Original Paper Cells Tissues Organs 2000;166:294–303 Gross Morphology and Topography of the Adult Intestinal Tract of the Tilapian Fish, Oreochromis niloticus L. Bonnie J. Smith a Stephen A. Smith a Bundit Tengjaroenkul a Terry A. Lawrence b a Department of Biomedical Sciences and Pathobiology and b Biomedical Media Unit, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, Va., USA Accepted after revision: April 27, 1999 Bonnie J. Smith Department of Biomedical Sciences and Pathobiology Virginia-Maryland Regional College of Veterinary Medicine Blacksburg, VA 24061-0442 (USA) Tel. +1 540 231 9024, Fax +1 540 231 7367, E-Mail bjsmith@vt.edu ABC Fax + 41 61 306 12 34 E-Mail karger@karger.ch www.karger.com © 2000 S. Karger AG, Basel 1422–6405/00/1663–0294$17.50/0 Accessible online at: www.karger.com/journals/cto Key Words Intestine W Morphology, gross W Tilapia W Fish W Oreochromis niloticus Abstract The intestinal tract of the Nile tilapia, Oreochromis niloti- cus L., follows a complex course involving multiple loops and coils arranged in a previously undescribed form. From cranial to caudal, five principal regions were identi- fied and designated as the hepatic loop (HL), proximal major coil (PMC), gastric loop (GL), distal major coil (DMC), and terminal segment (TS). The first four of these regions each possessed a reversal flexure and thus could be divided into proximal and distal limbs. Only the termi- nal segment was straight and undivided. The PMC and DMC were disposed in a spiral, cone-shaped mass (spiral intestine) – their proximal and distal limbs are thus desig- nated as centripetal and centrifugal limbs. These spiral limbs were arranged with each successive limb nested internal to the previous one. Beginning from the stom- ach, the complete course of the gut including designa- tions of the subdivisions of the major regions was as fol- lows: proximal limb of the HL, distal limb of the HL, cen- tripetal limb of the PMC, centrifugal limb of the PMC, proximal limb of the GL, distal limb of the GL, centripetal limb of the DMC, centrifugal limb of the DMC, and the TS. Though the topographical relations of the various gut loops permitted ready identification of each, external surface features were so similar among the segments that extirpated segments of gut could not be identified as to region of origin. The nesting of successive intestinal loops of the spiral intestine in this fish is novel among patterns previously described, and also among the more intricate of those that have been described. Copyright © 2000 S. Karger AG, Basel Abbreviations used in this paper DCfL distal centrifugal limb PCfL proximal centrifugal limb DCpL distal centripetal limb PCpL proximal centripetal limb DMC distal major coil PMC proximal major coil FL fish length TIL total intestinal length GL gastric loop TS terminal segment HL hepatic loop