Talk about Origin of Blood cells on general.

They are continuously destroyed because their life time is short in the blood and limited. So they reproduce using Hematopoiesis in order to maintain a stable blood count. Carried out in red bone marrow for adults present in cavities of spongy bone All originate from multipotent cells (MSCs AKA hemocytopblasts) which are small with reduced basophilic cytoplasm and a nucleus with one or more nuclei.