in patients with severe disease. The etiology of cardiac injury is multifactorial, and the extent is likely enhanced by preexisting cardiovascular disease. Disruption of homeostatic mechanisms secondary to pulmonary pathology ranks high on the list, and there is growing evidence that direct infection of cardiac cells can occur. Angiotensin-converting enzyme 2 (ACE2) plays a central role in COVID-19 and is a necessary receptor for viral entry into human cells. ACE2 normally not only eliminates angiotensin II (Ang II) by converting it to Ang-(1-7) but also elicits a beneficial response profile counteracting that of Ang II. Molecular analyses of single nuclei from human hearts have shown that ACE2 is most highly expressed by pericytes. Given the important roles that pericytes have in the microvasculature, infection of these cells could compromise myocardial supply to meet metabolic demand. Furthermore, ACE2 activity is crucial for opposing adverse effects of locally generated Ang II, so virus-mediated internalization of ACE2 could exacerbate pathology by this mechanism. While the role of cardiac pericytes in acute heart injury by SARS-CoV-2 requires investigation, expression of ACE2 by these cells has broader implications for cardiac pathophysiology.

Role of angiotensin-converting enzyme 2 and pericytes in cardiac complications of COVID-19 infection

The prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) quickly reached pandemic proportions, and knowledge about this virus and coronavirus disease 2019 (COVID-19) has expanded rapidly. This review focuses primarily on mechanisms that contribute to acute cardiac injury and dysfunction, which are common in patients with severe disease. The etiology of cardiac injury is multifactorial, and the extent is likely enhanced by preexisting cardiovascular disease. Disruption of homeostatic mechanisms secondary to pulmonary pathology ranks high on the list, and there is growing evidence that direct infection of cardiac cells can occur. Angiotensin-converting enzyme 2 (ACE2) plays a central role in COVID-19 and is a necessary receptor for viral entry into human cells. ACE2 normally not only eliminates angiotensin II (Ang II) by converting it to Ang-(1–7) but also elicits a beneficial response profile counteracting that of Ang II. Molecular analyses of single nuclei from human hearts have shown that ACE2 is most highly expressed by pericytes. Given the important roles that pericytes have in the microvasculature, infection of these cells could compromise myocardial supply to meet metabolic demand. Furthermore, ACE2 activity is crucial for opposing adverse effects of locally generated Ang II, so virus-mediated internalization of ACE2 could exacerbate pathology by this mechanism. While the role of cardiac pericytes in acute heart injury by SARS-CoV-2 requires investigation, expression of ACE2 by these cells has broader implications for cardiac pathophysiology.

Role of Pericytes in the Initiation and Propagation of Spontaneous Activity in the Microvasculature

The microvasculature is composed of arterioles, capillaries and venules. Spontaneous arteriolar constrictions reduce effective vascular resistance to enhance tissue perfusion, while spontaneous venular constrictions facilitate the drainage of tissue metabolites by pumping blood. In the venules of visceral organs, mural cells, i.e. smooth muscle cells (SMCs) or pericytes, periodically generate spontaneous phasic constrictions, Ca²⁺ transients and transient depolarisations. These events arise from spontaneous Ca²⁺ release from the sarco-endoplasmic reticulum (SR/ER) and the subsequent opening of Ca²⁺-activated chloride channels (CaCCs). CaCC-dependent depolarisation further activates L-type voltage-dependent Ca²⁺ channels (LVDCCs) that play a critical role in maintaining the synchrony amongst mural cells. Mural cells in arterioles or capillaries are also capable of developing spontaneous activity. Non-contractile capillary pericytes generate spontaneous Ca²⁺ transients primarily relying on SR/ER Ca²⁺ release. Synchrony amongst capillary pericytes depends on gap junction-mediated spread of depolarisations resulting from the opening of either CaCCs or T-type VDCCs (TVDCCs) in a microvascular bed-dependent manner. The propagation of capillary Ca²⁺ transients into arterioles requires the opening of either L- or TVDCCs again depending on the microvascular bed. Since the blockade of gap junctions or CaCCs prevents spontaneous Ca²⁺ transients in arterioles and venules but not capillaries, capillary pericytes appear to play a primary role in generating spontaneous activity of the microvasculature unit. Pericytes in capillaries where the interchange of substances between tissues and the circulation takes place may provide the fundamental drive for upstream arterioles and downstream venules so that the microvasculature network functions as an integrated unit.

