Leukopoiesis

Leukopoiesis is a form of hematopoiesis in which white blood cells (WBC, or leukocytes) are formed in bone marrow located in bones in adults and hematopoietic organs in the fetus. White blood cells, indeed all blood cells, are formed from the differentiation of pluripotent hematopoietic stem cells which give rise to several cell lines with more limited differentiation potential. These immediate cell lines, or colonies, are progenitors of red blood cells (erythrocytes), platelets (megakaryocytes), and the two main groups of WBCs, myelocytes and lymphocytes.

Myeloid stem cell products

The myeloid progenitor can differentiate in the bone marrow into granulocytes, macrophages (mature monocytes), mast cells (whose blood-borne progenitor is not well defined), and dendritic cells of the innate immune system. The granulocytes, also called polymorphonuclear leukocytes because of their oddly shaped nuclei, give rise to three short lived cell types including eosinophils, basophils, and neutrophils. A granulocyte differentiates into a distinct cell type by a process called granulopoiesis. In this process it first transforms from a common myeloblast (myeloid progenitor) to a common promyelocyte. This promyelocyte gives rise to a unique myelocyte that for the first time can be classified as a eosinophil, basophil, or neutrophil progenitor based on the histological staining affinity (eosinophilic, basophilic, or neutral granules). [Junqueira, Carneiro. Basic Histology, Text and Atlas.McGraw-Hill Companies. 2005. ISBN 978-0-07-144116-2] The unique myelocyte next differentiates into a metamyelocyte and then a band cell, with a "C" shaped nucleus, before becoming a mature eosinophil, basophil, or neutrophil. Macrophages come from monoblast progenitors that differentriate into promonocytes, which mature into monocytes. Monocytes eventually enter the tissues and become macrophages.

Lymphoid stem cell products

The common lymphoid progenitor differentiates into lymphocytes by first becoming a lymphoblast. It then divides several more times to become a prolymphocyte that has specific cell-surface antigens unique to a T cell or B cell. The common lymphoid progenitor can also differentiate into natural killer cells (NK) and dendritic cells (which are indistinguishable from those derived from myeloid progenitor, however, since there are more common myeloid progenitors than there are common lymphoid progenitors, the majority of the dentritic cells in the body develop from common myeloid progenitors). [Janeway,et al. Immuno Biology, The Immune System in Health and Science.Garland Science Publishing, New York, NY. 2005. ISBN 0-8153-4101-6] T and B lymphocytes are indistinguishable histologically but are distinguishable in the location of their differentiation. Indeed, the inactive B and T cells are so featureless with few cytoplasmic organelles and mostly inactive chromatin that until the 1960's textbooks could describe these cells, now the central focus of immunology, as having no known function. [Janeway,et al. Immuno Biology, The Immune System in Health and Science.Garland Science Publishing, New York, NY. 2005. ISBN 0-8153-4101-6] T cells are formed in bone marrow then migrate to the cortex of the thymus to undergo maturation in an antigen-free environment for about one week where a mere 2-4% of the T cells succeed. The remaining 96-98% of T cells die by apoptosis and are phagocytosed by macrophages in the thymus. So many thymocytes (T cells) die during the maturation process because there is intensive screening to make sure each thymocyte has the ability to recognize self-peptide:self-MHC complex (major histocompatability complex) and for self tolerance. Upon maturity, there are several forms of thymocytes including T helper (needed for activation of other cells such as B cells and macrophages), T cytotoxic (which kill virally infected cells), T memory (remember antigens previously encountered), and (arguably) T suppressor cells (which moderate the immune response of other leukocytes). B cells are formed and mature in bone marrow. These cells then leave the bone marrow and migrate to peripheral lymphoid tissues, such as a lymph node. Once in a secondary lymphoid organ the B cell can be introduced to an antigen that it is able to recognize. Through this antigen recognition and other cell interactions it becomes activated and becomes a plasma cell. This B cell end product is an antibody secreting cell that helps protect the body. NK cells, which lack antigen specific receptors, develop in the bone marrow and circulate in the blood until they recognize and kill abnormal cells such as cancer or virally infected cells.

External links

*
*

References