In the adults most blood cells are derived from the bone marrow.

During prenatal life, the first signs of embryonic hematopoiesis are found in the yolk sac 4 weeks after conception. During the second trimester the liver and the spleen become the primary sites of fetal hematopoiesis, to be gradually relocated to the bone marrow inside the developing skeleton. In adults, the hematopoietic bone marrow is confined to the axial skeleton, including the sternum, ribs, vertebrae, and pelvic bones (Fig. 6-2).

Figure 6-2 Blood-forming and lymphoid organs.

The hematopoietic bone marrow comprises hematopoietic cells and bone marrow stroma, which form the microenvironment essential for the growth and differentiation of the hematopoietic cells (Fig. 6-3). It also contains blood vessels, mostly in the form of sinusoids. These sinusoids have fenestrated walls so that the newly formed blood cells can easily enter the circulation. The bone marrow also contains thin bone trabeculae, which provide mechanical support and protection, and fat cells. With aging the hematopoietic elements decrease in number, and the fat cells become more numerous, replacing up to 70% of the total hematopoietic bone marrow in the elderly.

Figure 6-3 Bone marrow. The hematopoietic cells are located between the bone trabeculae. Cells of a single lineage tend to form groups, most prominent of which are the myeloid and erythroid nests. Megakaryocytes stand out because of their large size and multilobed nuclei. Blood vessels occur mostly in the form of sinusoids, which have fenestrated walls. RBC, red blood cell; WBC, white blood cell.

Blood cells develop from pluripotent hematopoietic stem cells under the influence of growth and differentiation factors.

The formation of blood cells is a highly regulated process that occurs in several steps involving both cell multiplication and differentiation. Several cell lineages are formed, ultimately giving rise to mature RBCs, platelets, and several distinct types of WBCs: neutrophils, eosinophils, basophils, monocytes, and lymphocytes (Fig. 6-4).

Figure 6-4 Cell lineages of hematopoiesis. Baso, basophil; BFU, burst-forming unit; CFU, colony-forming unit; E, erythrocyte; Eo, eosinophil; GM, granulocyte/macrophage; M, monocyte; Meg, megakaryocyte; NK, natural killer.

All hematopoietic cell lineages can be traced to a common ancestor—the pluripotent hematopoietic stem cell. Even though these cells account for less than 1% of all hematopoietic bone marrow cells, the adult bone marrow contains 25 to 500 million pluripotent stem cells. Some of these stem cells also enter into the circulating blood, from where they can be harvested for bone marrow transplantation. The pluripotent hematopoietic stem cells resemble small lymphocytes and can be identified only immunocytologically or by flow cytometry using labeled antibodies to their cell membrane marker—CD34.

Like all stem cells the totipotent hematopoeitic stem cells have a capacity for self-renewal but can also differentiate into two types of multilineage progenitor cells: lymphoid and myeloid progenitor cells. These progenitor cells give rise to unilineage progenitor cells, which in turn differentiate into committed precursor cells of mature lymphocytes, RBCs, neutrophils, monocytes, eosinophils, basophils, and platelets.

The replication of stem cells and progenitor cells and the differentiation of precursor cells depend on the action of growth and differentiation factors produced by the stromal cells, macrophages, and T lymphocytes. Numerous growth and differentiation factors have been isolated and characterized; many of these have been synthesized using recombinant DNA technology and are used in clinical practice to stimulate and or regulate hematopoiesis. Erythropoietin acts on the precursors of RBCs, thrombopoietin stimulates platelet production, and granulocyte colony–stimulating factor (G-CSF) acts on the differentiation of neutrophil precursors. Interleukin 3 (IL-3) behaves as a nonlineage-specific growth factor, acting on both myeloid and lymphoid lineages. Note that the terminal stages of lymphocyte maturation and differentiation are antigen-dependent and can be significantly amplified during immune reactions.

The differentiation of precursor cells is accompanied by the appearance and disappearance of specific cell surface molecules. Cell surface molecules known as clusters of differentiation (CD) have been especially useful for monitoring the maturation and differentiation of myeloid and lymphoid cells. For example CD34 is a marker of stem cells, CD4 is a marker for T helper cells, and CD8 is a marker for T cytotoxic cells, whereas CD20 is marker of B cells. Over 200 CD markers have been identified today, and many of them are used for diagnostic purposes in clinical hematopathology and immunology.

Normal hematopoiesis requires an adequate supply of energy, nutrients, and vitamins.

