Tissue Formation and Cell Communication
Many tissues in our bodies begin as a mass of identical cells, but they soon differentiate into multiple cell-types in a perfectly organized fashion — orchestrated for the tissue to perform its function. For example, stem cells in the central nervous system become either neurons or supportive glial cells that surround them. The specific patterning of neurons and glial cells is necessary for the nervous system to efficiently send and receive electrical impulses.
We see patterns like this often in nature. Hair follicles are evenly spaced on skin, our spine contains even columns separated regularly by disks, there are hair-like structures in our intestines that help us absorb nutrients. Patterns are everywhere in our bodies, and it seems like most of development can be described by patterning; however, we don’t fully understand how patterns form.
Two mechanisms for pattern formation are often discussed among researchers: first, cells receive cues from their environment, and second, they communicate with each other. The first mechanism was originally proposed by the renowned mathematician Alan Turing, who discovered that when two chemicals called “morphogens” react and diffuse in space, their concentrations can form many of the stripe and spot patterns that we see in animals. He proposed that morphogens diffuse throughout extracellular space, forming a pattern of chemicals, which then tell the cells to differentiate according to which chemical surrounds them. Oftentimes, however, an entire tissue contains many undifferentiated cells that receive the same extracellular signal. In these situations, cellular communication plays a key role in forming the tissue’s pattern. A cell knows what cell-type to become based on which cells are in its proximity.
Neither of these mechanisms, nor the combination of the two is enough to describe the extreme complexity of our bodies; however, by gaining a better understanding of them we can glimpse into how certain organs develop, how their development may fail, and how we can predict and correct abnormal development.
Let’s now focus on cell-communication, which is known to play a key role in the development of the central nervous system, the skin, the retina, and many more bodily systems. Due to groundbreaking mathematical work done by Dr. Marty Golubitsky and Dr. Ian Stewart, if we know which cells are talking to each other, we can determine the possible patterns that can form. One obstacle to applying this theory is that we don’t always know which cells communicate. In many cases, cells only communicate with their neighbors, but sometimes cells have arm-like protrusions called filopodia that allow them to communicate over long distances. Another obstacle is that the theory only tells us possible patterns, but it can be difficult to infer which pattern is realized in a biological system.
The realized pattern often depends on the complex interactions of chemicals within cells and signals between cells—that is, the chemical signaling pathway. By characterizing properties of chemical signaling pathways that cause one tissue pattern to form despite other possibilities, we can elucidate how patterning may fail and how it can be corrected at a molecular level, leading to potential drug targets. We may also gain a better understanding of chemical signaling pathways as certain of observed patterns in tissues are only possible provided that the chemical signaling pathway has certain properties.