Hematology is the study of the normal and pathologic aspects of blood and blood elements. Blood is a unique fluid compromised of many cellular elements as well as a liquid portion consisting of proteins, amino acids, carbohydrates, lipids and elements. The hematopoietic system is characterized by turnover and replenishment throughout life.
The pluripotent hematopoietic stem cell (HSC) is the progenitor of the cells in blood. The cellular elements that arise from this stem cell that circulates in blood include red blood cells, white blood cells, and platelets. Normal white blood cells in the peripheral circulation include neutrophils, monocytes, eosinophils, basophils and lymphocytes. Since the HSC also gives rise to cells of the lymphoid system, the study of hematology also includes the lymph nodes and lymph tissue.
1. Erythrocytes (red blood cell, RBC)
An erythrocyte is a specialized a nucleated cell that packages hemoglobin, the protein that is a respiratory gas transport vehicle that carries oxygen from the lungs to and carbon dioxide from tissues and back to the lungs to dispel. Erythrocytes undergo erythropoiesis whereby they mature from an early progenitor cell to the non – nucleated, biconcave disk, and the erythrocyte that with the absence of its nucleus and the flexibility of its membrane is able to bend to traverse 2 – 3 micron capillaries. A new RBC in the circulation is slightly bigger than older cells.
The red cell life span is 120 days, and normally there are about 5 million RBC/ μ L in whole blood in adult males and 4.5 million RBC/ μ L in adult females. Old RBCs lose their energy – producing (ATP) capacity, develop stiff membranes, and are removed from circulation by the macrophages of the mononuclear – phagocytic system of the spleen. Their hemoglobin is normally retained in the reticuloendothelial (RE) system but can be lost when there is brisk shortened red blood cell survival, i.e., hemolysis.
Neutrophils are also referred to as polymorphonuclear neutrophils, PMN or polys, segmented neutrophils, or segs. The neutrophil contains a nucleus that is usually a 3 – 4 lobed or “segmented” structure that stains a bluish color with Wright – Giemsa stain. An early form of a neutrophil is a “band” that shows an unsegmented nucleus. A neutrophil normally takes 12 – 13 days to be produced in bone marrow. Its life span in the circulation is about 12 hours and they can live in tissues for several days. The marrow pool of mature neutrophils is 30 – 40 times that seen in the circulation. Their functions are to phagocytize and digest bacteria, cellular debris, and dead tissue.
Monocytes are large, mononuclear cells with an indented (kidney – shaped) nucleus that form the circulating component of the mononuclear phagocyte system. The nucleolus in mature monocytes circulating in the peripheral circulation is usually not identified on blood by light microscopy. Monocytes spend 1 – 3 days in bone marrow and 8 – 72 hours in the peripheral blood. They have a similar functional role to neutrophils in host defense against organisms.
Eosinophils are characterized by their prominent orange – reddish (refractile) granules seen on Wright – Giemsa stain. Eosinophils usually have bilobed nuclei. Eosinophils increase in reaction to foreign protein and thus are seen in parasitic infection (especially larva of roundworms, helminths), allergic conditions, cancer and certain drugs.
Basophils are equally colorful with very dark, bluish prominent granules following Wright – Giemsa stain. Granules contain: histamine, heparin, and hyaluronic acid. Histamine release (basophil degranulation) is part of the allergic reaction. Normally basophils are 0 – 1% of WBC differential blood count. They are often increased in patients with chronic myelogenous leukemia and other myeloproliferative disorders
6. Platelets (thrombocytes)
Platelets bud off from the cytoplasm of the bone marrow megakaryocytes. The “mega” karyocyte in the bone marrow is recognized by its large size. Uniquely, the cell doubles its nuclear and cytoplasmic material but does not divide. Megakaryocyte growth and platelet segmentation is regulated by thrombopoietin. Platelets are anucleated cell fragments that contain remnant mRNA. They have a 7 – 10 day half – life and their first 1 – 2 days are spent in the spleen.
Platelets can be entrapped by an enlarged spleen as seen in congestive and inflammatory disorders. They play a central role in hemostasis as they contain many hemostatic cofactors and inhibitors in their granules. They also have a role in inflammation since they contain many growth factors. At the megakaryocyte level, plasma proteins can be adsorbed and packaged into platelet granules.