INTRODUCTION

Promoted by the observation that they are selectively lost in early diabetic retinopathy, pericytes have attracted the interest of researchers from many disciplines. Still, pericytes are enigmatic cells. Neither their origin, nor their normal function has been fully delineated, and the biological meaning and the causes and consequences of their loss in the diabetic retina are still under investigation. It is useful to review current knowledge about the cellular crosstalk between vascular cells, before adding a further level of complexity by addressing the cellular crosstalk between vascular and neuroglial cells (1, 2) Pericytes are functionally codependent on endothelial cell, and each cell type provides its counterpart with growth factors and contact-dependent signals that influence survival and/or proliferation (3). Diabetic pericyte loss may represent one of the prominent examples in which the survival impact of pericytes is critically lost for endothelial cells.

Origin and Differentiation

Resident retinal pericytes derive from mesodermal and from neural crest origin during development (4). The relative contribution of common embryonic stem cell precursor which may incorporate into vessels under the influence of pericyte-recruiting factors such as PDGF-B remains unclear (5). During postnatal vascular repair conditions, and during angiogenesis, endothelial cell transdifferentiation into pericytes has been suggested (6). Recently, evidence for a bone marrow origin of mural cells supporting adult angiogenesis has been presented (7). Whether endothelial precursor cells from nondiabetic origin can integrate into diabetic retinal capillaries and replace functional pericytes is also unclear (8).

Pericytes are able to transdifferentiate into other cell types. For example, in the rat brain, phagocytic pericytes assume microglia cell functions (9). The conversion of pericytes to tissue macrophages has been observed, suggesting an active contribution to a variety of clearance and defense functions (10). In the brain, a transitional cell phenotype of pericytes compatible with a fibroblast morphology and localization, but with a surface expression pattern of a pericyte (see later) has been reported. In particular, pericytes can differentiate into vSMC and fibroblasts, and the reverse transformation of vSMC and fibroblasts into pericytes is possible (10–15). In vitro observations suggest further transdifferential potential into adipogenic, chondrogenic, and osteogenic cells (16, 17), reflecting the heterogeneity of pericyte populations in general.

Morphology and Distribution

Pericytes are regular components of capillaries in almost all human tissues and organs (10). In contrast to arteries and arterioles, where the coverage consists of single or multilayers of vascular smooth muscle cells (vSMC), the capillary system is exclusively covered by individual pericytes. It has been suggested that vSMC and pericytes represent phenotypic variations of a continuous cell lineage, because of morphological similarities and the expression of common markers such as smooth muscle actin and desmin (3). In the capillary system, pericytes are readily identifiable by the protuberant position within the capillary basement membrane and the shape (18, 19) (Fig. 1). They

Fig. 2. Retinal digestion preparation of a nondiabetic rat retina. Note the approx. 1:1 ratio of pericytes and endothelial cells. PAS staining, original magnification 400x.

neighboring endothelial cells via various adhesive structures, such as gap junctions, tight junctions, adhesion plaques, and so-called peg and pocket contacts (26, 27). These communications are the basis for the variety of direct or indirect consequences of pericytes on endothelial survival and proliferation.

Gap junctions between endothelial cells and pericytes are membranous channels directly connecting the cytoplasms of both cell types. These junctions are involved in the exchange of nucleotides and small molecules between pericytes and endothelial cells (28). Major components of gap junctions are the connexins of which Cx-37,-42, and -57 have been identified in the eye. Gap junctions provide a substantial role in controlling endothelial cell proliferation during physiological angiogenesis (29, 30). Tight junctions are membrane proteins which interconnect endothelial cells and pericytes. Pericytes are frequently located adjacent to or over tight endothelial junctions, supporting a direct barrier-promoting role. These contacts form a diffusion barrier that controls paracellular fluid transport through the capillary wall. By this, the number of pericytes determines the number of tight junction proteins. Importantly, membrane protein families of tight junctions are the claudins and the occludins. Cell culture experiments demonstrated that pericytes induce occludin production of brain endothelial cells. Occludin expression is induced by the pericyte-derived Angiopoietin-1. In brain microvessels, the induction of occludin expression enhances the tightness of tight junctions. Further studies suggested that the attenuation or inhibition of the angiopoietin-1/Tie-2 signaling leads to dysfunction of blood brain barrier in disease (31). These data exemplify the importance of pericytes for the maintenance of endothelial barrier function. Another variant of cell contact between pericytes and endothelial cell is the adhesion plaque which is rich in fibronectin depositions. These cell contacts anchor the pericyte to the endothelium during the transfer of contractile forces such as during contraction or during propagation of shear stress (20, 32–34).