The lung primordium an outpouching from the foregut! Evidence-based dogma or myth?

Abstract

Background/Purpose This study examines and challenges the “evidence-based legitimacy” of the theory, “the lung primordium is an outpouching from the foregut.” Method A literature review was undertaken using computer database, journals, and relevant anatomical and embryological texts. Results The independent path of development taken by the tracheobronchial system and the oesophagus once identified as separate entities; the lack of morphologic, molecular, biological, and genetic supportive evidence for the “common-origin” theory; the distinct longitudinal line of demarcation between the nonsegmented muscles of the esophagus and the highly segmented cartilaginous structure of the tracheobronchial tree; the absence of a tracheoesophageal septum in the process of separation; the differences in epithelial lining; and the diametrically opposed mucociliary cascade of the upper airway vs the mucociliary escalator of the tracheobronchial tree all seriously challenge the authenticity of a common origin to these 2 entities. Conclusion To the extent that the foregut is seen as consisting of 2 separate semitubular splanchnopleuric entities ventrodorsally juxtaposed, it is true that the lung primordium as an outpouching of, and not from, the foregut. This must never be confused with the notion that the esophagus and tracheobronchial tree have a common origin. In fact, they develop from 2 completely separate segments of the trilaminar germ disk, but because of head fold development are brought together to create a common tracheoesophageal chamber that is later separated, facilitated by the prochordal membrane diverticulum.