Ultrastructural Demonstration of the Absorption and Transportation of Minute Chylomicrons by Subepithelial Blood Capillaries in Rat Jejunal Villi

Abstract

Chylomicrons from villous columnar epithelial cells are generally known to be transported only by central lymph vessels (CLV), whereas antigenic particulates derived from the intestinal lumen can also be transported by subepithelial blood capillaries (sBCs) in rat intestinal villi. The possibility of chylomicron absorption by sBCs was histoplanimetrically studied in the rat jejunum under a transmission electron microscope. The chylomicrons more abundantly presented in villous venules than in arterioles. The most frequent size (MFS) of chylomicrons was 75 to 90 nm in diameter in the areas near sBCs, while it was 45 to 60 nm in the epithelial intercellular spaces just above sBCs or the intermediate areas between sBCs. The MFS of chylomicrons was 45 to 60 nm in the intermediate areas between sBCs and in the epithelial intercellular spaces just above these areas. The MFS of chylomicrons in CLV was intermediate between that in the area adjacent to sBCs and that in the intermediate areas between sBCs. Chylomicrons were found in small vesicles in the endothelial cytoplasms of sBCs. No chylomicrons larger than 600 nm were observed in the lamina propria. These findings suggest that some of the chylomicrons smaller than 75 nm, which are probable intestinal very low-density lipoproteins (VLDL), are directly transported to the liver by hepatic portal blood in addition to CLV and that epithelial fat droplets larger than 600 nm are not discharged into lamina propria in rat jejunum under physiological conditions.

Emerging Role of Lymphatics in the Regulation of Intestinal Lipid Mobilization

“…Mature CM particles exit the enterocytes at the basolateral membrane by exocytosis into intercellular spaces and the lamina propria, before entering the lacteals, larger mesenteric lymphatics, and the thoracic duct, and ultimately enter the blood circulation. The presence of CM particles along this secretory pathway has been identified in the intercellular spaces, the lamina propria, and lymphatic vessels (Robertson et al, 2003;Takahara et al, 2013;Xiao et al, 2018b). These extracellular CMs, which appear to be retained for many hours after food ingestion (Xiao et al, 2018b), may also be a source of mobilized intestinal lipid and CMs in the post-absorptive period.…”

“…CMs are believed to move through the lamina propria via diffusion, largely influenced by convective movement of fluids (Tso and Balint, 1986). Subsequently, CMs enter the lymphatic system from the lamina propria via the lacteals (Takahara et al, 2013). Although a transcellular pathway has been described, the majority of CMs enter lacteals by a paracellular pathway via large, porous, intercellular junctions at the tip of the lacteal.…”

“…Although a transcellular pathway has been described, the majority of CMs enter lacteals by a paracellular pathway via large, porous, intercellular junctions at the tip of the lacteal. Entry of CMs into the lacteal is believed to be through size exclusion; thus, the majority of CMs with considerable size enter the lacteal while only a minor fraction of CMs with very small sizes may enter the subepithelial blood capillaries (Takahara et al, 2013). Recent evidence in mice suggests that mechanisms beyond simple size exclusion may underlie the uptake of CMs by the lacteals.…”

Immunohistochemical and histoplanimetrical study on the endothelial receptor involved in transportation of minute chylomicrons into subepithelial portal blood in intestinal villi of the rat jejunum

“…Until quite recently, chylomicrons discharged into the lamina propria from the villous columnar epithelial cells were thought to be transported only by the central lymph vessels (CLV) and never by blood capillaries in the intestinal villi [ 10 , 11 , 23 ]. However, a previous ultrastructural and histoplanimetrical study demonstrated that a portion of the minute chylomicrons less than 75 nm in diameter are transported by not only CLV but also subepithelial blood capillaries (sBC) and that minute chylomicrons encapsulated by cell membranes are transported into the lumina of the sBC through endothelial cells in the rat jejunum [ 41 ]. From these findings, the transportation of minute chylomicrons into the lumina of sBC is assumed to be performed by receptor-mediated transcytosis in the endothelial cells of sBC.…”

“…Therefore, the minute chylomicrons in the broad sense— i.e. , less than 75 nm in diameter [ 41 ]—are assumed to be recognized and bound by VLDL receptor or LDL receptor on the endothelial cells of sBC in the rat small intestine. The aim of the present study was to demonstrate the contribution of VLDL receptor or LDL receptor to the transportation of minute chylomicrons into the lumina of sBC in the rat jejunum.…”