Ca2+ Signalling in Pericytes

Microcirculation is the generic name for the finest level of the circulatory system and consists of arteriolar and venular networks located upstream and downstream of capillaries, respectively. Anatomically arterioles are surrounded by a monolayer of spindle-shaped smooth muscle cells (myocytes), while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are surrounded by a monolayer of morphologically different perivascular cells (pericytes). Pericytes are essential components of the microvascular vessel wall. Wrapped around endothelial cells, they occupy a strategic position at the interface between the circulating blood and the interstitial space. There are physiological differences in the responses of pericytes and myocytes to vasoactive molecules, which suggest that these two types of vascular cells could have different functional roles in the regulation of local blood flow within the same microvascular bed. Also, pericytes may play different roles in different microcirculatory beds to meet the characteristics of individual organs. Contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca²⁺concentration. In this chapter, we attempt to summarize the results in the field of Ca²⁺ signalling in pericytes especially in light of their contractile roles in different tissues and organs. We investigate the literature and describe our results regarding sources of Ca²⁺, relative importance and mechanisms of Ca²⁺ release and Ca²⁺ entry in control of the spatio-temporal characteristics of the Ca²⁺ signals in pericytes, where possible Ca²⁺ signalling and contractile responses in pericytes are compared to those of myocytes.

Gene network analysis: from heart development to cardiac therapy

Networks offer a flexible framework to represent and analyse the complex interactions between components of cellular systems. In particular gene networks inferred from expression data can support the identification of novel hypotheses on regulatory processes. In this review we focus on the use of gene network analysis in the study of heart development. Understanding heart development will promote the elucidation of the aetiology of congenital heart disease and thus possibly improve diagnostics. Moreover, it will help to establish cardiac therapies. For example, understanding cardiac differentiation during development will help to guide stem cell differentiation required for cardiac tissue engineering or to enhance endogenous repair mechanisms. We introduce different methodological frameworks to infer networks from expression data such as Boolean and Bayesian networks. Then we present currently available temporal expression data in heart development and discuss the use of network-based approaches in published studies. Collectively, our literature-based analysis indicates that gene network analysis constitutes a promising opportunity to infer therapy-relevant regulatory processes in heart development. However, the use of network-based approaches has so far been limited by the small amount of samples in available datasets. Thus, we propose to acquire high-resolution temporal expression data to improve the mathematical descriptions of regulatory processes obtained with gene network inference methodologies. Especially probabilistic methods that accommodate the intrinsic variability of biological systems have the potential to contribute to a deeper understanding of heart development.

Vascularisation for cardiac tissue engineering: the extracellular matrix

Cardiovascular diseases present a major socio-economic burden. One major problem underlying most cardiovascular and congenital heart diseases is the irreversible loss of contractile heart muscle cells, the cardiomyocytes. To reverse damage incurred by myocardial infarction or by surgical correction of cardiac malformations, the loss of cardiac tissue with a thickness of a few millimetres needs to be compensated. A promising approach to this issue is cardiac tissue engineering. In this review we focus on the problem of in vitro vascularisation as implantation of cardiac patches consisting of more than three layers of cardiomyocytes (> 100 μm thick) already results in necrosis. We explain the need for vascularisation and elaborate on the importance to include non-myocytes in order to generate functional vascularised cardiac tissue. We discuss the potential of extracellular matrix molecules in promoting vascularisation and introduce nephronectin as an example of a new promising candidate. Finally, we discuss current biomaterial- based approaches including micropatterning, electrospinning, 3D micro-manufacturing technology and porogens. Collectively, the current literature supports the notion that cardiac tissue engineering is a realistic option for future treatment of paediatric and adult patients with cardiac disease.

Angioarchitecture and morphology of temporomandibular joint of Monodelphis domestica

The opossum Monodelphis domestica presents movement of the temporomandibular joint (TMJ) reflecting adaptation to eating habits similar to movement in humans, but the structure of the TMJ is not yet known. Thus, nine young M. domestica, of both sexes were weighed, anesthetized with xylazine (10 mg kg(-1) ), and ketamine (70 mg kg(-1) ) and processed for: 1. The analyses of the macroscopic angioarchitecture after latex injection, as well as the topography of the TMJ; 2. The analysis of microvascularization after injection of Mercox resin and corrosion of soft tissue with NaOH using scanning electron microscopy and; 3. The histological evaluation of the TMJ with an optical microscope. Macroscopic analysis of the latex injected vessels revealed the distribution of the arteries from the common carotid artery, receiving branches of the superficial temporal and maxillary arteries. The mandibular condyle has the long axis in the lateral-lateral direction, and is convex in the anterior-posterior direction. Its topography was determined in relation to the eye and external acoustic meatus. With scanning electron microscopy, microvascularization consists of arterioles of varying diameter (85-15 µm) of the meandering capillary network in the retrodiscal region, and a network of straight capillaries in the TMJ anterior region. Via light microscopy the TMJ has similar histological features to those of humans. These macroscopic, microscopic and ultrastructural data from TMJ of the M. domestica could be a suitable model for TMJ physiology and pathophysiology studies for then speculate on possible human studies. Microsc. Res. Tech. 79:806-813, 2016. © 2016 Wiley Periodicals, Inc.

