The Free Border of The Intestinal Epithelial Cell of Vertebrates

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Abstract
1. The structure of the free border of the epithelial cells of the intestine has been investigated in various adult and larval Amphibia and in a reptile, bird, and mammal. 2. The structure is essentially the same in all (except in frogtadpoles in the stage with external gills). This structure is shown diagrammatically in Text-figs. 1 and 2. 3. The free border consists essentially of three layers: a. A superficial layer, pierced by pores, next the lumen of the intestine. b. A canal layer, pierced by spindle-shaped canals continuous with the pores of the superficial layer. These canals traverse the layer at right angles to its surface. c. A granular layer, containing many spherical granules, next the cytoplasm of the cell. 4. The intercellular band or Kittleiste is situated at the level the granular layer. 5. The continuous substance of the canal layer--that is, the substance that intervenes between the canals--appears to be composed of proteins. The granular layer contains lipines. 6. The theory that the free border contains ‘rods’ is found to be erroneous. 7. It is suggested that unhydrolysed fats may enter the intestinal epithelial cell through the pores and canals in the form of extremely fine droplets. 8. Two hypotheses are put forward, the one mechanical and the other electro-endosmotic, to account for the passage of substances from the cavity of the intestine into the epithelial cells.

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