Ultrastructural Basis of Intestinal Absorption

抄録

The mechanisms of intestinal absorption were reviewed and discussed from the viewpoint of fine morphology of the small-intestinal epithelium. After reviewing the fine structural specializations of intestinal absorptive cells, particular emphasis was given to the morphological aspects of the first entry of gut contents into the intestinal epithelium.
1. The absorption of gut contents into the absorptive cells involves two kinds of mechanisms: one is the transport across the plasma membrane of the microvilli, by which mainly micromolecules are absorbed into the cells, and the other is the transport by endocytosis at the base of the microvilli, by which macromolecules can be taken up into the cells.
2. Freeze-fracture replica studies on the microvillous membranes suggest the existence of transmembrane channels formed by assembling of three or more protein subunits, through which hydrophilic micromolecules may be transported into the cytoplasm of absorptive cells. Since disaccharidases are known to be localized in the microvillous membranes as a complex composed of two or three disaccharidase molecules, the possibility is suggested that some of the protein subunits forming the transmembrane channels might be members of disaccharidases, and thus, the sugar absorption across the microvillous membranes may be closely coupled with the terminal digestion by disaccharidases.
3. The endocytotic uptake of macromolecules such as horseradish peroxidase or ferritin is seen as a common function of the absorptive cells in the small intestine of suckling mammals or stomachless fishes, but not common in the normal small intestine of adult mammals. The absorptive cells in the adult, however, have a potential ability of endocytotic uptake of substances which might be enhanced under some pathological conditions like malabsorption syndrome or in some experimental conditions such as ligation or organ culture of the small intestine.
4. The intercellular junction between adjacent epithelial cells is not permeable to such macromolecules as intact proteins, but permeable to micromolecules such as ionic lanthanum. This characteristic permeability of the tight junction may well be reflected in the structural organization of intramembranous junctional particles which is revealed by freeze-fracturing of the tight junctional region. Thus, the intercellular transport of intestinal contents is usually restricted, if any, to very small amounts of micromolecules.