Ultrastructure and permeability of blood microvessels: precapillary arteriole
ORIGINAL PAPERS
Abstract
Introduction. Intestinal villi, whose surface area is greatly increased by microvilli, represent a key structural element facilitating nutrient absorption in the small intestine. However, current knowledge regarding transepithelial exchange remains fragmentary; the study of the microcirculatory bed as a morphofunctional unit responsible for hematolymphatic transfer is therefore of particular relevance. The aim of this study was to investigate the ultrastructure and permeability of the precapillary arteriole in the intestinal villus. Materials and methods. The jejunal mucosa of white rats and humans at various stages of prenatal and postnatal ontogenesis served as the object of investigation. Scanning and transmission electron microscopy, corrosion casts, intravascular injection of horseradish peroxidase followed by light microscopic analysis, and serial semithin sections were employed. Permeability was assessed using a tracer method with chylomicrons and horseradish peroxidase under fasting conditions (control) and following dietary load. Results. The wall of the precapillary arteriole (diameter 14.7–17.2 μm) consists of endothelium, basement membrane, and pericytes; smooth muscle cells are absent. Endothelial cells contain numerous microfilaments and microtubules. Intercellular junctions are represented by tight and gap junctions, which form an effective barrier: the peroxidase reaction product fills the intercellular cleft only up to the level of specialized contacts. Under functional load, the number of perfused capillaries increases by 48%. Conclusion. Experiments with retrograde peroxidase transport confirmed the possibility of bidirectional macromolecule movement across the endothelial barrier; however, its efficiency is considerably inferior to direct transfer from the bloodstream.
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