Change in diameters of the small intestine according to embryonic and early fetal growth.

No previous studies have examined the diameter of the small intestine successively from the oral to the anal side of the small intestine. Therefore, the objectives of this study were to determine the successive intestinal diameters from the oral to the anal side (proximal to the distal) of the intestine, evaluate changes in diameter associated with growth, examine the effects of positional variation along the intestinal tract, investigate dynamic positional change from the extraembryonic coelom to the abdominal cavity, and assess the impact of complex tertiary intestinal loop formation. To this end, 14 human embryonic and fetal specimens with crown-rump lengths (CRLs) ranging from 25.6 to 69.0 mm were selected for high-resolution magnetic resonance imaging acquisition. The small intestines of the specimens were located in the extraembryonic coelom (herniation phase), transitioning phase, or abdominal cavity (return phase). The small intestine and mesentery were reconstructed in three dimensions, and the resulting morphological changes were observed and analyzed. Successive intestinal diameters from the oral to anal side of the small intestine were determined. Specifically, we observed the following: (1) gradual changes in the diameter of the position from the oral to the anal side in the jejunum-ileum, (2) the difference between the duodenum and jejunum-ileum, and (3) the difference between the superior part of the duodenum derived from the foregut and the remaining parts derived from the midgut. (4) Notably, the dynamic positional change from the extraembryonic coelom to the abdominal cavity, along with the rapid elongation and complex intestinal loop formation-a conspicuous phenomenon in the embryonic and early fetal periods-had little effect on the changes in diameter. This study indicates that increased diameter may serve as a useful indicator of intestinal development and differentiation, independent of tertiary intestinal loop formation and positional changes into and out of the abdominal cavity.