Collective cell migration during human mammary gland organoid morphogenesis.

Organ morphogenesis is driven by cellular migration patterns, which become accessible for observation in organoid cultures. We demonstrate here that mammary gland organoids cultured from human primary cells, exhibit oscillatory and collective migration patterns during their development into highly branched structures, as well as persistent rotational motion within the developed alveoli. Using high-resolution live-cell imaging, we observed cellular movement over the course of several days and subsequently characterized the underlying migration pattern by means of optical flow algorithms. Confined by the surrounding collagen matrix, characteristic correlated back-and-forth movements emerge due to a mismatch between branch invasion and cell migration speeds throughout the branch invasion phase. In contrast, alveolar cells exhibit continuous movement in the same direction. By modulating cell-cell adhesions, we identified collective migration as a prerequisite for sustaining these migration patterns both during the branching elongation process and after alveolus maturation.

Generation of ductal organoids from normal mammary luminal cells reveals invasive potential.

Here we present an experimental model for human luminal progenitor cells that enables single, primary cells isolated from normal tissue to generate complex branched structures resembling the ductal morphology of low-grade carcinoma of no special type. Thereby, we find that ductal structures are generated through invasive branching morphogenesis via matrix remodeling and identify reduced actomyosin contractility as a prerequisite for invasion. In addition, we show that knockout of E-cadherin causes a dissolution of duct formation as observed in invasive lobular carcinoma, a subtype of invasive carcinomas where E-cadherin function is frequently lost. Thus, our model shows that invasive capacity can be elicited from normal luminal cells in specific environments, which results in low-grade no special type morphology. This assay offers a platform to investigate the dynamics of luminal cell invasion and unravel the impact of genetic and non-genetic aberrations on invasive morphology. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Surface-tension-induced budding drives alveologenesis in human mammary gland organoids.

Organ development involves complex shape transformations driven by active mechanical stresses that sculpt the growing tissue 1,2. Epithelial gland morphogenesis is a prominent example where cylindrical branches transform into spherical alveoli during growth3-5. Here we show that this shape transformation is induced by a local change from anisotropic to isotropic tension within the epithelial cell layer of developing human mammary gland organoids. By combining laser ablation with optical force inference and theoretical analysis, we demonstrate that circumferential tension increases at the expense of axial tension through a reorientation of cells that correlates with the onset of persistent collective rotation around the branch axis. This enables the tissue to locally control the onset of a generalized Rayleigh-Plateau instability, leading to spherical tissue buds6. The interplay between cell motion, cell orientation and tissue tension is a generic principle that may turn out to drive shape transformations in other cell tissues.

The role of nonlinear mechanical properties of biomimetic hydrogels for organoid growth.

Cell-driven plastic remodeling of the extracellular matrix (ECM) is a key regulator driving cell invasion and organoid morphogenesis in 3D. While, mostly, the linear properties are reported, the nonlinear and plastic property of the used matrix is required for these processes to occur. Here, we report on the nonlinear and plastic mechanical properties of networks derived from collagen I, Matrigel, and related hybrid gels and link their mechanical response to the underlying collagen structure. We reveal the predominantly linear behavior of Matrigel over a wide range of strains and contrast this to the highly nonlinear and plastic response of collagen upon mechanical load. We show that the mechanical nonlinear response of collagen can be gradually diminished by enriching the network stepwise with Matrigel. This tunability results from the suppression of collagen polymerization in the presence of Matrigel, resulting in a collagen network structure with significant smaller mesh size and consequent contribution to the mechanical response. Thus, the nonlinear plastic properties and structure of the ECM is not simply the addition of two independent network types but depends on the exact polymerization conditions. The understanding of this interplay is key toward an understanding of the dependencies of cellular interactions with their ECM and sheds light on the nonlinear cell-ECM interaction during organogenesis.

Controlling Non-Equilibrium Structure Formation on the Nanoscale.

Controlling the structure formation of gold nanoparticle aggregates is a promising approach towards novel applications in many fields, ranging from (bio)sensing to (bio)imaging to medical diagnostics and therapeutics. To steer structure formation, the DNA-DNA interactions of DNA strands that are coated on the surface of the particles have become a valuable tool to achieve precise control over the interparticle potentials. In equilibrium approaches, this technique is commonly used to study particle crystallization and ligand binding. However, regulating the structural growth processes from the nano- to the micro- and mesoscale remains elusive. Here, we show that the non-equilibrium structure formation of gold nanoparticles can be stirred in a binary heterocoagulation process to generate nanoparticle clusters of different sizes. The gold nanoparticles are coated with sticky single stranded DNA and mixed at different stoichiometries and sizes. This not only allows for structural control but also yields access to the optical properties of the nanoparticle suspensions. As a result, we were able to reliably control the kinetic structure formation process to produce cluster sizes between tens of nanometers up to micrometers. Consequently, the intricate optical properties of the gold nanoparticles could be utilized to control the maximum of the nanoparticle suspension extinction spectra between 525 nm and 600 nm.