Christopher Mason in Delancey Place:
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.
More here.