A study of the ultrastructure of the small iris vessels in the vervet monkey (Ceropithecus aethiops)

Not All Pericytes Are Born Equal: Pericytes from Human Adult Tissues Present Different Differentiation Properties

Pericytes (PCs) have been recognized for a long time only as structural cells of the blood vessels. The identification of tight contacts with endothelial cells and the ability to interact with surrounding cells through paracrine signaling revealed additional functions of PCs in maintaining the homeostasis of the perivascular environment. PCs got the front page, in the late 1990s, after the identification and characterization of a new embryonic cell population, the mesoangioblasts, from which PCs present in the adult organism are thought to derive. From these studies, it was clear that PCs were also endowed with multipotent mesodermal abilities. Furthermore, their ability to cross the vascular wall and to reconstitute skeletal muscle tissue after systemic injection opened the way to a number of studies aimed to develop therapeutic protocols for a cell therapy of muscular dystrophy. This has resulted in a major effort to characterize pericytic cell populations from skeletal muscle and other adult tissues. Additional studies also addressed their relationship with other cells of the perivascular compartment and with mesenchymal stem cells. These data have provided initial evidence that PCs from different adult tissues might be endowed with distinctive differentiation abilities. This would suggest that the multipotent mesenchymal ability of PCs might be restrained within different tissues, likely depending on the specific cell renewal and repair requirements of each tissue. This review presents current knowledge on human PCs and highlights recent data on the differentiation properties of PCs isolated from different adult tissues.

A contemporary concept of the blood-aqueous barrier

This review traces the evolution of the concept of the blood-aqueous barrier (BAB) during the past 20 years. The Classical model simply stipulated that the tight junctions of the iris vasculature and ciliary epithelium excluded plasma proteins from the aqueous humor (AH). It failed to reconcile the presence of AH protein levels equal to 1% of that found in plasma. Moreover, models of barrier kinetics assumed that the processes of AH secretion and plasma protein entry were directly linked. Thus, elevations of AH protein levels could only be explained by a pathological breakdown of the BAB. Over the last 20 years it has been shown that the plasma proteins in normal AH by-pass the posterior chamber entirely. Instead, these proteins diffuse from the capillaries of ciliary body stroma, into the iris stroma and then into the anterior chamber. This creates a reservoir of plasma-proteins in the iris stroma that is not derived from the iris vessels. This reservoir is prevented from diffusing posteriorly by tight junctions in the posterior iris epithelium. The one-way valve created by the pupil resting on the anterior lens capsule, combined with the continuous, forward flow of AH through the pupil, prevents protein reflux into the posterior chamber. Importantly, in the new paradigm, secretion of AH and the entry of plasma proteins into AH, are semi-independent events. This opens the possibility that AH protein levels could increase in the absence of breakdown of the BAB. Clinical consequences of this new paradigm of the BAB are discussed.

Capillary pericytes: perspectives and future trends

A complete understanding of the microcirculation requires full knowledge of the structure and function of each of the constituent cells, including pericytes. Vascular endothelium and smooth muscle cells have been investigated intensively during the last two decades, but much less is known about the metabolism and function of capillary pericytes. However, the development of new electron microscopy techniques and the application of new cell culture and molecular biology techniques should allow for the rapid elucidation of the cellular biochemistry and the microvascular function and pathology of this ubiquitous capillary cell.

Contractile proteins in pericytes. I. Immunoperoxidase localization of tropomyosin

