Embryological study of the development of thin membranous dense connective tissues around the esophagus using two fetal cadavers

Objective/Aim We investigated the developmental process of thin membranous dense connective tissue (TMDCT) surrounding the esophagus. Methods Horizontal mediastinal sections at 1-mm intervals were prepared from 2 fetal cadavers fixed in 10% formalin at 4 and 8 months of gestation. The sections were stained with hematoxylin and eosin, elastica van Gieson, and Azan. The structures of the dense connective tissues in the cervical and mediastinal regions were compared with those in adult cadavers. Results In the superior mediastinum, at 4 months of gestation, homogeneous non-dense fibrous collagenous fibers were observed around the trachea, esophagus, and great vessels. At 8 months of gestation, the difference in collagen fiber density became evident around the great vessels and in parts of the peritracheal and periesophageal regions, forming structures homologous to the vascular and visceral sheaths observed in adults. In the middle-to-lower mediastinum, homogeneous collagen fibers at 4 months of gestation became more organized and denser at 8 months of gestation, especially around the descending aorta and azygos vein. A membranous structure extending bilaterally from the esophagus toward the pulmonary hila was also identified. Discussion and conclusion During the mid-to-late fetal development period, regional differentiation of collagen fiber density in the mediastinum becomes apparent, forming the structural basis for the vascular and visceral sheaths in adults. Vascular pulsations and esophageal peristaltic movements are presumed to be the driving forces behind this process.