Seeing tissue as a ‘phase of matter’: exploring statistical mechanics for the cell

Local and global dynamics in collective movements of embryonic cells

Several important morphogenetic processes belong to the category of collective cell movements (CCM), by which we mean coordinated rearrangements of many neighboring cells. The causes of the dynamic order established during CCM are still unclear. We performed statistical studies of rates and angular orientations of cell rearrangements in two kinds of embryonic tissues, which we categorized as "committed" (in the sense of being capable of autonomous CCM) as opposed to "naïve" tissues, which are those that require external forces in order to exhibit full scale CCM. In addition, we distinguished two types of cell rearrangements: first, those in which mutual cell-cell shifts characterizing the local dynamics (LD); and, second, those which moved in reference to common external coordinates (global dynamics, GD). We observed that in most cases LD rates deviated from normal distributions and do so by creating excesses of extensively converging and moderately diverging cells. In contrast, GD was characterized by nearly random behavior of slowly moving cells, combined with increased angular focusing of the fast cells trajectories as well as bimodal distribution of cell rates. When committed tissues were opposed by external mechanical forces, then they tended to preserve the inherent CCM patterns. On the other hand, the naïve ones reacted by creating two orthogonal cells flows, one of these coinciding with the force direction. We consider CCM as a self-organizing process based on feedbacks between converging and diverging cell shifts, which is able to focus the trajectories imposed by external forces.

Vascular networks due to dynamically arrested crystalline ordering of elongated cells

Recent experimental and theoretical studies suggest that crystallization and glass-like solidification are useful analogies for understanding cell ordering in confluent biological tissues. It remains unexplored how cellular ordering contributes to pattern formation during morphogenesis. With a computational model we show that a system of elongated, cohering biological cells can get dynamically arrested in a network pattern. Our model provides an explanation for the formation of cellular networks in culture systems that exclude intercellular interaction via chemotaxis or mechanical traction.