In these studies we have compared the relative amounts and isoforms of tropomyosin in capillary and postcapillary venule pericytes, endothelial cells, and vascular smooth muscle cells in four rat microvascular beds: heart, diaphragm, pancreas, and the intestinal mucosa. The results, obtained by in situ immunoperoxidase localization, indicate that (a) tropomyosin is present in capillary and postcapillary venule pericytes in relatively high concentration; (b) the tropomyosin content of pericytes appears to be somewhat lower than in vascular smooth muscle cells but higher than in endothelia and other vessel-associated cells; and (c) pericytes, unlike endothelia and other nonmuscle cells, contain detectable levels of tropomyosin immunologically related to the smooth muscle isoform. These results and our previous findings concerning the presence of a cyclic GMP-dependent protein kinase (Joyce, N., P. DeCamilli, and J. Boyles, 1984, Microvasc. Res. 28:206-219) in pericytes demonstrate that these cells contain significant amounts of at least two proteins important for contraction regulation. Taken together, the evidence suggests that pericytes are contractile elements related to vascular smooth muscle cells, possibly involved, as are the latter, in the regulation of blood flow through the microvasculature.

Rod-shaped bodies and crystalloid inclusions in ocular vascular endothelia of adult and developing Macaca mulatta

In addition to the usual organelles, endothelial cells of ocular blood vessels and Schlemm's canal in adult and fetal Macaca mulatta, contain two types of inclusions: rod-shaped bodies and crystalloids. The rod-shaped bodies are similar to those first described in arterial endothelia of the rat. They are between 0.2 to 0.35 micrometer in diameter, up to 2.5 micrometers in length, and are membrane bounded. The crystalloid inclusions are up to 1.2 micrometers in diameter and are associated with the granular endoplasmic reticulum. Their crystalline lattice consists of subunits, 28 nm in diameter, which are either arranged in rows separated by amorphous matrix or tightly packed in a honeycomb-like lattice. The rod-shaped bodies are much more numerous than the crystalloids; however, both kinds of inclusions are consistently found in both adult and fetal vessels. The represent normal components of ocular vascular endothelia which appear very early in the development of the eye.

Rubeosis iridis and glaucoma associated with sickle cell retinopathy: a light and electron microscopic study

The ultrastructure of rubeosis iridis in sickle cell-hemoglobin C disease is described for the first time. Findings included open interendothelial cell junctions, intraendothelial cytoplasmic attenuations (fenestrations), and pericyte formation. The ultrastructural appearance of rubeosis iridis gives no clue to the underlying etiology and is similar to that reported in rubeosis associated with diabetes mellitus, central retinal vein occlusion, and uveitis. The electron microscopic findings explain the functional incompetence of rubeotic vessels that are manifested by transmural leakage of fluorescein.

Fluorescein angiography of the iris. A correlation of microangiopathy in the iris and retina

Fluorescein angiography of the iris using filters that eliminated pseudofluorescence was performed. In 10 of 40 non-diabetic healthy eyes, a fine-grained leakage of dye occurred from capillaries of the pupillary margin (PML). In 87 of 94 diabetic eyes with or without retinopathy a PML was present. The vascular changes of the iris were compared with those of the retina as disclosed by fundus fluorescein angiography. In 14 of 18 eyes without retinopathy PML was observed. In 25 of 28 eyes with non-proliferative retinopathy there was a PML, 12 of those in addition showed extravasation to the iris stroma. All 48 eyes with proliferative retinopathy showed PML. In addition, 19 of these had extravasation to the iris stroma and 26 iris neovascularization. When iris neovascularization is present, larger fibrovascular proliferations are usually found in the vitreous space, often associated with recurrent vitreous haemorrhages or retinal detachment. Iris angiography yields valuable information on the vitreo-retinal diabetic sequelae and is useful for selection of such eyes for vitrectomy.

Ultrastructure of the microvessels of the iris in mammals with special reference to their permeability

The ultrastructure of the microvessels of the guinea pig, rat, rabbit and cat was studied in an effort to discover the morphological basis of the permeability of the iris vessels. The microvessels of the iris revealed a non-fenestrated endothelium. A relatively wide intercellular space and macula occludens were seen at the intercellular junctions of the endothelial cells in the capillaries and postcapillary venules. It is concluded that these intercellular spaces may form the small pore system which makes the diffusion through the iris vessels possible. Transcapillary exchange of high molecular substances is made possible by the vesicular transport system of the micropinocytosis vesicles.