How is pancytopenia developed during aplastic anemia?

Production stops: The body continuously recycles & replaces different cells in the blood. Red cells last ~100days, platelets several hours, other cells vary. The healthy system acheives a steady state of producing cells as fast as they break down. If an event such as a toxic exposure (chloramphenacol) or tumor invasion shuts down the "factory", no new material will be made & you get low on everything.

Aplastic anemia: Aplastic anemia is a disease in which the bone marrow, and the stem cells that reside there, are damaged. This causes a deficiency of all three blood cell types: red blood cells (anemia), white blood cells (leukopenia), and platelets (thrombocytopenia).