Morphogenesis from cells to tissue gives rise to the complex architectures that make our organs. How cells and their dynamic behavior are translated into functional spatial patterns is only starting to be understood. Recent advances in quantitative imaging revealed that, although highly heterogeneous, cellular behaviors make reproducible tissue patterns. Emerging evidence suggests that mechanisms of cellular coordination, intrinsic variability and plasticity are critical for robust pattern formation. While pattern development shows a high level of fidelity, tissue organization has undergone drastic changes throughout the course of evolution. In addition, alterations in cell behavior, if unregulated, can cause developmental malformations that disrupt function. Therefore, comparative studies of different species and of disease models offer a powerful approach for understanding how novel spatial configurations arise from variations in cell behavior and the fundamentals of successful pattern formation. In this chapter, I dive into the development of the vertebrate nervous system to explore efforts to dissect pattern formation beyond molecules, the emerging core principles and open questions.

Coordinated cell re-organization is critical to ensure correct tissue morphogenesis for a number of important embryonic and tissue repair events, however the mechanisms that govern cells coordination during collective movements, particularly in situations where cells are spatially restricted by their neighbours, are not well understood. Here we assessed cell re-organization in monolayers of retinal epithelial cells (ARPE-19) to determine if cells that coordinate with their neighbours exhibit differential migration properties to non-coordinating cells and participate differently in local cell re-organization of the tissue sheet. From global tracking analysis, we determined that the movement profiles of cells were indistinguishable regardless of whether or not they were a part of multicellular streams. Using high magnification live imaging of cell membranes, we also characterized the localized geometry and organization of a monolayer (cell area, number of nearest neighbours, aspect ratio, internal cell angles) during cell re-organization in both streaming and non-streaming regions. Consistent with our global migration analysis, we observed no differences in cell sheet geometry and organization in streaming versus non-streaming regions. We did however observe that cells executed T1-like transitions to exchange position within the space-limited monolayer and that exchange events consistently involved at least one non-streaming cell. Our data suggests a model in which cell movement within the sheet is limited by neighbour exchange events and likely cells transition between streaming and non-streaming regimes to facilitate these neighbour exchange events while maintaining the integrity of the sheet.