The immune system originates with a primordial cell of the fetus known as the myeloid progenitor stem cell.
Stem cells are undifferentiated or partially differentiated cells that can differentiate into different types of cells and multiply indefinitely to produce more of the same stem cell in multicellular organisms. They are the ancestor of all cell types in a lineage. [1] Although they are present in both embryonic and adult organisms, they differ slightly in each. They are typically distinguished from precursor or blast cells, which are typically dedicated to developing into one cell type, and progenitor cells, which cannot divide endlessly.
In mammals, the inner cell mass is composed of approximately 50–150 cells and occurs during days 5–14 of the blastocyst stage of embryonic development. These can function as stem cells. They eventually differentiate in vivo into every form of cell in the body (making them pluripotent).
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