There are numerous types of cells that make up the bone marrow, each with their own specific function.  I’m going to focus on the three most commonly affected by chemotherapy: neutrophils (white blood cells), erythrocytes (red blood cells, measured by hemoglobin or hematocrit) and platelets (blood clotting cells).  These cells are born primarily in the bone marrow and then eventually leave the bone marrow to circulate in the peripheral blood.  When your blood is drawn in the clinic, the number that comes back reflects the number of each particular cell in the peripheral blood.  Each of these cells starts out as an immature stem cell that has the potential to be any type of blood cell.  However, what type of cell the immature cell becomes depends on the type of growth factor that acts upon it.  For example, if the body senses it needs more red blood cells, it will release more erythropoietin which will tell more of the stem cells to become erythrocytes which will mature in red blood cells.  

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