The microcirculation is a major topic in current physiology textbooks and is frequently explained with schematics including the precapillary sphincters and metarterioles. We re-evaluated the validity and applicability of the concepts precapillary sphincters and metarterioles by reviewing the historical context in which they were developed in physiology textbooks. The studies by Zweifach up until the 1950s revealed the unique features of the mesenteric microcirculation, illustrated with impressive schematics of the microcirculation with metarterioles and precapillary sphincters. Fulton, Guyton and other authors introduced or mimicked these schematics in their physiology textbooks as representative of the microcirculation in general. However, morphological and physiological studies have revealed that the microcirculation in the other organs and tissues contains no metarterioles or precapillary sphincters. The metarterioles and precapillary sphincters were not universal components of the microcirculation in general, but unique features of the mesenteric microcirculation.

The ultrastructure of the rat intestinal interstitium was analyzed from the viewpoint of mechanical dynamics to stabilize the intestinal villi, crypts and mucosal folds. In the rat, the small intestine lacks circular folds, but the large intestine possesses spiral folds. The intestinal villi, the largest in the duodenum, decreased in size in the jejunum and ileum successively, and were absent in the large intestine. The intestinal interstitium consisted of lamina propria mucosae (LPM) and tela submucosa (TSM) separated by muscularis mucosae (MM), the LPM was subdivided into an upper part within the villi and a lower part among the crypts in the small intestine. The light microscopic density of interstitium in the intestinal wall was lowest in the upper LPM, moderately dense in the lower LPM and highest in the TSM, and that among the intestinal region was highest in the duodenum and decreased successively in the jejunum and ileum. In the large intestine, the TSM bulged to form spiral folds with very low density. The intestinal epithelium in the villi possessed wide intercellular spaces and that in the crypts had closed intercellular spaces. At electron microscopic level, the upper and lower LPM contained subepithelial supportive meshwork that consisted of collagen fibrils and myofibroblast processes. The lower LPM and TSM contained conspicuous bundles of collagen fibrils and, in addition, TSM contained minor populations of scattered collagen fibrils near the smooth muscle layer (SML). The diameter of collagen fibrils was the largest in the bundles of TSM, and decreased from the duodenum through the jejunum and ileum to the large intestine. On the basis of these observations, we hypothesize that the intestinal villi are mechanically stabilized by the balance between the expansive interstitial pressure and inward pull by the subepithelial supportive meshwork. This hypothesis explains the hitherto neglected fact that the intestinal epithelium possesses wide intercellular spaces only in the villi, and accounts for the counterforce against the perpendicular smooth muscle cells, which are supposed to contract the intestinal villi.

The ultrastructure of the rat intestinal interstitium with regard to the mechanical components was analyzed from a functional viewpoint utilizing serial horizontal as well as longitudinal sections through the lamina propria mucosae, including both villi and crypts. The axial smooth muscle cells in the villi (villus-axial SMs) exhibited different configurations at various levels of the wall. They were separated from the voluminous fluid-filled spaces by sheet-like processes of fibroblasts in the upper part of the intravillous interstitium, formed a sheet around the central lymphatics, and were covered by the sheet-like processes of fibroblasts in the lower part of the intravillous interstitium. These villus-axial SMs were poorly developed and associated with the lymphatic walls in the upper part of the pericryptal interstitium; they were tapered and connected to microtendons composed of fascicles of longitudinal collagen fibrils in the lower part of pericryptal interstitium. At the apical termination, the villus-axial SMs were connected to myofibloblasts, which sent off many processes into the subepithelial meshwork layer of fine cell processes and extracellular matrices. The villus-axial SMs possibly develop longitudinal tension against the intravillous hydraulic pressure developing from the transepithelial absorption through the intestinal epithelium.

Podocytes are specialized epithelial cells used for glomerular filtration in the kidney. They can be divided into the cell body, primary process and foot process. Here, we describe two useful methods for the three-dimensional(3D) visualization of these subcellular compartments in rodent podocytes. The first method, field-emission scanning electron microscopy (FE-SEM) with conductive staining, is used to visualize the luminal surface of numerous podocytes simultaneously. The second method, focusedion beam SEM (FIB-SEM) tomography, allows the user to obtain serial images from different depths of field, or Z-stacks, of the glomerulus. This allows for the 3D reconstruction of podocyte ultrastructure, which can be viewed from all angles, from a single image set. This is not possible with conventional FE-SEM. The different advantages and disadvantages of FE-SEM and FIB-SEM tomography compensate for the weaknesses of the other. The combination renders a powerful approach for the 3D analysis of podocyte ultrastructure. As a result, we were able to identify a new subcellular compartment of podocytes, "ridge-like prominences" (RLPs).

