Left-right (L-R) asymmetry is a very common feature of living systems, appearing in diverse forms such as the handedness of snail shells, the growth patterns of bacterial colonies, and even the looping of the developing heart. Although these structures exhibit clear left-right asymmetry at macroscopic scales, the physical mechanisms underlying its emergence are not yet fully understood. One hypothesis is that a weak microscopic chiral bias is amplified through collective interactions and mechanical processes to produce large-scale chiral structures. However, how such amplification occurs remains an open question.

In this talk, I will discuss our recent work on the spontaneous formation of large-scale chiral patterns in a population of migrating Dictyostelium cells. In experiments where cells are continuously supplied from a central source, we observe the formation of a rotating, high-density annulus and multi-armedspiral structures extending into the surrounding low-density region. To understand the emergence of these patterns, we develop theoretical and computational models of self-propelled particles with intrinsic cellular chirality, local alignment interactions, and continuous particle supply.By combining experimental observations, numerical simulations, and theoretical analysis, we identify an emergent mechanical (density) gradient that couples collective alignment with microscopic chirality, introducing a chiral bias into the emerging polar order and ultimately giving rise to macroscopic chiral organization. Our findings suggest a possible physical mechanism for chiral amplification and provide a framework for investigating the emergence of left–right asymmetry during biological development.