Effect of extent of resection on intestinal muscle adaptation.

The response of intestinal muscle to resection has received less attention than mucosal adaptation but may be important in relation to altered motility and improved intestinal absorption. Our aim was to determine the effect of extent of resection on intestinal muscle adaptation. Distal resections of 25% (n = 5), 50% (n = 5), and 75% (n = 5) of the intestine were performed. Transverse intestinal sections were taken at resection and 12 weeks later to evaluate changes in mucosa and circular and longitudinal muscle thickness. Muscle cell number and size and muscle length were also measured. Mucosal thickness increased (P < 0.05) after all resections, (1007 +/- 253 microns vs 1259 +/- 181 microns, 25%; 1019 +/- 191 microns vs 1366 +/- 293 microns, 50%; and 927 +/- 88 microns vs 1432 +/- 213 microns, 75%). Longitudinal muscle thickness (169 +/- 35 microns vs 254 +/- 45 microns, 50%; 207 +/- 71 microns vs 353 +/- 103 microns, 75%) and length (180 +/- 10 cm vs 203 +/- 16 cm, 50%; 90 +/- 16 cm vs 110 +/- 21 cm, 75%) increased (P < 0.05) following 50% and 75% resections but not after a 25% resection. Circular muscle length increased after 75% resection alone (4.4 +/- 0.2 cm vs 5.8 +/- 0.4 cm, P < 0.05). There was no significant change in circular muscle thickness after any resection. Muscle cell size and number per unit area were unchanged in all groups. We concluded that: (1) Intestinal muscle adapts after intestinal resection and this response is related to the extent of resection; in contrast, mucosal adaptation was evident following even the least extensive resection. (2) Increased thickness and length of the longitudinal muscle layer are the most prominent changes. (3) This increased muscle thickness results from hyperplasia rather than hypertrophy.