  Robust self-organization of subcellular structures is a key principle
governing the dynamics and evolution of cellular life. In fission yeast cells
undergoing division, the mitotic spindle spontaneously emerges from the
interaction of microtubules, motor proteins and the confining cell walls, and
asters and vortices have been observed to self-assemble in quasi-two
dimensional microtubule-kinesin assays. Their is no clear microscopic picture
of the role of the active motors driving this pattern formation, and the
relevance of continuum modeling to filament-scale structures remains uncertain.
Here we present results of numerical simulations of a discrete filament-motor
protein model confined to a pressurised cylindrical box. Stable spindles,
nematic configurations, asters and high-density semi-asters spontaneously
emerge, the latter pair having also been observed in cytosol confined within
emulsion droplets. State diagrams are presented delineating each stationary
state as the pressure, motor speed and motor density are varied. We further
highlight a parameter regime where vortices form exhibiting collective rotation
of all filaments, but have a finite life-time before contracting to a
semi-aster. Quantifying the distribution of life-times suggests this
contraction is a Poisson process. Equivalent systems with fixed volume exhibit
persistent vortices with stochastic switching in the direction of rotation,
with switching times obeying similar statistics to contraction times in
pressurised systems. Furthermore, we show that increasing the detachment rate
of motors from filament plus-ends can both destroy vortices and turn some
asters into vortices. Based on our findings we argue the need for a deeper
understanding of the microscopic activities underpinning macroscopic
self-organization in active gels and urge further experiments to help bridge
these lengths.
