Cell growth: stress and proliferation are related!!!

Recently, I have spent countless sleepless nights thinking to myself, how can I understand epithelial monolayer mechanics better so I can help the sick and the hungry of the world? Well, thanks to Terry and BE115, I think I've found an article that'll help me end my Pursuit of Sleepyness.

Title: Emergent patterns of growth controlled by multicellular form and mechanics
By: Celeste M. Nelson, and others. (U. Penn)
Link to the article

So in BE115, we've been culturing lots of Fibroblast cells. We do bunch of stuff to them and we finally seed them to a piece of plastic, and a week later, BOOM, There they are, gloriously filling up the whole dish. But our observation stops there. We kind of wave our hands in the air and convince ourselves that because of contact-inhibition, there are no more proliferation that'll interest us. And rightly so! With sunny beach few miles away and snowy slopes and fine wine country only a few hours away, who has time to think about these things? Well, Apparently, people in Pennsylvania do. It's winter, it's snowed up to your hip and it's so cold. If you spit, it'll freeze and shatter by the time it got to the ground. So what do these good Pennsylvanians do? They sit in their toasty labs and do research on multicellular form and mechanics. Heaven bless Pennsylvania!

Here's a summary of their work.

Tissue structures can influence patterns of proliferation, and in return, it can influence tissue structures. I.e. there's a kind of feedback mechanism. Traditionally, scientists have thought that most important aspects of tissue structural fates lie with soluble growth factors call morphogens. They are distributed asymmetrically in our body, and these in turn can influence various important bodily changes such as tissue segregation, gastrulation, epithelial folding, tubulogenesis, and differential cell growths.

Apparently, though, physical stuffs like shape and stress of the tissue can influence how these changes come about as well.

To test this, they made several microfabricated wells of different shapes and sizes:

1) A square well
2) A rectangular well
3) A circular well
4) A pyramidal mesh

They seeded these with bovine pulmonary artery endothelial cells (which is apparently contact-inhibited), and they tried to measure proliferative rates. (They have markers that fluoresce during DNA synthesis.) And they found that proliferation is greatest at the (see figure 1):

1) Corners. (for square and rectangular wells)
2) Short edges. (for rectangular wells)
3) Perimeter (for square, rectangular, and circular wells)
4) Valleys (for pyramidal mesh)

These are all regions of high tractional stress. But just for completeness sake, they made some FEA models to predict that levels of local tractional stresses and proliferation match. And apparently they do, very well! (See figure 2)

They tried to test their theory that stress is related to (or even causes) proliferation with several other non-computational methods. First, they used elastomeric force sensor arrays (those microfabricated devices with small poles everywhere) and found that, indeed, locations of high stress coincided with regions of high proliferation. (figure 3 B, D)

They also found that when you increase or decrease the internal stress by varying the composition of cytoskeletal components with pharmacological factors, proliferation increased and decreased as expected. (Figure 3, F, H)

Overall, I think this was an excellent paper demonstrating how external factors such as stress and shape can influence the growth of a colony of cells. This is of course very important, since being able to understand and replicate tissue genesis for organ replacement can significantly better everyone's standard of living.

And off to sleep I go.

See you everyone.

David.