  During the early developmental process of organisms, the formation of the
left-right laterality requires a subtle mechanism, as it is associated with
other principal body axes. Any inherent chiral feature in an egg cell can in
principal trigger this spontaneous breaking of chiral symmetry. Individual
microtubules, major cytoskeletal filaments, are known as chiral objects.
However, to date there lacks convincing evidence of a hierarchical connection
of the molecular nature of microtubules to large-scale chirality, particularly
at the length scale of an entire cell. Here we assemble an in-vitro active
layer, consisting of microtubules and kinesin motor proteins, on a glass
surface. Upon inclusion of methyl cellulose, the layered system exhibits a
long-range active nematic phase, characterized by the global alignment of
gliding MTs. This nematic order spans over the entire system size in the
millimeter range and, remarkably, allows hidden collective chirality to emerge
as counterclockwise global rotation of the active MT layer. The analysis based
on our theoretical model suggests that the emerging global nematic order
results from the local alignment of MTs, stabilized by methyl cellulose. It
also suggests that the global rotation arises from the MTs' intrinsic
curvature, leading to preferential handedness. Given its flexibility, this
layered in-vitro cytoskeletal system enables the study of membranous protein
behavior responsible for important cellular developmental processes.
