Epithelial branch elongation is a central developmental process during branching morphogenesis in diverse organs. This fundamental growth process into large arborized epithelial networks is accompanied by structural reorganization of the surrounding extracellular matrix (ECM), well beyond its mechanical linear response regime. Here, we report that epithelial ductal elongation within human mammary organoid branches relies on the non-linear and plastic mechanical response of the surrounding collagen. Specifically, we demonstrate that collective back-and-forth motion of cells within the branches generates tension that is strong enough to induce a plastic reorganization of the surrounding collagen network which results in the formation of mechanically stable collagen cages.
Fig:Human mammary gland organoids invade the ECM by non-continuous contractions: a Schematic overview of 3D culture: single primary human basal mammary epithelial cells are cultured in floating collagen gels. b Characteristic organoid morphology at three developmental stages (Establishment n = 36 organoids, Branch elongation n = 75 organoids, Alveologenesis n = 111 organoids). Nuclei are visualized using sirDNA. c The organoid diameter of the long axis during the different stages reveals an increase in diameter during the elongation phase (Establishment n = 36 organoids, Branch elongation n = 75 organoids, Alveologenesis n = 111 organoids). Box plots indicate median (red line), 25th, 75th percentile (blue box) and 5th and 95th percentile (whiskers) as well as outliers (single points). d Live-cell imaging reveals an anisotropic deformation field with strong deformations in front of the branches and no deformation at the sides of the branches (n = 24 organoids). The near field (n.f.) is defined as area between the branch tip and the ECM 300 µm away from it. e The deformation is decreasing with increasing angle to the branch (n = 14 organoids). Error bars, mean ± s.d. fThe bead displacement is non-continuous over time with contractions towards the branches and relaxations into the opposite direction (n = 14 organoids). g ECM contractions and relaxations slowly diminish with increasing distance to the organoid. Between each line 25 min passed, highlighting the alternations between contractions of the ECM towards the branches and relaxations away from them (n = 23 organoids). h The cumulative bead displacement in front of branches is increasing over time (red, n = 5 organoids), while the branch elongation is discontinuous in time (gray, n = 7 organoids). Scale bars, 200 µm (b), 70 µm (d). Organoids were derived from three biologically independent donors (Supplementary Table). P values are from a two-tailed Mann–Whitney test and provided in Supplementary Table.
Such matrix encasing in turn directs further tension generation, branch outgrowth and plastic deformation of the matrix. The identified mechanical tension equilibrium sets a framework to understand how mechanical cues can direct ductal branch elongation.
Buchmann, B., Engelbrecht, L.K., Fernandez, P. et al. Mechanical plasticity of collagen directs branch elongation in human mammary gland organoids. Nat Commun 12, 2759 (2021). https://doi.org/10.1038/s41467-021-22988-2