Three‐dimensional analysis of the segmental arrangement of lower lung lobes in human fetuses: is this arrangement a miniature version of adult morphology?

Abstract

Knowledge of the lung segment system is essential for understanding human anatomy and has great clinical relevance. The arrangement of 11 segments, including the S* or subsuperior segment, and its individual variations, are considered to be the same in fetal and adult lungs. The present study assessed the topographical anatomy of lower segmental and subsegmental bronchi by computer‐assisted three‐dimensional imaging of serial sagittal sections of both lungs of 22 embryos and fetuses of gestational age 6–7 weeks (crown–rump length 15.0–28.5 mm). Long inferior courses of B8b (basal) and B10c (medial) were observed in sagittal sections of both lungs. B8a (lateral) and B10b (lateral) in the right lungs were consistently underdeveloped, with S9 occupying most of the lateral half of the lower lobe. In some samples, B6b (lateral) did not reach the lateral surface. The lateral dominance of S9 was also seen in the left lungs. Some B* candidates were present, but B7 candidates were absent. Lateral and posterior expansions of S6b, S8a and S10b to cover S9 were observed in additional midterm and near‐term lung sections, indicating that the original S9 dominance was 'corrected' by an increase in lung volume. Delayed growth of the lower lateral subsegments might induce mechanical stress, resulting in aberrant notches or fissures, such as those separating an independent posterior lobe. The segmental arrangement of fetal lungs was not stable, but was altered over a long fetal period after the complete subsegmental division of the bronchi, except for the minor bronchi B* and B7.

Left/right difference in the course and division of the pulmonary arterial branches in the lung upper lobe: A study using human embryos and early fetuses

“…Moreover, the rotation of the whole upper lobe seemed not to occur in the embryonic left lung but in the right lung after midterm when the increased volume of the lower lobe provides a driving force (Cai et al, 2020). Thus, S3 is corrected from a lateral to an anterior position and the earliest first division of the PA to the medial aspect of the lung.…”

“…This mechanism explains many events those had been called as migration or moving (reviewed by Gasser, 2006). Cai et al (2020) did not rule out a classical concept of the broncho-arterial unit. They noted that A4 and A5 divided at a site close to the first division of the middle or lingual lobar bronchus.…”

“…However, according to the present result, the initial upper bronchi B1, B2a, B2b, and B3 did not accompany an artery, respectively, due to the delayed arterial development. Moreover, the rotation of the whole upper lobe seemed not to occur in the embryonic left lung but in the right lung after midterm when the increased volume of the lower lobe provides a driving force (Cai et al ., 2020). Thus, S3 is corrected from a lateral to an anterior position and the earliest first division of the PA to the medial aspect of the lung.…”

“…Recently, our group (Cai et al, 2020) described a delay in the arterial development in the lower lobe: All segmental arteries except for A7 appear after bronchial development has advanced to the F I G U R E 1 Topographical anatomy of the bronchial tree and pulmonary arterial branches in the lung of an embryo at 6 weeks (stage 18). Sagittal sections.…”

“…Recently, our group (Cai et al ., 2020) described a delay in the arterial development in the lower lobe: All segmental arteries except for A7 appear after bronchial development has advanced to the sub‐subsegmental level. They concluded that a fetal lung is not a mini‐version of the adult morphology.…”