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Hematopoiesis describes the pathway from a hemocytoblast, a cell that gives rise to blood cells, to each formed element.

Hematopoiesis is the process by which new blood cells are formed in the red bone marrow. Red bone marrow is the tissue found inside the ends of long bones that gives rise to new red blood cells. In a healthy adult approximately 1011 to 1012 new blood cells are produced daily in order to maintain steady conditions in blood flow.

Hematopoietic stem cells are often associated with newborn babies because the umbilical cord is filled with them, but they are also found in the blood and red bone marrow. Stem cells are important because they can develop into each formed element (solid portion of the blood, such as red blood cells, white blood cells, and platelets).

Stem cells also have the ability to develop into different types of mature blood cell types and tissues. Because they have this ability, stem cells can be used to treat diseases such as lymphoma, leukemia, and some immune disorders. All blood cells follow one of two lines of development: the myeloid line or the lymphoid line. Each of these lines starts from a stem cell or hematopoietic stem cell (hemocytoblast). The cells that develop on the lymphoid line are only lymphocytes. Lymphocytes are white blood cells, T cells and B cells that are responsible for immune response. Cells that develop on the myeloid line include all other types of blood cells. The cell types include thromboocytes (platelets), which help blood clot; basophils; neutrophils; eosinophils; monocytes, which are a type of white blood cell; and myeloblasts, which are immature blood cells.
The process of hematopoiesis begins with a hematopoietic stem cell splitting into lymphoid cells-white blood cells that are responsible for immune response-and myeloid cells. Myeloid cells become erythrocytes (red blood cells) and platelets (thrombocytes, which help blood clot), among other cell types.