Stem cell diagram illustrates a human fetus st...

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The University of Utah has an excellent description of how our cells communicate both within and between each other. This communication starts with a stem cell and causes each cell to modify itself into a type of cell (e.g. blood, skin, liver, heart). Once a cell becomes a type like our nervous system all later communication and changes are within the constraints of those already made. Nervous system cells don’t communicate like blood cells. Our heart cells don’t suddenly become a cuticle cell and start growing fingernails. These epigenetic switches become permanent and influence the remaining life of the cell and its ancestors. Other epigenetic chatter changes a cell dynamically and can be adjusted to react to environmental changes. Hormones get released by one part of the body and can influence cells in another part. This chatter makes us who we are. It causes us first to be mammals, then humans, and then a unique individual.