Figure 6. Transient and permanent intercalations are distinguished at the basal tissue surface.

(A) A tissue field was wrapped around a lumen to form a 2D stratified epithelium modeling a 3D mammary end bud. Example of a cell failing to contact the basal surface (blue), with protrusion strength of 1.6× and posterior gradient strength of 1.06× field strength. (A′) Example of a successful intercalation (blue), with protrusion strength of 1.5× and posterior gradient strength of 1.4× field strength.

(B) Protrusion strength and gradient strength were varied across FEM simulations to define the mechanical characteristics of successful boundary contact, within the context of the in silico TEB model from (A). Tension magnitudes are defined as in (Figures 3I–J).

(C) Quantification of basal area dynamics (C) prior to and (C′) after radial intercalation. The YZ maximum intensity projection displays the cell’s basal area in green.

(D) YZ-axis projection showing membrane (tdTomato, red) and 3D surface rendering (green) of basal area during (D) transient and

(E) permanent intercalation.

(F) Quantification of basal area (μm², from D) during transient and

(G) permanent (μm², from E) intercalation.

(H) Quantification of the maximum basal area (μm², mean ± SD) during intercalation for transient (106.5 ± 37.8μm²) and permanent (193.2 ± 66.1μm²). N=22 cells, 19 orgs, r=11. Mann-Whitney test, p<0.001.

(I) Quantification of the rate of basal expansion (%/hour, mean ±S D) during intercalation for transient (73.9 ± 28.4%/hr) and permanent (35.7 ± 30.7%/hr). N=22 cells, 19 orgs, r=11. Mann-Whitney test, p<0.01.

(J) A FEM was generated of a tissue with high basal surface tension (dark green cells) to test candidate mechanisms for basal area of an intercalating cell. A cell (blue) was captured at the basal surface through a combination of posterior cell tension and focal disruption of the high basal tissue tension. (J′) Principle tension components (γ_MI, γ_EI, γ_ME, yellow arrows) acting at the two intercellular junctions. M, Matrix; I, Intercalating cell; E, Epithelial cell. Angles θ and ϕ are the angles γ_EI and γ_ME, respectively, make with the action line of γ_MI as illustrated in Figure S5D. The intercalated cell basal surface could then undergo 1 of 3 conditions:

(K) γ_MI<γ_EIcosθ+γ_MEcosϕ results in expansion of the basal surface (+0.75), a permanent intercalation.

(L) γ_MI=γ_EIcosθ+γ_MEcosϕ results in stasis of the basal surface (+0.75), a permanent intercalation.

(M) γ_MI>γ_EIcosθ+γ_MEcosϕ results in contraction of the basal surface (+0.75), a transient intercalation.