Do the lung segments exist continuously from the early stage of the embryonic period as units?

Objectives: Although various types of segmentectomy are frequently performed for resecting lung tumours at present, there is no clear answer to the question what kind of segmentectomy would be more efficient for performing lymphadenectomy. Learning the embryological mechanism of the segment formation could be one of the methods for selecting the surgical procedure. To investigate the developmental mechanism of the lung, this study focused on 'sharing structure', a unique 3D structure consisting of the bronchi and pulmonary arteries. In the structure, two arteries from different directions, after straddling the bronchus in the central part, share one bronchial tree at the peripheral part.

Methods: Using computed tomography data obtained before segmentectomy, this study observed the 'sharing structure' in 193 left and right upper lobe cases. This study investigated the relationship between the segmental arterial types and the straddled bronchi, which were straddled by the pulmonary arteries found in the centre of the sharing structure.

Results: In the right upper lobes, the straddled bronchi were anterior segmental bronchi. In the left upper lobes, however, the straddled bronchi of the lingular interlobar pulmonary artery type contained no anterior segmental bronchi. But, the straddled bronchi of lingular mediastinal pulmonary artery type contained anterior segmental bronchi in all cases.

Conclusions: Although pulmonary arteries in almost all sharing structures in the right upper lobes straddled anterior bronchi, those in mediastinal type and interlobar type in the left upper lobe were found to straddle the anterior and apicoposterior bronchi, respectively. These findings indicated that the interlobar type was speculated to be rotating mediastinal type backward in the embryonic period. This study strongly suggested a new concept that 'the lung segments never continuously exist from the early stage of the embryonic period as units, but they are only simple units artificially named by their prevailing bronchial branching patterns'. Therefore, during segmentectomy including lymphadenectomy for pulmonary tumours, the retrieval of the branching patters of pulmonary arteries could allow the segmentectomy to become more efficient with considering the formations of lung lobes.