The precise spatial arrangement and orientational alignment of cells are fundamental to the proper functioning of tissues and are established during embryonic development. However, in tissues comprised of multiple cell types, the interplay of genetic expression, biochemical signalling, and mechanical forces in shaping these patterns remains largely unexplored.

In the context of auditory epithelial development, our research reveals that the sensory hair cells and non-sensory supporting cells play a crucial role in the regulation of junction-type specific and asymmetric mechanical forces. Through a mechanical analysis of this tissue, tightly accompanied by experiments in the group of Prof. Raj Ladher, I uncover that variations in contractility amongst junctions create patterns of tissue-wide forces that are responsible for establishing both positional order and planar polarity. These forces arise from junctional contractility, driven by the localised enrichment of mono- and di-phosphorylated forms of the regulatory light chains of non-muscle myosin II.

Junction-type-specific forces underpin the orderly arrangement of cells within the tissue. These forces promote the precise intermixing of hair cells, where each hair cell is exclusively surrounded by supporting cells. In our theoretical description, the additional apical-basal asymmetry of this force is at the origin of a quasi-stratification of the tissue, marked by a significant increase in the apical surface area of hair cells. The combination of cell intermixing and expansion of the apical hair cell area leads to the emergence of hexatic order among hair cells, as well as proximal-distal gradients in the arrangement of hair cells and supporting cells.

Furthermore, planar polarity emerges as a consequence of the localised attenuation of these junctional forces by intrinsic asymmetries in hair cells. Our findings shed light on the pivotal role of junctional asymmetries in coupling positional order to planar polarity, revealing a potentially fundamental aspect of development and tissue repair in environments comprising diverse cell types.