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Myeloid tissue, in thebone marrow sense of the wordmyeloid (myelo- +-oid), istissue of bone marrow, of bone marrowcell lineage, or resembling bone marrow, andmyelogenous tissue (myelo- +-genous) is any tissue of, or arising from, bone marrow; in these senses the terms are usually used synonymously, as for example withchronic myeloid/myelogenous leukemia.
Inhematopoiesis,myeloid cells, or myelogenous cells areblood cells that arise from aprogenitor cell forgranulocytes,monocytes,erythrocytes, orplatelets[1][2] (thecommon myeloid progenitor, that is, CMP orCFU-GEMM), or in a narrower sense also often used, specifically from the lineage of themyeloblast (themyelocytes,monocytes, and their daughter types). Thus, although all blood cells, evenlymphocytes, are normally born in the bone marrow in adults, myeloid cells in the narrowest sense of the term can be distinguished fromlymphoid cells, that is, lymphocytes, which come from common lymphoid progenitor cells that give rise toB cells andT cells.[2] Those cells'differentiation (that is,lymphopoiesis) is not complete until they migrate tolymphatic organs such as thespleen andthymus for programming byantigen challenge. Thus, amongleukocytes, the termmyeloid is associated with theinnate immune system, in contrast tolymphoid, which is associated with theadaptive immune system. Similarly,myelogenous usually refers to nonlymphocytic white blood cells,[3] anderythroid can often be used to distinguish "erythrocyte-related" from that sense ofmyeloid and fromlymphoid.[4]
The wordmyelopoiesis has several senses in a way that parallels those ofmyeloid, and myelopoiesis in the narrower sense is the regulated formation specifically of myeloid leukocytes (myelocytes), allowing that sense ofmyelopoiesis to be contradistinguished fromerythropoiesis and lymphopoiesis (even though all blood cells are normally produced in the marrow in adults).
Myeloidneoplasms always concern bone marrow cell lineage and are related tohematopoietic cells. Myeloid tissue can also be present in theliver andspleen[5] infetuses, and sometimes even in adults as well, which leads toextramedullary hematopoiesis.
There is one other sense ofmyeloid that means "pertaining to thespinal cord", but it is much less commonly used.Myeloid should not be confused withmyelin, referring to an insulating layer covering theaxons of manyneurons.
