&Bullet; physics 14, p81
Cells often contain droplets that are involved in various cell functions. The environments in which these droplets form are complicated – the cell nucleus, for example, is filled with a complex molecular structure called chromatin, which, among other things, can affect the merging of droplets. Now, Yaojun Zhang from Princeton University and colleagues have proposed a description of how chromatin influences droplet formation in cells  . The result underlines the crucial role of the environment in the formation of cellular structures.
In their experiments, the team created liquid droplets of different sizes in the nucleus of a living cell. Conventional theoretical understanding of this situation, where droplets of one immiscible liquid are dispersed in another, predicts that the larger droplets should swallow the smaller ones and grow quickly. But Zhang and colleagues found that their droplets barely grew. They hypothesized that the presence of long, fibrous structures – the chromatin – could inhibit the expected droplet growth. Previous work had suggested that polymer networks can affect droplet formation in other scenarios.
The team then ran molecular dynamics simulations to get a more detailed picture of how networks of cross-linked chromatin might affect droplet formation and growth. They found that cross-linked chromatin can act as a mechanical growth barrier, with the fiber network pushing against droplets that become large enough to stretch it, which limits the droplet size. The team suggests that this effect could be responsible for suppressing the growth of large droplets that they observed in experiments. Taking into account factors such as pressure and chemical potential, the team found that the mechanical effects of chromatin could also affect the number, size, and placement of the droplets.
Today, physics published another cell story that highlights theoretical predictions showing that the number of molecular components in a cell directly determines the number of types of droplets that can form (Synopsis: Predicting the Population of Self-Assembling Droplets in Cells).
–Erika K. Carlson
Erika K. Carlson is Corresponding Editor for physics based in New York City.
- Y. Zhang et al., “Mechanical frustration of phase separation in the cell nucleus due to chromatin”, Phys. Rev. Lett.126, 258102 (2021).