HEMATOLOGY



They contain primary lysosomal ZSuIes and give rise to neutrophilic eosino­philic or basophilic myelocytes, which display the specific granules that identify each myeloid subtype. Myelocytes form the most abundant type of myeloid “cell in the marrow and represent the last proliferating cell pool; their descendants, metamyelocytes, bands, and neutrophils (or eosinophils or basophils) are incapable of cell division but show increasing functional activity concomitant with maturation of membrane functions: chemotaxis, phagocytosis, bactericidal action. After extrusion from the bone marrow, granulocytes occupy two geographic pools in free equilibrium: a marginated pool of cells adhering to blood vessel walls and a circulating pool of roughly equal size. Once in the peripheral blood, granulocytes, like erythrocytes, do not normally re-enter the bone marrow. They follow a “one-way traffic” pattern, spending a relatively short time in the peripheral blood (average, 6 hours) and exiting into the tissues.
The megakaryocyte, the only giant, multinu­cleated cell in the hematopoietic lineage, also de­rives from the primitive stem cell, and undergoes endomitosis, reaching DNA contents of 32 to 64 n. Specific surface markers characteristic of ma­ture platelets appear at very early stages in mega­karyocyte differentiation and persist through maturation. Megakaryocytes tend to stay near marrow sinusoids, where they mature, developing internal membranes that mark off platelet fields. Each megakaryocyte eventually breaks up, releas­ing some 5000 platelets at the edge of a marrow sinusoid into the blood. Some megakaryocytes circulate and may release their platelets in the lung. Although platelets are anucleate fragments of megakaryocytes, they possess a highly organ­ized structure and glycogen energy stores (and some mitochondria) and circulate for 7 to 10 days.
Marrow lymphocytes are mainly of B cell ori­gin, arising also in spleen and lymph nodes. Prim­itive T cell precursors arise in the marrow, travel to the thymus, where they undergo further dif­ferentiation, and thence to spleen, lymph nodes, or marrow as fully functional mature T cells. Lym­phocytes are generally much longer-lived than other marrow cells and may survive for years.
Bone marrow functions include hematopoiesis, the differentiation of antibody-producing plasma cells, and the monitoring of hematopoietic cell quality. Normal bone marrow has an 8- to 10-fold capacity to increase blood cell production in response to humoral signals, which include erythropoietin, colony-stimulating factors, and prostaglandins. Normal bone marrow function depends upon intact marrow architecture, the availability of key nutrients including iron, folic acid, and vitamin B12> and regulatory hormones.
Two types of injury can induce marrow failure: damage that affects the capacity for stem cells to undergo differentiation (e.g., in aplastic anemia) and damage that alters the marrow microenviron-ment (e.g., invasion of marrow by infection or tumor).





HEMATOLOGY