Quantitative fine structure of capillaries in subregions of the rat subfornical organ

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Abstract
The differentiated cytology across subregions of the rat subfornical organ (SFO) prompted our hypothesis that ultrastructural features of capillary endothelial cells would vary topographically and quantitatively within this small nucleus. We used electron microscopic and computer‐based morphometric methods to assess fine structural dimensions of the capillary endothelium in four distinct subregions of the SFO from Long‐Evans and homozygous Brattleboro rats. Three types of capillary were present. Type III capillaries (resembling those of endocrine glands) had an average wall thickness of 0.17 μm, 54% thinner than those of Type I and II capillaries. Pericapillary spaces around Type III capillaries measured 56 μm2, 100% larger than for Type I vessels (resembling those of skeletal muscle). Only Type III capillaries contained fenestrations (9 per μm2 of endothelial cell) and were the predominant type of capillary in central and caudal subregions of the SFO. Type I capillaries, prevalent in the transitional subregion between the central and rostral parts of the SFO, had 10 cytoplasmic vesicles per μm2 of endothelial cell area, a number not different from that of Type III capillaries but 3× the frequency found in Type II vessels. Type II capillaries (those typical of “blood‐brain barrier”endothelium) had low vesicular density (3 per μm2), no fenestrations, and no pericapillary spaces. Luminal diameters and the densities of mitochondria and intercellular junctions were not different among capillary types or subregions in the SFO. Furthermore, there were no morphometric differences for any capillary dimensions between Long‐Evans and Brattleboro rats. These studies reveal quantitative differences among capillary networks in four distinct compartments of the rat SFO and provide morphometric dimensions for the three types of capillary existing in this cerebral sensory nucleus.