Three-dimensional morphogenesis of the omental bursa from four recesses in staged human embryos.

The omental bursa (OB) is a complex upper abdominal structure in adults. Its morphological complexity stems from embryonic development. Approximately 200 years ago, the first theory regarding OB development was reported, describing that the OB developed from changes in the position of the stomach and its dorsal mesentery. Thereafter, the second theory reported that the OB originated from three recesses: the right pneumato-enteric recess (rPER), hepato-enteric recess (HER), and pancreatico-enteric recess (PaER). However, the first theory, focusing on the rotation of the stomach, is still described in certain modern embryology textbooks. These two coexisting embryological theories deter the understanding of the anatomical complexity of the OB. This study aimed to unify these two theories into realistic illustrations. Approximately 10 samples per stage among Carnegie stage (CS) 13 and CS21 were microscopically observed and histological serial sections of the representative samples were aligned using the new automatic alignment method. The aligned images were segmented computationally and reconstructed into 3D models. The rPER and the HER encompassed the right half circumference of the esophagus and the stomach at CS13 and CS14, the PaER spread dorsal to the stomach and formed a discoid shape at CS15 and CS16, the infracardiac bursa (ICB) was separated by the diaphragm at CS17 and CS18, and the fourth recess, which we called the greater omental recess (GOR), extended caudally from the PaER among CS19 and CS21. The present results indicate that the fourth recess is also the origin of the OB. These two theories over 200 years can be generally unified into one embryological description indicating a new recess as the origin of the OB.