Why are taeniae, haustra, and semilunar folds differentiated in the gastrointestinal tract of mammals, including man?
When a small change, either contraction or dilation, of the surface area of a digestive tube coincides with a considerable change in internal volume, this can be considered an effective influence of surface on volume. This study discusses the effects of anatomical differences between those types of digestive tracts where the longitudinal musculature is reduced to small bands, the taeniae, and those where such differentiations are not found. With the help of a geometric model the efficiency of transport (eta = net volume expelled aborally / volume of the tube) from haustra in tubes with two, three, four, and six taeniae was determined. It could be shown that efficiency of transport from haustra decreases when the number of taeniae increases. The second model applies the program "Surface Evolver" (author K.A. Brakke). The program is applied to investigate the relationships between changes in surface area of tubes with different numbers of taeniae (0, 2, 3, 4, and 6) and the respective changes of the internal volumes of tubes. Both haustra formation and formation of semilunar folds between haustra are considered. Tubes with two or three taeniae reduce their internal volume more efficiently than all others, namely, with a relatively small reduction of the surface area and contraction of the Tunica muscularis at a relatively low rate. In addition to chemical and mechanical decomposition of digesta the gastrointestinal tube has three tasks: propagation, storage, and retention of digesta. A tube without taeniae, i.e., with a complete longitudinal muscular layer, propagates contents with peristaltic movements. Storage can take place in areas of the tract with a dilated cross-section. Finally, folds can retain digesta. In many sections of the gastrointestinal tract folds are differentiated as permanent structures. However, folds formed with relatively little contraction of the musculature, i.e., little change in the surface area, represent an effective means of retention and thus of flow regulation. Tubes with taeniae and semilunar folds are adaptations for this effective type of regulation of digesta transit through the tract.
Langer
Peter
P
Institut für Anatomie & Zellbiologie, Justus-Liebig-Universität, Aulweg 123, D-35385 Giessen, Germany. peter.langer@anatomie.med.uni-giessen.de
Takács
Aron
A
D000818:Animals; D005769:Gastrointestinal Motility / Q000502:physiology; D041981:Gastrointestinal Tract / Q000033:anatomy & histology* / Q000502:physiology; D006801:Humans; D008322:Mammals / Q000033:anatomy & histology* / Q000254:growth & development / Q000502:physiology; D008954:Models, Biological; D009024:Morphogenesis; D009132:Muscles / Q000033:anatomy & histology* / Q000502:physiology; D013045:Species Specificity