A comparison of the size of fenestrations in the internal elastic lamina of young and old porcine aortas as seen with the scanning electron microscope

Advanced age decreases local calcium signaling in endothelium of mouse mesenteric arteries in vivo

Aging is associated with vascular dysfunction that impairs tissue perfusion, physical activity, and the quality of life. Calcium signaling in endothelial cells (ECs) is integral to vasomotor control, exemplified by localized Ca(2+) signals within EC projections through holes in the internal elastic lamina (IEL). Within these microdomains, endothelium-derived hyperpolarization is integral to smooth muscle cell (SMC) relaxation via coupling through myoendothelial gap junctions. However, the effects of aging on local EC Ca(2+) signals (and thereby signaling between ECs and SMCs) remain unclear, and these events have not been investigated in vivo. Furthermore, it is unknown whether aging affects either the number or the size of IEL holes. In the present study, we tested the hypothesis that local EC Ca(2+) signaling is impaired with advanced age along with a reduction in IEL holes. In anesthetized mice expressing a Ca(2+)-sensitive fluorescent protein (GCaMP2) selectively in ECs, our findings illustrate that for mesenteric arteries controlling splanchnic blood flow the frequency of spontaneous local Ca(2+) signals in ECs was reduced by ∼85% in old (24-26 mo) vs. young (3-6 mo) animals. At the same time, the number (and total area) of holes per square millimeter of IEL was reduced by ∼40%. We suggest that diminished signaling between ECs and SMCs contributes to dysfunction of resistance arteries with advanced age.Listen to this article's corresponding podcast at http://ajpheart.podbean.com/e/aging-impairs-endothelial-ca2-signaling/.

Arterial internal elastic lamina holes: relationship to function?

Internal elastic lamina (IEL) hole (fenestration) characteristics and myoendothelial gap junction (MEGJ) density were examined in selected resistance and conduit arteries of normal and diseased rat and mouse models, using conventional, ultrastructural and confocal microscopy methods. Selected vessels were those commonly used in functional studies: thoracic aorta, proximal and distal mesenteric, caudal, saphenous, middle-cerebral and caudal cerebellar artery. Rat and mouse strains and treatment groups examined were Dahl, Sprague Dawley, Wistar Kyoto, Wistar, spontaneously hypertensive (SHR), deoxycorticosterone (DOC) treated rat; and apolipoprotein E knockout, C57/BL6 and BALB/c mice. Vessel size (as IEL circumference), IEL hole and MEGJ density were quantified. In mesenteric arteries, the width of IEL holes and the percent of IEL occupied by holes were also determined. IEL hole density varied significantly within and between mesenteric artery beds, even among normotensive rat strains. Among the hypertensive rats (SHR and DOC), hole density in some vessels was higher in the normotensives than in the hypertensives within each strain, whereas in Dahl rats, hole density was similar between hypertensives and normotensives. Hole density was not correlated with the formation of intimal lesions in superior mesenteric artery. There was no positive general correlation between IEL hole and MEGJ density in resistance and conduit vessels. However, there was a positive correlation between the size of some resistance arteries and MEGJ density, although such a relationship did not hold for conduit vessels or during development, and there was no such relationship between vessel size and IEL hole density. Whilst IEL holes are obviously required for MEGJ communication, their presence is not an indication of contact-mediated communication, but rather may be related to the presence of sites for the low resistance passage of diffusion-mediated release of vasoactive endothelial and smooth muscle substances.

Role of elastic fibers in cooling-induced relaxation

The objective of this work was to confirm the main role of elastic fibers in differing responses of certain vessels during cooling from 37 to 8 degrees C. Previous results have shown that the nature of the vessel (conduit vessel vs muscular vessel) determines the different behavior (dilatation vs contraction) of isolated vessel segments when temperature decreases from 37 to 8 degrees C. In this work, it has been demonstrated that vessels with a great amount of elastic fibers show a dilatation when cooling. On the other hand, muscular vessels with fewer elastic fibers, such as the renal artery, undergo a contraction. The output of calcium from intracellular stores causes contraction of the renal artery during cooling. In this vessel, vasodilatation occurs only when mechanisms of smooth muscle contraction are inactive, as is the case with vessels that have undergone a cold storage period of 48 h. The results presented in this work confirm that there are two main effects, which directly depend on the vessel origin. In conduit arteries, the decrease of temperature induces a vascular relaxation, dependent on the elastic component of the vessel wall. In muscular vessels, the predominant effect is cooling-induced contraction due to an increase of intracellular calcium. This cooling-induced contraction needs the vessel to be in optimal conditions with an active metabolism of the muscular cells. These results are a crucial issue in the sense of explaining several biomedical mechanisms where hypothermia is implicated. The type of vessel implicated in procedures, such as isolated organ perfusion, extracorporeal circulation, and bypass surgery, must be taken into account.

