Quantitative approaches to uncover physical mechanisms of tissue morphogenesis

Abstract

Morphogenesis, the creation of tissue and organ architecture, is a series of complex and dynamic processes driven by genetic programs, microenvironmental cues, and intercellular interactions. Elucidating the physical mechanisms that generate tissue form is key to understanding development, disease, and the strategies needed for regenerative therapies. Advancements in imaging technologies, genetic recombination techniques, laser ablation, and microfabricated tissue models have enabled quantitative descriptions of the cellular motions and tissue deformations and stresses with unprecedented temporal and spatial resolution. Using these data synergistically with increasingly more sophisticated physical, mathematical, and computational models will unveil the physical mechanisms that drive morphogenesis.

Figure 1

Quantitative mapping of morphodynamics. Example strategies for labeling and tracking individual cells include using (A) membrane localized fluorophores [34], (B) chimeric embryos to track distinct cell populations [18], and (C) Cre/lox recombination techniques to track large numbers of individual cells [19]. Reconstruction of OPT imaging data produces (D) 3D surface renderings of morphology that can be used to compare initial limb bud morphologies (t₀) with those at later stages (t₁) [41]. Quantitative image processing reveals cellular rearrangements such as (E) rosette formation during kidney tube elongation [34] and (F) the branch morphology and lineage tree for ramified architectures such as the developing lung airways [42]. Images adapted with permission.

Figure 2

Elucidating the forces that drive morphogenesis. (A) The overall wound geometry and the dynamics of the displacement of sub-cellular structures following laser ablation can be used to determine the nature and anisotropy of the stress field within a tissue, as shown with the displacement of GFP-tagged myosin networks in the ventral furrow of the Drosophila embryo [47]. Culture models are used to reduce the complexity of the microenvironment and provide more quantitative approaches to determine cellular and tissue-level forces. (B) Monolayer stress microscopy is used to define the stresses within a monolayer of cells collectively migrating on a polyacrylamide gel [55]. Similarly, micromolding techniques and 3D traction force microscopy can be combined to (C) calculate the interfacial traction forces of an epithelial tissue of (D, E) various geometries embedded in a collagen gel [60]. Images adapted with permission.

Figure 3

(A) Physical, mathematical, and computational models work synergistically with experiments that measure cell and tissue mechanics to reveal physical mechanisms of morphogenesis and guide additional experiments. (B) Determining the mechanics of head-fold formation in the chicken embryo is one example of studies that integrate experimental data and computational models [16]. A finite element model was constructed with various cellular behaviors imposed on distinct tissue zones. (C) The imposed forces produce shape changes in the simulated tissue that mimic those seen in the embryo. (D) The computational model enables in silico hypothesis testing to determine the relative and combined roles for various cellular behaviors in the tissue deformations observed. Once a set of cellular behaviors that produce the overall tissue morphology are identified, the numerical results are further compared with experiments to verify that these physical mechanisms uniquely generate the observe tissue deformations. An example of one such verification is the comparison of the 2D Lagrangian transverse (E_xx) longitudinal (E_yy) and shear strain (E_xy) contours calculated from the head fold model and measured in the embryo. NP; neural plate. Images adapted with permission.