Fig. 2

Morphological changes before a, b and during PS elevation (c). The PSs were observed in 7 coronal planes (P1–7) along the AP axis- from E12.5 to late E13.5. P1 was the extremely posterior region where the PSs first emerged and horizontally grew without obstruction from the tongue, implying the inherent spontaneous horizontal growth capacity. P2 and P3 were the posterior, P4 and P5 were the middle, and P6 and P7 were the anterior regions where the PSs were initially vertically constricted between the lateral borders of the mandibular bone and tongue at E12.5 and early E13.5. Black arrows, the new lingual bulging protrusions of PSs; green arrows, the original distal tip of PSs; red arrows, the disappearing frenulum lingualis