This image shows a more magnified view of red bone marrow.  Red marrow contains a type of stem cell called a hemocytoblast from which all formed elements originate.  Some hemocytoblasts differentiate into myeloid stem cells from which erythrocytes, granulocytes, and monocytes will develop while other hemocytoblasts differentiate into lymphoid stem cells from which lymphocytes will develop. Lastly, some hemocytoblasts differentiate into giant cells called megakaryocytes which then fragment in the red marrow to produce platelets. The development of each cell type is regulated by specific growth factors.