The process of morphogenesis in C. elegans embryos is largely driven by epidermal cells. Unlike in Drosophila and zebrafish embryonic development, no cell division or cell rearrangement is involved in C. elegans morphogenesis. Cell shape changes within the epidermis, which is characterized by junction lengthening along the anterior/posterior direction, play a key role in this process. The nature and the mechanical forces stimulating junction lengthening, as well as the cellular mechanisms involved in their lengthening are not understood.
Our lab observed that junction elongation along anterior/posterior (A/P) direction increases after muscle becomes active, and fails in muscle defective embryos. To better understand which role muscles play in driving epidermal cell junction lengthening during embryo elongation, we examined the global and local movement patterns using Single Plane Illumination Microcopy (SPIM), focusing on epidermal adherens junctions and muscle nuclei. We analyzed so far four wild-type embryos, five muscle defective embryos and twelve Rho-kinase mutant embryos. We found that wild-type embryos rotated strongly soon after muscle became active, and equally frequently to an outward or inward direction. However, muscle defective and Rho-kinase mutant embryos, which stop elongation at the 2-fold stage, scarcely rotated, suggesting that rotations are important for embryo elongation. Using cross-correlation analysis to compare the perimeter and area changes of each seam cell, we observed that the head, body and tail mechanically behaved as partially independent entities. This local pattern was lost in muscle defective and Rho-kinase mutant embryos. We next sought to understand how such movements could account for the polarized extension of junctions along the A/P direction, keeping in mind that C. elegans embryos are radially symmetric. By measuring the distance between two dorsal or ventral muscle nuclei, respectively, we found that muscles on both sides generally contract alternatively, accounting for the rotation movement. Analysis of seam cell roughness and aspect ratio showed that junctions were stretched along the A/P direction when seam cells were positioned outwards during rotation. This asymmetric muscle activity defines the source of polarity in C. elegans embryo and provides the local driving force of epidermis stretching. Furthermore, we propose that C. elegans embryos extend in a ratchet mode due to the alternate pattern of muscle contractions.