Organ Specificity and Functional Relevance of the Arterial Structure: A Comparative Study in the Kidney and the Skeletal Muscle with Electron Microscopy

The differences in circulation among various organs are well known, but the structural differences have only been poorly investigated. In the present study the wall structure of arteries was observed with electron microscopy in rat kidney and skeletal muscle. The wall thickness was almost equal in both organs at about 40 μm in luminal diameter, while it was relatively thin in the larger arteries and thick in the smaller arteries in the kidney compared with that in the skeletal muscle. The smooth muscle cells were regularly arranged in parallel in circular or slightly spiral orientation in the kidney, whereas those in the skeletal muscle were irregularly arranged in heterogeneous orientations. Extracellular matrices were more abundant in the arterial media in the skeletal muscle than in the kidney. The inner elastic lamina was continuous in the kidney, and arranged in longitudinal bundles in the skeletal muscle. The adventitial collagen fibers were abundant and dense in the skeletal muscle, and were scattered in small bundles in the fluid-filled spaces in the kidney. While the arteries in the skeletal muscle are under severe mechanical stress during muscle contraction and dilate dramatically during exercise, those in the kidney regulate glomerular pressure almost constantly.

Spontaneous activity in the microvasculature of visceral organs: role of pericytes and voltage-dependent Ca(2+) channels

The microvasculature plays a primary role in the interchange of substances between tissues and the circulation. In visceral organs that undergo considerable distension upon filling, the microvasculature appears to display intrinsic contractile properties to maintain their flow. Submucosal venules in the bladder or gastrointestinal tract generate rhythmic spontaneous phasic constrictions and associated Ca(2+) transients. These events are initiated within either venular pericytes or smooth muscle cells (SMCs) arising from spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) and the opening of Ca(2+) -activated chloride channels (CaCCs) that trigger Ca(2+) influx through L-type voltage-dependent Ca(2+) channels (VDCCs). L-type VDCCs also play a critical role in maintaining synchrony within the contractile mural cells. In the stomach myenteric layer, spontaneous Ca(2+) transients originating in capillary pericytes appear to spread to their neighbouring arteriolar SMCs. Capillary Ca(2+) transients primarily rely on SR Ca(2+) release, but also require Ca(2+) influx through T-type VDCCs for their synchrony. The opening of T-type VDCCs also contribute to the propagation of Ca(2+) transients into SMCs. In visceral microvasculature, pericytes act as either spontaneously active contractile machinery of the venules or as pacemaker cells generating synchronous Ca(2+) transients that drive spontaneous contractions in upstream arterioles. Thus pericytes play different roles in different vascular beds in a manner that may well depend on the selective expression of T-type and L-type Ca(2+) channels.

Cardiomyocyte proliferation in cardiac development and regeneration: a guide to methodologies and interpretations

The newt and the zebrafish have the ability to regenerate many of their tissues and organs including the heart. Thus, a major goal in experimental medicine is to elucidate the molecular mechanisms underlying the regenerative capacity of these species. A wide variety of experiments have demonstrated that naturally occurring heart regeneration relies on cardiomyocyte proliferation. Thus, major efforts have been invested to induce proliferation of mammalian cardiomyocytes in order to improve cardiac function after injury or to protect the heart from further functional deterioration. In this review, we describe and analyze methods currently used to evaluate cardiomyocyte proliferation. In addition, we summarize the literature on naturally occurring heart regeneration. Our analysis highlights that newt and zebrafish heart regeneration relies on factors that are also utilized in cardiomyocyte proliferation during mammalian fetal development. Most of these factors have, however, failed to induce adult mammalian cardiomyocyte proliferation. Finally, our analysis of mammalian neonatal heart regeneration indicates experiments that could resolve conflicting results in the literature, such as binucleation assays and clonal analysis. Collectively, cardiac regeneration based on cardiomyocyte proliferation is a promising approach for improving adult human cardiac function after injury, but it is important to elucidate the mechanisms arresting mammalian cardiomyocyte proliferation after birth and to utilize better assays to determine formation of new muscle mass.

