Quantification of small microvascular system morphology in five rat brain areas

Introduction

The local microvascular system consists of small arterioles, capillaries, and small venules. In the main, it is this system that is involved when investigating microvascular physiology by quantitative autoradiography (QAR) and other imaging modalities. In a previous animal study, this system was divided into small, medium, and large components based on luminal diameter ¹ . The purpose of the present work was to analyze the ultrastructure of these same three groups of microvessels in tissue samples from the previous study, correlate luminal diameter with microvessel type (arteriole, capillary, and venule), and calculate luminal (blood) volumes and surface area of arterioles, capillaries, and venules for five representative brain area.

Methods

Brain tissue from 5 spontaneously hypertensive (SHR) and 5 Wistar-Kyoto (WKY) rats were fixed; regions of interest (ROIs) were embedded in EPON resin, cut into 2 micron thick sections, and stained with 1% Toluidine blue as described before ¹ . Using these sections for guidance, sets of ultrathin sections from five ROIs were prepared. Small microvessels were chosen at random within them and viewed by electron microscopy. The microvessels were sized as: small, d ≤7.5 μm; medium, 7.5< d ≤12 μm; and large, 12< d ≤50 μm. The ROI's were the sensorimotor cortex (SMC), inferior colliculus (IC), paraventricular nucleus of the hypothalamus (PVN), the genu of the corpus callosum (GCC), and the subfornical organ (SFO), one of the brain areas with leaky capillaries.

Results

First, there were no differences between SHR and WKY rats for any of the measured end points, and the data from the two strains have been combined. Of the 181 small microvessels studied in all five ROIs, only one had a bit of discontinuous vascular smooth muscle and appeared to be arteriolar. All the rest (99% of those with d ≤7.5 μm) were capillaries. Pericytes were visible within the basal lamina in 80% of these capillaries. For the SMC, IC, PVN, and GCC, 25% of the 72 medium-sized microvessels examined had vascular smooth muscle and were, thus, arterioles. The remaining 75% of these microvessels (7.5< d ≤12 μm) were classified as very small venules; pericytes were present in the walls of 84% of them. For these same four ROIs, 23% of the 39 large microvessels (12< d ≤50 μm) studied were muscular and were, therefore, arterioles. The rest were venules, of which 85% were pericytic. As for the SFO, both the medium (n=14) and large (n=9) microvessels lacked smooth muscle and were venules.

Conclusions

Essentially all of the microvessels with d ≤7.5 μm are capillaries. Approximately 80% of surface area and 65% of the “blood” volume of the small microvascular system is in the capillaries ¹ . About 75% of the medium and large microvessels of this system are venules; the rest are arterioles. These two populations of microvessels have 35% of the morphological blood volume and 20% of the surface area, with 75% of both being venular.