Structural changes in rat aortic intima due to transmural pressure

Huang et al. (1997) propose a new hypothesis and develop a mathematical model to explain rationally the in vitro and in situ measured changes (Tedgui and Lever, 1984; Baldwin and Wilson, 1993) in the hydraulic conductivity of the artery wall of rabbit aorta with transmural pressure. The model leads to the intriguing prediction that this hydraulic conductivity would decrease by one half if the thin intimal layer between the endothelium and the internal elastic lamina volume-compresses approximately fivefold. This paper presents the first measurements of the effect of transmural pressure on intimal layer thickness and shows that the intimal matrix is, indeed, surprisingly compressible. We perfusion-fixed rat thoracic aortas in situ with 2 percent glutaraldehyde solution at 0, 50, 100, or 150 mm Hg lumen pressure and sectioned for light and electron microscopic observations. Electron micrographs show a dramatic, nonlinear decrease in average intimal thickness, i.e., 0.62 +/- 0.26, 0.27 +/- 0.14, 0.15 +/- 0.10, and 0.12 +/- 0.07 (SD) micron for 0, 50, 100, and 150 mm Hg lumen pressure, respectively. The volume strain of the intima is more than 20 times greater than the radial strain of the artery wall due to hoop tension and two orders of magnitude greater than the consolidation of the artery wall as a whole assuming constant medial density (Chuong and Fung, 1984). Moreover, in both light and electron microscopic observations, it is easy to find numerous sites where the endothelium puckers into the fenestral pores at high lumen pressure, as predicted by the theory in Huang et al. (1997). In contrast, the average diameter of a fenestral pore increases only 10 percent as the lumen pressure is increased from 0 to 150 mm Hg. These results indicate that the thin intimal layer comprising less than 1 percent of the wall thickness can have a profound effect on the filtration properties of the wall due to the large change in Darcy permeability of the layer and the large reduction in the entrance area of the flow entering the fenestral pores, though the pores themselves experience only a minor enlargement due to hoop tension.

Changes in morphology of elastin fibers during development of the tunica intima of monkey aorta

The normal development of elastin fibers in the thoracic aorta was studied in fetal, young, and adult monkeys. Tissue was examined by scanning electron microscopy (SEM) after NaOH treatment and by transmission electron microscopy (TEM). The NaOH treatment of fixed tissues effectively removed collagen fibers and enabled three-dimensional visualization of the elastin fibers. In intact fetal aortae, the internal elastic lamina (IEL) was situated immediately beneath the endothelium. This IEL consisted of superficial, longitudinally arranged bundles of elastin fibrils and an underlying solid sheet containing round fenestrations. In neonates, diffuse intimal thickening was observed. In the young and young-adult monkeys, the aortae exhibited intimal thickening with slender but split IEL. One of the most important findings of this study was that elastin fibers in the intimal thickening, as well as smooth muscle cells, ran in a longitudinal fashion. This was in contrast with the elastic laminae of the media which were mainly oriented circumferentially. Subendothelial elastin fibers in this intimal thickening combined with longitudinally arranged microfibrils which formed close associations with endothelial stress fibers. In some adult monkey aortae with well-developed intimal thickening, a complex meshwork of slender elastin fibers was also found beneath the endothelium. The development of the intimal elastin fibers is discussed in relation to hemodynamic forces.

An in vitro model for cell-cell interactions

Heterotypic cell-cell interactions appear to be involved in the control of development and function in a wide variety of tissues. In the vasculature, endothelial cells and mural cells (smooth muscle cells or pericytes) make frequent contacts, suggesting a role for intercellular interactions in the regulation of vascular growth and function. We have previously grown endothelial cells and mural cells together in mixed cultures and found that heterocellular contact led to endothelial growth inhibition. However, this mixed culture system does not lend itself to the examination of the effects of contact on the phenotype of the individual cell types. We have therefore developed a co-culture system in which cells can be co-cultured across a porous membrane, permitting intercellular contact while maintaining pure cell populations. Co-culture of endothelial cells and smooth muscle cells across membranes with pore sizes of 0.02, 0.4, 0.6, and 0.8 microns maintained the two cell types as homogeneous populations, whereas smooth muscle cells migrated across the membrane through pores of 2.0 microns. Vascular cell co-culture across membranes with 0.8-microns pores resulted in the inhibition of endothelial cell proliferation and the generation of conditioned media which inhibited endothelial cell growth The arrangement of the cells in this co-culture system mimics the in vivo orientation of vascular cells in which mural cells are separated from the abluminal surface of the endothelium by a fenestrated internal elastic lamina or basement membrane. Because this co-culture system maintains separable populations of cells in contact or close proximity allowing for biochemical and molecular analyses of pure populations, it should prove useful for the study of cell-cell interactions in a variety of systems.

The architecture of adventitial elastin in the canine infrarenal aorta

Although the artery wall consists of three distinct layers, only the structures of the intima and media have been well characterized. The adventitia has generally been overlooked. Our examination focused on the organization of elastin and collagen which are the major components of this tunic. Canine infrarenal aortas were excised, stretched to their in vivo length, then pressure fixed in formalin. Transverse, longitudinal, and frontal sections were prepared with specific elastin and collagen stains. Areas of adventitia in these sections were examined with LM, and interconnections between collagen and elastin were photographed at various magnifications. Subsequently, the slides were fractured for attachment to SEM stubs, and the coverslips were demounted. The identical areas were then examined with SEM using the LM micrographs as a guide to identify elastin and collagen. Whole mount aortic ring preparations were digested in formic acid for 72 and 96 h at 45 degrees C to confirm adventitial elastin architecture. The adventitia was organized in alternating lamellae of collagen and elastin. The elastin lamellae consisted of continuous sheets of elastin with a longitudinal fibrillar substructure. Finer circumferential elastin fibers were also identified. These attached to both longitudinal elastin and adjacent collagen lamellae. Collagen lamellae were arranged in broad corrugated bands of fibrils. The unique architecture of the adventitia may explain some of the visco-elastic properties of the aorta in both normal and pathologic states.