Figure 3.

Proposed model uniting mechanical stretching with oriented cell division and Fgfr2b signaling during branch elongation. (A) Proximal airways and branch stalks (1) experience strong and directed mechanical stress (solid lines), while distal buds (2) experience weaker, more variable stress (dashed lines). The response of cells to stress is under Fgfr2b regulation. Note how stalk cells show clear apical-basal polarity, whereas tip cells are more rounded. The blue oval represents localized source of Fgf10 expression. (B) One of the responses to mechanical stress is spindle orientation during mitosis; proximal stalk cells (1) display a fixed spindle orientation resulting in divisions parallel to the airway long axis, or spindle angles around 0°, while distal cells (2) display rotating spindles resulting in more randomly oriented cell divisions, such as 45° and 90°.