Matrix

A computer programmer and hacker named Neo realizes something is wrong with the world he lives in. He begins searching for the truth, and a man named Morpheus who knows it. Eventually Neo takes a red pill and wakes up to reality: His entire world was a simulation, and he and other humans are trapped in liquid-filled pods run by intelligent machines. Neo’s life, as portrayed by Keanu Reeves in the classic movie The Matrix, had unfolded in this artificial reality. It depends almost entirely on the environment around it.

Like Neo in his liquid-filled pod, cells in humans and other animals depend on their environment for survival. In animal tissue, including our own, cells are surrounded by a three-dimensional mesh of proteins and polysaccharides (carbohydrates) called the extracellular matrix. This mesh of macromolecules is so important that its most abundant component, a protein called collagen, makes up an astounding 30 percent by mass of all the protein in the human body.

The extracellular matrix serves as a structural support network for cells, and biologists used to think of it much like the cement in rebar—as a ubiquitous but inert material that did little more than position cells in space and provide physical support. But after investigating its biomechanics and molecular properties for a few decades, they’ve realized that it does much more.

The composition of proteins in the extracellular matrix varies in different tissues, and this confers distinct physical properties. For example, the protein elastin generates extra elasticity in skin, blood vessels, lungs. What’s more, changes in extracellular matrix protein composition can play a role in disease, as they do when the lung’s airways stiffen and become narrower in chronic obstructive pulmonary disease.

In addition, the extracellular matrix is in constant communication with the cells it houses, clueing the cells in to changes in their environment. And cells communicate right back, sometimes changing the protein composition of the matrix. In addition, during development and whenever tissues must grow or regenerate, matrix molecules work in a complex dance with small proteins called growth factors to tell cells when to multiply and how to develop into tissues. Biologists are still working out the details of this dance, and they’re critically important to tissue engineers trying to grow replacement biological tissues and organs. In fact, tissue engineers have spent an enormous amount of time creating artificial extracellular matrices called scaffolds, and seeding them with growth factors, all to get cells to knit together properly into lab-grown tissues and organs.

All this complexity is distinct in meaning from the matrices that students plug through to solve engineering problems in college. Those matrices are rectangular arrays of numbers or abstract quantities that can be added, multiplied or transformed. Matrix mathematics sits under the hood in MATLAB or computational fluid dynamics programs, helping scientists and engineers crunch a large volume of numbers. The results of those calculations can help predict how air flows over an airplane wing or turbine blade, the mechanical stresses on elements of a building or bridge, and much more.