Calcium signalling in pericytes

Recent advances in pericyte research have contributed to our understanding of the physiology and pathophysiology of microvessels. The microvasculature consists of arteriolar and venular networks located upstream and downstream of the capillaries. Arterioles are surrounded by a monolayer of spindle-shaped myocytes, while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are encircled by a monolayer of morphologically diverse pericytes. There are physiological differences in the response of pericytes and myocytes to vasoactive molecules, suggesting that these two vascular cell types could have different functional roles in the regulation of local blood flow. The contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca(2+) concentration. In this short review, we summarize our results and those of other authors on the contractility of pericytes and their Ca(2+) signalling. We describe results regarding sources of Ca(2+) and mechanisms of Ca(2+) release and Ca(2+) entry in control of the spatiotemporal characteristics of the Ca(2+) signals in pericytes.

Coronary microvascular pericytes are the cellular target of sunitinib malate-induced cardiotoxicity

Sunitinib malate is a multitargeted receptor tyrosine kinase inhibitor used in the treatment of human malignancies. A substantial number of sunitinib-treated patients develop cardiac dysfunction, but the mechanism of sunitinib-induced cardiotoxicity is poorly understood. We show that mice treated with sunitinib develop cardiac and coronary microvascular dysfunction and exhibit an impaired cardiac response to stress. The physiological changes caused by treatment with sunitinib are accompanied by a substantial depletion of coronary microvascular pericytes. Pericytes are a cell type that is dependent on intact platelet-derived growth factor receptor (PDGFR) signaling but whose role in the heart is poorly defined. Sunitinib-induced pericyte depletion and coronary microvascular dysfunction are recapitulated by CP-673451, a structurally distinct PDGFR inhibitor, confirming the role of PDGFR in pericyte survival. Thalidomide, an anticancer agent that is known to exert beneficial effects on pericyte survival and function, prevents sunitinib-induced pericyte cell death in vitro and prevents sunitinib-induced cardiotoxicity in vivo in a mouse model. Our findings suggest that pericytes are the primary cellular target of sunitinib-induced cardiotoxicity and reveal the pericyte as a cell type of concern in the regulation of coronary microvascular function. Furthermore, our data provide preliminary evidence that thalidomide may prevent cardiotoxicity in sunitinib-treated cancer patients.

How calcium signals in myocytes and pericytes are integrated across in situ microvascular networks and control microvascular tone

The microcirculation is the site of gas and nutrient exchange. Control of central or local signals acting on the myocytes, pericytes and endothelial cells within it, is essential for health. Due to technical problems of accessibility, the mechanisms controlling Ca2+ signalling and contractility of myocytes and pericytes in different sections of microvascular networks in situ have not been investigated. We aimed to investigate Ca2+ signalling and functional responses, in a microcirculatory network in situ. Using live confocal imaging of ureteric microvascular networks, we have studied the architecture, morphology, Ca2+ signalling and contractility of myocytes and pericytes. Ca2+ signals vary between distributing arcade and downstream transverse and precapillary arterioles, are modified by agonists, with sympathetic agonists being ineffective beyond transverse arterioles. In myocytes and pericytes, Ca2+ signals arise from Ca2+ release from the sarcoplasmic reticulum through inositol 1,4,5-trisphosphate-induced Ca2+ release and not via ryanodine receptors or Ca2+ entry into the cell. The responses in pericytes are less oscillatory, slower and longer-lasting than those in myocytes. Myocytes and pericytes are electrically coupled, transmitting Ca2+ signals between arteriolar and venular networks dependent on gap junctions and Ca2+ entry via L-type Ca2+ channels. Endothelial Ca2+ signalling inhibits intracellular Ca2+ oscillations in myocytes and pericytes via L-arginine/nitric oxide pathway and intercellular propagating Ca2+ signals via EDHF. Increases of Ca2+ in pericytes and myocytes constrict all vessels except capillaries. These data reveal the structural and signalling specializations allowing blood flow to be regulated by myocytes and pericytes.

