Structural and functional differences in various divisions of the rabbit colon.

The rabbit colon displays a diversity of form and function along its proximo-distal axis. Morphologically, four regions can be discerned based on macroscopic and microscopic criteria: 1) the initial portion of the colon immediately distal to the cecum (P1), in which wart-like protrusions characterize the surface topography, is 10 cm in length and endowed with three teniae. 2) The adjoining portion of the colon (P2) possesses one tenia, is about 20cm in length and also displays the wart-like protrusions in slightly less prominent form. 3) Fusus coli, a short segment approximately 4 cm in length, is free of teniae, but exhibits longitudinal folds on its inner aspect. Electron microscopically, it shows a paucity of microvilli in direct contrast to the two afore-mentioned regions. These three portions together constitute the proximal colon. 4) The fourth region of the colon, the distal colon, reaches a length of 80-100 cm and shows no obvious second-order enlargements of its surface, displaying scanning electron microscopically ridges in looped configurations. Physiological parameters also showed differences depending on the region of the colon observed. Water content of the ingesta increases slightly during passage of the proximal colon, decreasing in the fucus coli and distal colon. Na concentration was highest in the area P1-P2, decreasing distally. K was low in area P2 and then rose toward the fusus, only to fall again distally. Nitrogen values decrease considerably during passage of P2 but only slightly distally. Transmural electrical potential differences also exhibit a characteristic, discontinuous gradient.