In comparison to tissues harvested from WKY rats, SHR mesenteric tissues contain smaller microvascular networks, consistent with previously documented microvascular rarefaction in other organs of the SHR (Chen et al., 1981
; Greene et al., 1990
; Hutchins and Darnell, 1974
; Kobayashi et al., 2005
; Prewitt et al., 1982
). 15/15 SHR isolated networks from 6 tissues were classified as small
defined by being contained to a 10× field of view. A 7th tissue contained 5 larger networks, atypical of the architectures associated with the other tissues and spanning vascular areas greater than a 10 × field of view. In contrast to the small
SHR networks, 5/8 WKY networks spanned multiple fields of view. 5/6 WKY tissues contained these dramatically larger networks. Overall fewer WKY networks were observed because, based on our definition, an isolated network could consist of multiple interconnected sub-networks.
Inspection of microvascular architectures within SHR and WKY networks of comparable area served to identify alterations more complex than just a loss of microvessels (). Typical small SHR networks had arterial/venous (A/V) anastomoses located proximal to the capillary level and prior to multiple arterial or venule branching (; ). As a consequence, these networks had less of a tree-shape as compared to WKY networks (). Individual arterial and venous vessels were determined based on position within the microvascular network, relative vessel diameter, and endothelial cell morphology. The larger, but less frequent, SHR microvascular networks displayed an increased capillary density per vascular area () compared to WKY networks ().
Figure 1 Representative examples of SHR and WKY adult mesenteric microvascular networks identified by PECAM labeling. Panel A: Typical SHR microvascular networks display arterial/venous anastomoses and a non-branching or non-tree like architecture. Panel B: In (more ...)
Figure 2 Representative examples of SHR and WKY adult mesenteric microvascular networks labeled with PECAM (blue) and NG2 (dark red). Panel A) In typical SHR networks, perivascular cells along both arterioles and venules lack positive NG2 labeling. At the capillary (more ...)
SHR mesenteric microvascular networks also displayed an altered perivascular cell NG2 expression pattern. In typical small SHR networks, NG2 labeling did not identify smooth muscle cells, wrapping around arterioles or venules (). In comparably sized WKY networks, NG2 expression identified smooth muscle cells wrapping around arterioles and pericytes along most, if not all, capillaries (). In the SHR networks NG2-positive labeling is present only on a subset of capillaries (). Pericyte specific NG2-positive cell labeling along capillaries was confirmed by identification of the characteristic elongated, wrapping morphology (; Murfee et al., 2005
). These NG2 labeling patterns seemed to be associated with the SHR and WKY architectural differences. In one example of a typical SHR small network observed in a WKY tissue, NG2 expression patterns were similar to the SHR.
Figure 3 Vascular pericyte identification along capillaries in SHR microvascular networks. Panel A) NG2 positive (dark red) labeling identifies pericytes (arrows), which wrap and elongate along the PECAM positive (blue) endothelial lined capillaries. Panel B) (more ...)
In addition to the blood vascular endothelial cells, PECAM labeling in rat mesenteric tissues also identified lymphatics endothelial cells. Lymphatic vessels were distinguishable from blood vessels by diameter, decreased PECAM labeling intensity, and endothelial cell morphology (Murfee et al., 2007
). We observed significant structural differences in initial lymphatic vessels, including increased diameters and altered branching patterns, in SHR and WKY mesenteric tissues compared to tissues harvested from adult Wistar rats (). The Wistar strain represents an alternative normotensive control given that it is the origin for both the SHR and the WKY and that the WKY, in spite of its lower blood pressure relative to the SHR, is still slightly hypertensive (DeLano et al., 2005
Figure 4 Representative examples of terminal lymphatic vessels labeled with PECAM in adult SHR, WKY, and WISTAR mesenteric tissues. In general, lymphatic vessels in SHR (Panel A) and WKY (Panel B) networks demonstrate increased diameters and increased branching (more ...)