Are the precapillary sphincters and metarterioles universal components of the microcirculation? An historical review

The microcirculation is a major topic in current physiology textbooks and is frequently explained with schematics including the precapillary sphincters and metarterioles. We re-evaluated the validity and applicability of the concepts precapillary sphincters and metarterioles by reviewing the historical context in which they were developed in physiology textbooks. The studies by Zweifach up until the 1950s revealed the unique features of the mesenteric microcirculation, illustrated with impressive schematics of the microcirculation with metarterioles and precapillary sphincters. Fulton, Guyton and other authors introduced or mimicked these schematics in their physiology textbooks as representative of the microcirculation in general. However, morphological and physiological studies have revealed that the microcirculation in the other organs and tissues contains no metarterioles or precapillary sphincters. The metarterioles and precapillary sphincters were not universal components of the microcirculation in general, but unique features of the mesenteric microcirculation.

Myocardial accumulation and localization of the inducible 70-kDa heat shock protein, Hsp70, following exercise

Exercise increases the 70-kDa heat shock protein (Hsp70) in the myocardium, and this exercise-induced increase is associated with significantly improved cardiac recovery following insult. However, while heat shock has been shown to elevate Hsp70 primarily in the cardiac vasculature of the myocardium, the localization following exercise is unknown. Male Sprague-Dawley rats performed continuous treadmill running at 30 m/min for 60 min (2% incline) on either 1 or 5 consecutive days. At 30 min and 24 h following exercise, hearts were extirpated, and the left ventricle was isolated, OCT-cork mounted, and sectioned for immunofluorescent analysis. Whereas immunofluorescent analysis revealed little to no Hsp70 in control hearts and 30 min postexercise, the accumulation of Hsp70 24 h after a single exercise bout or 5 days of training was predominantly located in large blood vessels and, in particular, colocalized with a marker of smooth muscle. Furthermore, higher core temperatures attained during exercise led to more abundant accumulation in smaller vessels and the endothelium. It is concluded that the accumulation of myocardial Hsp70 following acute exercise predominantly occurs in a cell type-specific manner, such that changes in the cardiac vasculature account for much of the increase. This accumulation appears first in the smooth muscle of larger vessels and then increases in smaller vessels and the endothelium, as core temperature attained during exercise increases. This finding supports the observations after heat shock and further suggests that the vasculature is a primary target in exercise-induced cardioprotection.

The dynamic structure of arterioles

Arterioles are the blood vessels in the arterial side of the vascular tree that are located proximal to the capillaries and, in conjunction with the terminal arteries, provide the majority of resistance to blood flow. Consequently, arterioles are important contributors to the regulation of mean arterial pressure and tissue perfusion. Their wall consists of cellular and extracellular components that have been traditionally classified as conforming three layers: an intima containing endothelial cells sited on a basement membrane; a media made of an internal elastic lamina apposed by one or two layers of smooth muscle; and an adventitia composed mostly of collagen bundles, nerve endings and some fibroblasts. These components of the arteriolar wall are dynamically interconnected, providing a level of plasticity to the arteriolar wall that blurs the traditional boundaries of a rigid layered classification. This MiniReview focuses on the structural conformation of the arteriolar wall and shows how wall components interact spatially, functionally and temporally to control vascular diameter, regulate blood flow and maintain vascular permeability.

Isolation, bulk cultivation, and characterization of coronary microvascular pericytes: the second most frequent myocardial cell type in vitro

Densely arranged pericytes engird the endothelial tube of all coronary microvessels. Since the experimental access to these abundant cells in situ is difficult, a prerequisite for broader investigation is the availability of sufficient numbers of fully differentiated pericytes in homogenous culture. To reach this goal, we applied strictly standardized cell isolation techniques, optimized culture methods and specific histological staining. Approximately 1,000-fold enriched pericytes were proteolytically detached from highly purified coronary microvascular networks (density gradient centrifugation) of eight mammalian species including human. Addition of species-autologous fetal or neonatal serum (10-20% vol/vol) was a precondition for longer term survival of homogenous pericyte cultures. This ensured optimal growth (doubling time <14 h) and full expression of pericyte-specific markers. In 3-mo, 10(10) pericytes (15 g) could be cultivated from 1 bovine heart. Pericytes could be stored in liquid N(2), recultured, and passaged repeatedly without loss of typical features. In cocultures with EC or vascular smooth muscle cells, pericytes transferred fluorescent calcein to each other and to EC via their antler-like extensions, organized angiogenetic sprouting of vessels, and rapidly activated coagulation factors X and II via tissue factor and prothrombinase. The interconnected pericytes of the coronary system are functionally closely correlated with the vascular endothelium and may play key roles in the adjustment of local blood flow, the regulation of angiogenic processes, and the induction of procoagulatory processes. Their successful bulk cultivation enables direct experimental access under defined in vitro conditions and the isolation of pericyte specific antigens for the production of specific antibodies.

