Hematopoietic stem cells (HSCs) develop from hemangioblasts within the mesoderm. During hematopoiesis, HSCs respond to a wide array of extracellular signals and differentiate into cells of the erythroid, lymphoid, and myeloid lineages. R&D Systems offers tools to study changes in the expression of molecules that are considered markers of differentiated blood cell lineages. For example, human HSCs are characterized by high levels of CD34 expression. Cells that differentiate into lymphocytes, such as B Cells and T Cells, can be identified by detecting increased expression MS4A1/CD20 and CD4, respectively. Antibodies are also available to monitor the differentiation of HSCs into myeloid cells such as granulocytes, macrophages, megakaryocytes, and platelets.