Blood Cell Differentiation from Hematopoietic Stem Cells (HSCs). HSCs normally divide to generate either more HSCs (self-renewal) or progenitor cells, which are precursors to various blood cell types. HSCs are found mainly in bone marrow, although T cells develop in thymus, and some other cell types develop from blood monocytes. Once HSCs partly differentiate into progenitor cells, further differentiation into one or a few types of blood cell is irreversible. Solid lines indicate known pathways; dash lines indicate pathways about which there is uncertainty.