The ultrastructure and size distributions of collagen fibrils in Glisson's sheath were investigated in the rat liver to analyse the mechanical environment around the fibrils and their possible cells of origin. Glisson's sheath was found to contain 2 populations of collagen fibrils with different diameters and distinct localisations, namely fibroblast-associated and bile epithelium-associated. Fibroblast-associated collagen was composed of fibrils arranged in bundles and constituted the majority of the collagen in Glisson's sheath. Bile epithelium-associated collagen was represented by small dispersed groups of fibrils just beneath the basement membrane of the bile duct. The basement membrane of the bile duct was frequently reduplicated into a few or as many as 10 layers of laminae densae, with scattered collagen fibrils between these laminae. The diameters of the fibrils of both groups of collagen increased in relation to the calibre of the bile duct, whereas at any given place in Glisson's sheath bile epithelium-associated collagen fibrils had a smaller diameter compared with those of the fibroblast-associated fibrils. The increment in fibril diameter along the bile duct is considered to be correlated with the increase in mechanical stress acting on Glisson's sheath. The difference in diameter between the 2 populations as well as the incorporation of fibrils between the laminae densae of the basement membrane of the bile duct supports the view that the bile epithelium-associated collagen is produced by the epithelial cells of the bile duct, thus having a different origin from that of fibroblast-associated collagen. These findings provide the first evidence that the epithelial cells of the interlobular bile duct produce fibril-forming collagen. Furthermore, it is suggested that cholestasis stimulates the epithelial cells of interlobular bile duct to increased synthesis of fibril-forming collagen that is also produced by these cells under physiological conditions.

Effects of Spirulina on serum lipid components and glucose tolerance in rats were studied.Experiment diets were prepared with 20% Spirulina water-soluble fraction (SS), with 20% Spirulina waterinsoluble fraction (SI) or with 18% casein (CN). Each group of 8 male Sprague-Dawley rats were fed with SS, SI and CN diets for 4 weeks in individual cages. Fasting serum glucose levels were measured on the 7th and 28th days, and glucose tolerance on the 28th day. The nitrogen balance was also examined during the last 3 days. Total-cholesterol, HDL-cholesterol and triglycerides in serum were measured on the 28th day when the rats were sacrificed. No effect was observed on either growth or feed efficiency among all groups. The highest ratio of HDL-cholesterol to other cholesterols and the low fasting serum glucose levels were observed in the SS group. In the glucose tolerance test, the elevation of glucose seemed to be more suppressed in the SI group than in the SS group.

Periarterial connective tissue with a moderate amount of collagen fibrils is known to be a specialized domain in the renal interstitium. This study aimed to clarify the microscopic architecture of the periarterial connective tissue as a mechanical supportive structure of the intrarenal arteries. Transmission and scanning electron microscopy revealed two populations of collagen fibrils in the periarterial connective tissue. The major one was composed of many bundles of collagen fibrils running in longitudinal directions, whereas the minor one was represented by a few circumferential bundles adjacent to the smooth muscles. The amount of collagen fibrils was obviously variable and correlated with the arterial caliber. The correlation between abundance of collagen fibrils and the arterial caliber was confirmed by morphometric analysis of the collagen fibril area per arterial perimeter on electron micrographs. The size of individual collagen fibrils was measured in periarterial connective tissue of arteries with various calibers. A positive correlation between the diameter of collagen fibrils and arterial caliber was confirmed, indicating the supportive function of collagen fibrils in the periarterial connective tissue. The accumulated morphological findings supported the hypothesis that the collagen fibrils in the periarterial connective tissue develop longitudinal tension with their tensile strength, whereas the smooth muscle cells in the media develop circumferential tension with active regulation of contracting force.

SummaryWe studied the effects of fish oil and apple polyphenol combined with a high cholesterol diet in rats, and assessed serum and liver lipids concentrations, serum oxidative stress and fecal bile acid excretion. Young male rats were fed a diet containing the control (Control), apple polyphenol (AP), fish oil (FO) or fish oil1apple polyphenol (FO1AP) for 4 wk. The control diet contained a lard component. Posterior abdominal wall fat and testicle peripheral fat weights decreased in the FO1AP group compared to the AP group. The concentration of total cholesterol in the serum and liver decreased in the FO group and the FO1AP group compared to the Control and the AP groups. The concentration of adiponectin and biological antioxidant potential in the serum increased in the FO group compared to the other groups. The diacron-reactive oxygen metabolites in serum decreased in the FO group and the FO1AP group compared to the Control and the AP groups. The bile acid excretion in feces increased in the AP group, the FO group and the FO1AP group compared to the Control group. These results suggested that the combination of fish oil and apple polyphenol in the diet improved serum and liver lipids, which should assist in the prevention and improvement of metabolic syndrome.