Abstract
Purpose :
During early lens development, a thickened sheet of epithelial cells invaginates inward to eventually form a roughly spherical vesicle. While apical constriction and localized placodal growth have been hypothesized to drive invagination, neither contribution appears wholly sufficient to explain how this process occurs. Because cells centripetally move toward the center of the lens placode at the onset of its formation and during invagination, it is hypothesized that these movements drive epithelial bending.
Methods :
To investigate the mechanisms that drive this process, fluorescent microscopy was employed on live chicken embryos. Incubation with a fluorescent dye that labels F-actin enabled the visualization of apical junctions and permitted the observation of several epithelial cell behaviors in real-time. Additionally, treatment of chick embryos with pharmacological inhibitors and mechanical injury protocols as well as the analysis of knock-out mouse embryos were utilized.
Results :
Similar to convergent extension-like movements in elongating tissues, the peripheral-most lens placodal cells form epithelial rosettes such that circumferentially orientated junctions contract to a point, then resolve in a perpendicular direction. This causes cells to move centripetally, elongating the tissue toward the placodal center, which is thought to cause invagination through epithelial buckling. Further investigation into circumferential junctional contraction led to the observation that it occurs along arcs consisting of several cells with aligned circumferential junctions near the lens placodal border. F-actin and non-muscle myosin labeling are more intense in these aligned junctions and their contraction is Rho-kinase and non-muscle myosin dependent. Placodal injury and fate mapping experiments further suggest that the formation of circumferentially orientated actomyosin arcs is dependent on both the differentiation status of placodal cells and their mechanical environment.
Conclusions :
Together, these results have provided novel details into the mechanisms that drive lens placodal invagination. The formation, alignment, and contraction of circumferentially orientated bicellular junctions are thought to be a critical factor in driving epithelial invagination.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.