The expression of soluble guanylate cyclase in the vasculature of rat skeletal muscle

Nitric oxide (NO) has various roles in the skeletal musculature in both normal and pathological conditions. NO primarily activates soluble guanylate cyclase (sGC) and mediates subsequent intracellular signaling in target cells. We sought to identify the target cells of NO in the rat skeletal musculature, using subtypes of sGCalpha1 and sGCbeta1 antibodies. Immunohistochemistry revealed that both antibodies stained the same cells with round or oval shapes, having several long processes. The sGC-immunopositive cells co-expressed NG2 chondroitin sulfate proteoglycan, a marker of pericytes. The sGC-immunopositive cells were associated with capillaries and formed cellular networks with elongated cytoplasmic processes. sGCalpha1 and sGCbeta1 were not found in muscle sarcolemma that were stained by anti-dystrophin, or neuromuscular junctions, as detected by anti-synaptophysin. Based on these findings, we concluded that sGC immunoreactivity was specifically distributed in capillary pericytes. Pericytes in the skeletal musculature have been shown to be target cells of NO and are involved in the microvascular blood flow.

Cholesterol depletion alters coronary artery myocyte Ca(2+) signalling in a stimulus-specific manner

Although there is evidence that caveolae and cholesterol play an important role in myocyte signalling processes, details of the mechanisms involved remain sparse. In this paper we have studied for the first time the clinically relevant intact coronary artery and measured in situ Ca(2+) signals in individual myocytes using confocal microscopy. We have examined the effect of the cholesterol-depleting agents, methyl-cyclodextrin (MCD) and cholesterol oxidase, on high K(+), caffeine and agonist-induced Ca(2+) signals. We find that cholesterol depletion produces a stimulus-specific alteration in Ca(2+) responses; with 5-HT (10microM) and endothelin-1 (10nM) responses being selectively decreased, the phenylephrine response (100microM) increased and the responses to high K(+) (60mM) and caffeine (10mM) unaffected. Agonist-induced Ca(2+) signals were restored when cholesterol was replenished using cholesterol-saturated MCD. In additional experiments, enzymatically isolated myocytes were patch clamped. We found that cholesterol depletion caused a selective modification of ion channel function, with whole cell inward Ca(2+) current being unaltered, whereas outward K(+) current was increased, due to BK(Ca) channel activation. There was also a significant decrease in cell capacitance. These data are discussed in terms of the involvement of caveolae in receptor localisation, Ca(2+) entry pathways and SR Ca(2+) release, and the role of these in agonist signalling.

Smooth muscle persists in the muscularis externa of developing and adult mouse esophagus

Following initial patterning as differentiated smooth muscle (SM) cells, the muscularis externa of the murine esophagus is replaced by skeletal muscle, but the mechanism underlying this process is controversial. The hypothesis that committed SM cells transdifferentiate into striated muscle is not consistent with fate mapping studies. Similarly, apoptosis does not fully explain the process. Using immunohistochemical techniques and transgenic mice that express eGFP and Cre-recombinase exclusively in SM, we have identified a population of remnant SM cells that persist throughout the developing and mature murine esophagus. These cells display an atypical phenotype, are not associated with microvasculature, but are often apposed to cKit positive, interstitial cells of Cajal. The absolute length of the SM component of the developing esophagus remains constant during a period when total esophageal length increases 4-fold, resulting in a small maintained distal segment of smooth muscle. Esophageal SM cells fail to express myogenin during development, and striated muscle cell precursors expressing myogenin fail to express specific SM cell markers, indicating that they did not transdifferentiate from SM cells. Moreover, smooth muscle-specific myogenin inactivation has no effect on esophageal skeletal myogenesis. Taken together, our results provide an alternative hypothesis regarding the fate of SM cells in the developing murine esophagus, which does not invoke apoptosis or transdifferentiation.

Angiogenesis: The Major Abnormality of the Keratin-14 IL-4 Transgenic Mouse Model of Atopic Dermatitis

OBJECTIVE

Angiogenesis plays an important role in psoriasis, but its role in atopic dermatitis is unknown. The authors examined the dermal microvasculature of an IL-4 transgenic mouse model of atopic dermatitis to determine whether angiogenesis was present.

