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Every multicellular organism begins as one cell, which contains all of the intricate instructions to synthesize, organize, and regulate not only this cell but the development and maintenance of all cells that will inevitably comprise the organism. All of these instructions are encoded in the first cell's DNA. This underscores the complexity of the genome and how each cell's expression must be controlled in specific ways depending on its function. The cells hailing from each tissue in the human body (e.g., muscle, lung, heart, liver) harbor a unique epigenetic signature, which enables the maintenance of tissue-specific functions through the control of gene regulation, as just discussed.
"Our knowledge of the total number of unique cells, or cell types, is still growing. Previous estimates put the number of unique cell types in the human body at ~300, but new estimates from the Human Cell Atlas have shown that we may have thousands of cell types and subtypes, each harboring a unique function for a specific physiological state or response to stimuli. But even cells of the same cell type will not be identical. A cell's 'presentation' of molecules on their surface can radically change depending on internal variables such as genetic mutations or altered states of their epigenome, transcriptome, and proteome, as well as external stimuli including drugs and interactions with other cells. This novel presentation is most pronounced with a neoantigen, when a cancer cell creates an entirely new molecule on the surface of a cell. Given its unique presentation, which wouldn't be found in normal cells, this offers a unique target for safer cancer therapies.
"The human body has about 30 trillion human cells plus another 30-40 trillion bacterial cells, for a total of about 70 trillion cells. If your body were a democracy, the human cells would often be the minority or equal party. You (as a human) would never win an election. Your loss of control would likely result in you rolling around in the soil or lying in a bathtub full of yogurt, which I do sometimes on Sundays. Regardless of how you spend your Sundays, there are a lot of microbes in, on, and around your body. There are in fact so many microbes that they compose the bulk of the cells on Earth. This is a humbling and exciting statistic, and one which is vividly apparent for anyone who has ever had explosive diarrhea.
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Christopher E. Mason (The Next 500 Years: Engineering Life to Reach New Worlds)