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Myeloid stem cells—these make RBCs, myeloid cells (granulocytes, eosinophils, basophils, monocytes), and megakaryocytes (that make platelets).
As mentioned, there are growth factors that play a role in the development of these cells. For RBCs, the main growth factor is erythropoietin or EPO. This is a hormone secreted by fibroblast cells in the kidneys when they sense low oxygen tension. EPO goes to the bone marrow and triggers the division of myeloid stem cells to make cells along the RBC or “erythrocyte” line. This hormone is used in many medical situations that lead to reduced RBC counts in the body. Another growth factor is thrombopoietin, which is a glycoprotein hormone that makes megakaryocytes/platelets. It is produced by the kidneys and liver. Cytokines are made by many cells of the body and can act locally to stimulate the proliferation of cells to make more WBCs. The two subtypes of cytokines include colony-stimulating factors and interleukins. Colony-stimulating factors act completely locally to trigger myeloblasts to make neutrophils, basophils, and eosinophils. These are called “granulocyte CSFs.” They are used medically for cancer patients to increase the WBC count. Interleukins are signaling molecules made by several cell types to encourage the maturation of cells during inflammation and the immune response. There are many different interleukin types.
ERYTHROCYTES The erythrocyte, or RBC, is the most common formed element in the blood. These cells make up one-fourth of the total number of cells in the body. These are very small cells because they need to squeeze through tiny capillaries to give oxygen to the tissues. They pick up nearly all the oxygen in the lungs, carrying it to tissues; they only pick up 24 percent of the carbon dioxide, however. They do not leave the vessels (unlike WBCs, which extravasate out of the vessels). Erythrocytes mature in the bone marrow and lose their nucleus in the process. Immature RBCs are called reticulocytes and account for 1-2 percent of the total RBC count. The cells cannot use cellular respiration because they don’t have mitochondria and utilize anaerobic respiration in order to function. They have a unique structure, called the biconcave disc, which defines these cells. The main protein in the cell is oxygen-carrying hemoglobin. The shape gives it a greater 227