METHODS

Transmission and scanning electron microscopy and confocal microscopy studies were performed.

RESULTS

Transmission electron microscopy showed sprouting, transcapillary pillars of intussusception, thickened endothelial cells with large nuclei, and increased interendothelial junctional cleft number and length. Compared to nontransgenic littermates, there was a significant increase in the lengths and numbers of the interendothelial junctional clefts, along with a decrease in the length ratios of tight junction to interendothelial junctional clefts in both the early and late disease stages. In the early and late skin lesions, scanning electron microscopy of vascular corrosion casts showed disorganization of the capillary network hierarchy with increased density of capillary sprouts. Confocal microscopy of the animals with early and late skin lesions showed significant reduction in tight junction protein claudin-5.

CONCLUSIONS

Angiogenesis is the major pathologic feature in this model of atopic dermatitis. The chronic skin inflammation is intertwined with and may cause the angiogenesis, but the angiogenesis itself is likely to be important in this disease process.

Multicellular simulation predicts microvascular patterning and in silico tissue assembly

Remodeling of microvascular networks in mammals is critical for physiological adaptations and therapeutic revascularization. Cellular behaviors such as proliferation, differentiation, and migration are coordinated in these remodeling events via combinations of biochemical and biomechanical signals. We developed a cellular automata (CA) computational simulation that integrates epigenetic stimuli, molecular signals, and cellular behaviors to predict microvascular network patterning events. Over 50 rules obtained from published experimental data govern independent behaviors (including proliferation, differentiation, and migration) of thousands of interacting cells and diffusible growth factors in their tissue environment. From initial network patterns of in vivo blood vessel networks, the model predicts emergent patterning responses to two stimuli: 1) network-wide changes in hemodynamic mechanical stresses, and 2) exogenous focal delivery of an angiogenic growth factor. The CA model predicts comparable increases in vascular density (370+/-29 mm/mm3) 14 days after treatment with exogenous growth factor to that in vivo (480+/-41 mm/mm3) and approximately a twofold increase in contractile vessel lengths 5-10 days after 10% increase in circumferential wall strain, consistent with in vivo results. The CA simulation was thus able to identify a functional patterning module capable of quantitatively predicting vessel network remodeling in response to two important epigenetic stimuli.

Three-dimensional cytoarchitecture of angiogenic blood vessels in a gelatin sheet implanted in the rat skeletal muscular layers

To demonstrate the structure of angiogenic blood vessels three-dimensionally, a gelatin sponge sheet immersed in a vascular endothelial growth factor (VEGF) solution was implanted in the rat dorsal muscular layer, and examined by light microscopy and scanning electron microscopy (SEM) 5 days to 2 weeks after implantation. Light microscopy of anti-collagen IV antibody immunostained specimens enabled a determination of the basement membrane tube of newly formed blood vessels in the implanted sponge sheet. The tubes were 5-40 microm in diameter, and sometimes tapered to a slender cord within the vascular network. The SEM study of 30% KOH treated tissues revealed two types of tapering ends of newly formed blood vessels. One consisted of endothelial cells with microprojections, and lacked any investment of pericytes over the length of 5-20 microm. The other type was a tapering tip of the endothelial tube covered with pericytic processes. The presence of long processes of pericytes extending beyond the tip of the endothelial tube and connecting to the adjacent vessel wall indicates that this type was produced by endothelial tube regression. Thus, the present study supports the ideas that endothelial tube formation is followed by pericyte coverage at the sprouting tip, and that endothelial tube regression precedes pericyte detachment at the regressing site.

Developmental changes in the structure of the rat fetal lung, with special reference to the airway smooth muscle and vasculature

