Tissue growth constrained by extracellular matrix drives invagination during optic cup morphogenesis

Abstract

In the early embryo, the eyes form initially as relatively spherical optic vesicles (OVs) that protrude from both sides of the brain tube. Each OV grows until it contacts and adheres to the overlying surface ectoderm (SE) via an extracellular matrix (ECM) that is secreted by the SE and OV. The OV and SE then thicken and bend inward (invaginate) to create the optic cup (OC) and lens vesicle, respectively. While constriction of cell apices likely plays a role in SE invagination, the mechanisms that drive OV invagination are poorly understood. Here, we used experiments and computational modeling to explore the hypothesis that the ECM locally constrains the growing OV, forcing it to invaginate. In chick embryos, we examined the need for the ECM by (1) removing SE at different developmental stages and (2) exposing the embryo to collagenase. At relatively early stages of invagination (Hamburger–Hamilton stage HH14−), removing the SE caused the curvature of the OV to reverse as it ‘popped out’ and became convex, but the OV remained concave at later stages (HH15) and invaginated further during subsequent culture. Disrupting the ECM had a similar effect, with the OV popping out at early to mid-stages of invagination (HH14− to HH14+). These results suggest that the ECM is required for the early stages but not the late stages of OV invagination. Microindentation tests indicate that the matrix is considerably stiffer than the cellular OV, and a finite-element model consisting of a growing spherical OV attached to a relatively stiff layer of ECM reproduced the observed behavior, as well as measured temporal changes in OV curvature, wall thickness, and invagination depth reasonably well. Results from our study also suggest that the OV grows relatively uniformly, while the ECM is stiffer toward the center of the optic vesicle. These results are consistent with our matrix-constraint hypothesis, providing new insight into the mechanics of OC (early retina) morphogenesis.

Signaling and Gene Regulatory Networks in Mammalian Lens Development

“…While the majority of extracellular signaling plays positive roles in lens cell formation, the repressive mechanisms seem to be underepresented (Table 1), and various cross-talks between individual pathways remain poorly understood [131, 190, 236]. Other levels of unresolved complexities relate to the contradictory findings that optic cup formation requires ectoderm-derived signals [137], anopthalmia is caused by various mutations of genes either in the ectoderm or optic vesicle [3, 51, 54, 79, 82, 83, 237], and that optic cup formation proceeds without any rudimentary lens in retinal organoid cultures [125, 238]. These and other outstanding questions previously discussed will be resolved in the upcoming years.…”

“…This contact is established by the constraining effects of ECM [123] and the formation of F-actin-rich basal temporal filopodia acting as physical tethers [124]. This model was further extended by findings which showed that the ECM causes lens cells to thicken locally in order to facilitate subsequent invagination [99, 125], a process primarily driven by both apical constriction to produce wedge-shaped cells [122, 126] and by BMP signaling [127]. In the invaginating lens placode, the cells near the lens pit circumferentially contract the adherens junctional complexes joining them together, and this actomyosin cytoskeleton remodeling requires RhoA small GTPase, Rho-kinase (Rock), PDZ domain-containing protein Shroom3, and p120-catenin δ1 [122, 126].…”

BMP signaling controls buckling forces to modulate looping morphogenesis of the gut

“…intestinal looping | buckling | biomechanics | Bmp | morphogenesis D ifferential growth represents one of the core physical mechanisms driving morphogenesis throughout the vertebrate embryo (1)(2)(3)(4)(5)(6)(7)(8)(9). Investigations into the mechanics of differential growth have often illustrated key physical parameters (e.g., tissue stiffness, growth rates, and initial geometry) that determine the resultant tissue shape.…”

An energy landscape approach to understanding variety and robustness in tissue morphogenesis

“…This characteristic behavior of multicellular tissue would be modeled as a change in stress-free configuration. The second is a mechanical constraint, such as extracellular matrix surrounding a growing tissue, that determines the direction of invagination (Huang et al 2011;Oltean et al 2016). Further analysis is required to investigate the effects of mechanical constraints modeled as boundary conditions on tissue deformation.…”