How Your Cells Decide Whether to Be Part of the Brain, or the Colon

Your body has trillions of cells in it, including web-like neurons in our brains that help us think, bundles of muscle cells that power our movements, and electricity-conducting cells in our heart that help pump blood around. But, if we rewind far back enough, they started out from the same thing, but how?

Just like how people experiment with different trends, and explore different types of music when they’re young, our cells also go through a bit of an identity crisis, our cells are deciding whether they want to be a skin or heart cell. At one point everyone was just a tiny single cell, called a zygote or the fertilized egg. The cell, which is about as wide as a strand of hair, then divided into a few more identical cells, and eventually some of these cells began to deviate from their neighbours and take on new identities.

A new study probed deeper into individual cells in mouse embryos juuust as those first few differences start happening. At this point the activity in the cells is noisy as they decide what type to be. This decision point is not super clean, it’s deemed “noisy,” but this noise isn’t actual sound. It comes in the form of excess and seemingly random transcription of DNA. If you think of DNA as the set of instructions - like a lesson plan your teacher writes on the board - each cell only copies out the instructions that are specifically relevant to its functions.

For example, a light sensing cell in the eye would copy parts of DNA that instructs it how to make proteins that can sense light, whereas a blood cell wouldn’t need those genes. Normally each cell type has a characteristic pattern of gene expression, but during the noisy time all the cells are filled with random bits of RNA, like a big mixer. Then, they settle down and pick just the genes they’ll end up needing.