Structural changes in the developing rat lung were studied by a combined use of light microscopy including immunohistochemistry for a-smooth muscle actin (alpha-SMA) and scanning electron microscopy (SEM) using the KOH-collagenase digestion method. In the embryonic stage (E11-E13), the lung bud appeared as an outgrowth from the ventral wall of the foregut which grew caudally into the splanchnic mesoderm to form a pair of bronchial buds at the end. At E13, the airway smooth muscle cells first appeared around the bifurcation of the trachea. These smooth muscle cells were restricted to the dorsal surface of the tracheal epithelium, suggesting a difference in character between the dorsal and ventral sides of the mesenchymal cells in this region. During the pseudoglandular stage (E13-E18.5), the bronchial buds repeatedly gave off branches in the mesenchymal tissue. The smooth muscle cells in the bronchioles were spindle-shaped and arranged completely circularly around the epithelial tube, except that the terminal bud of bronchioles lacked the smooth muscles. The neck of the terminal bud was constantly surrounded by flat and irregularly-shaped immature smooth muscle cells, representing an early event in the smooth muscle cell differentiation from mesenchymal cells. In the canalicular to saccular stages (E18.5 to birth), the terminals of bronchioles became saccular, thus forming prospective alveolar acini. At birth, the alveolar wall became thinner than before birth, and the individual smooth muscle cells in bronchioles were elongated like a tape. As to the blood vessel differentiation, various sized sinusoidal spaces indicating the primitive blood vessels were already present in the mesenchymal tissue at E11.5. The endothelial cells of these sinusoidal spaces were irregularly shaped and sometimes extended their processes into the lumen. The network of tubular vessels appeared from E14.5. These vessels had tapering ends as well as transluminal trabeculae, suggesting that capillary growth proceeds by both the sprouting and partitioning (i.e., intussusception) of vessels in the pseudoglandular stage.

Fine structure of the mouse portal vein in relation to its peristaltic movement

The hepatic portal vein has been known to make a spontaneous peristaltic movement in some mammals, including the mouse and rat. To investigate the fine structure of the portal vein in relation to its physiological characteristics, we observed the mouse portal vein by using various histological techniques including conventional light microscopy, videomicroscopy, transmission and scanning electron microscopy, and real-time confocal laser scanning microscopy. The mouse hepatic portal vein was provided with a spiral fold which was produced by the inner layer, i.e. the endothelium and smooth muscles of the wall protruding into the lumen. Longitudinal smooth muscle cells spanned the interval of the fold, like a spirally arranged palisade around the vessel wall. The longitudinal muscle fibers ended at the spiral fold, being partly connected with a network of irregularly shaped smooth muscle cells. This network, hitherto unknown, was recognized to be restricted to the fold in distribution and characterized by numerous gap junctions connecting the muscle cells. Real-time confocal laser scanning microscopy using a Ca2+ sensitive fluorescent dye revealed that a transient and periodic increase in Ca2+ concentration occurred in the longitudinal smooth muscle cells and was transmitted spirally from the intestinal to the hepatic side. These findings indicate that, during the peristaltic movement, the contraction of smooth muscle cells is transmitted along the longitudinal smooth muscles of the portal vein wall toward the liver, presumably controlled by the network of the irregularly-shaped smooth muscle cells in the fold of the portal vein. Light microscopic observation in some specimens indicated an occurrence of cardiac muscle cells outside the smooth muscle layer. Restricted to the site of the porta hepatis in distribution, their involvement in the peristaltic contraction of the portal vein seemed unlikely.

Three-dimensional analysis of nephrogenesis in the neonatal rat kidney: light and scanning electron microscopic studies

In order to clarify the process of renal development more precisely than previously, the present study observed the rat neonatal kidney by scanning electron microscopy (SEM) of KOH digested tissue as well as by light microscopy of plastic sections. In the subcapsular region, aggregation of the mesenchymal cells was closely associated with the upper side of the ureteric duct ampulla. These mesenchymal cells projected a number of fine irregular processes at the basal portion facing the ureteric duct. A spherical cluster transformed from the mesenchymal cell aggregation was found on the lower side of the terminal ampulla, and was differentiated into the renal vesicle. Some cells at the top of the renal vesicle formed a cone-shaped projection and invaded the ureteric duct ampulla, forming a connection with it. In the advanced stage, a shallow transverse cleft appeared on the outer lateral side of the renal vesicle, and a second cleft was formed on the opposite side close to the junction between the renal vesicle and the ampulla. As the two clefts deepened, the vesicle assumed the well-known S-shaped body. In the advanced S-shaped body, the lower limb became cup-shaped, while the segment between the middle and lower limbs of the "S" elongated to form a tubular structure (i.e., the prospective proximal tubule and Henle's loop). The upper limb of the "S" also increased its length to form a distal tubule. The middle limb of the "S", however, was attached firmly to the cup-shaped lower limb (i.e., the prospective renal corpuscle) and was considered to become the macula densa of the mature nephron. In the maturing renal corpuscle, irregularly shaped cells were observed as a sheet-like aggregation at its vascular pole and were continuous with the vascular smooth muscle cells. These findings will help toward a better understanding of the morphological complexities of nephrogenesis.