Formation and maturation of hematopoietic stem cells are tightly controlled by growth and differentiation factors. This process is also critically dependent on the normal supply of energy, nutrients, and vitamins. The most important among these are as follows:

Figure 6-5 The role of vitamin B₁₂ and folic acid in DNA synthesis. Vitamin B₁₂ is essential for the formation of tetrahydrofolate (THF) from folic acid. THF in its methylated form acts as a coenzyme for DNA synthesis.

The life span of normal blood cells varies.

The blood cells can be divided into three groups: RBCs (erythrocytes), WBCs (leukocytes), and platelets, or thrombocytes. The RBCs and platelets do not have nuclei, whereas the WBCs do. On the basis of the shape of their nuclei the WBCs can be further divided into segmented cells (neutrophils and eosinophils) and nonsegmented, or mononuclear, cells (monocytes and lymphocytes).

The normal life span of blood cells varies. The process of formation of blood cells and their maturation inside the bone marrow varies, and the duration the mature cells remain in the bone marrow is also cell lineage-specific. Likewise, the half-life of cells in the circulation varies. Remember that mature RBCs spend all their life inside the blood vessels, whereas WBCs can exit into the tissues and live there as well.

Red blood cells. Erythrocytes live in the circulation the longest—on average 120 days. This is important because RBCs can be removed from the blood and stored a few days or even a week or two for transfusion. Old and damaged RBCs are mostly lyzed by the splenic macrophages, but a small portion of them are hemolyzed inside the blood vessels. Thus we can distinguish between extravascular and intravascular hemolysis.

Figure 6-6 Lysis of senescent red blood cells (RBCs). Extravascular hemolysis occurs in the spleen and results in the formation of bilirubin, which is processed in the liver. Intravascular hemolysis leads to the formation of hemoglobin degradation products, which bind to specific plasma proteins carrying them into the liver. Unbound hemoglobin dimers are excreted in the urine, accounting for hemoglobinuria and hemosiderinuria. Both forms of hemolysis are associated with an increased excretion of urobilinogen in urine. Met Hb, methemoglobin.

Iron released in extravascular or intravascular hemolysis is reused to synthesize Hb. Globin is also reused.

Granulocytes. White blood cell precursors in the bone marrow form two compartments: a proliferative compartment and a maturation-storage compartment. In the first the cells remain for less than one day. It has been estimated that the maturing neutrophils remain in the maturation-storage department of the bone marrow for 7 to 10 days. Eosinophils remain there 2.5 days and basophils for only 12 hours.

Neutrophils enter the circulation from the bone marrow pool maturation-storage compartment, which contains the equivalent of neutrophils in reserve for 4 to 8 days. The half-life of neutrophilic granulocytes in circulation is 6 to 12 days. Neutrophils enter the blood in the form of segmented neutrophils or band cells. In the circulation they form two distinct sets: the marginating pool (storage portion) and the circulating pool. Neutrophils also can enter into the tissue, where they live, on average, for 2 to 3 days (Fig. 6-7).

Figure 6-7 Typical life span of neutrophils, platelets, and red blood cells and their bone marrow precursors. As shown in the diagram, all these cells originate from stem cells, but their life span in the bone marrow and outside of it varies. In the bone marrow the neutrophils are found in a proliferation–maturation compartment and a storage compartment. Platelets and erythrocytes are released as soon as they are formed and do not form a storage compartment. In peripheral blood the neutrophils and platelets form two compartments: a storage pool and a functional compartment. All red blood cells are in circulation and there is no red blood cell storage compartment. Under normal circumstances only neutrophils (and other leukocytes) emigrate into tissues.

Approximately 6% of all circulating WBCs are nonsegmented (band cells), whereas all others undergo nuclear segmentation. As they age their nuclei become more segmented, and the oldest ones have five segments. This segmentation can be used to estimate the average age of neutrophils in the Arneth index, which is a curve based on the number of segments of neutrophilic nuclei (Fig. 6-8). Left shift indicates that fewer nuclei are segmented; that is, there is a prevalence of young PMNs, whereas right shift indicates that there are more aging PMNs.

Figure 6-8 Arneth index. The blood contains young neutrophils (band cells) and old neutrophils (cells with five segmented nuclei), but most neutrophils have nuclei with three segments. Left shift of the curve indicates more young neutrophils in the circulation, and right shift more aging neutrophils.

Platelets. The half-life of platelets in circulation is 8 to 10 days. Platelets leaving the bone marrow initially enter the spleen where they remain for approximately 2 days. Two thirds of platelets enter the circulation, whereas one third remain as the active reserve pool of the spleen. Aging platelets are phagocytosed by splenic or hepatic phagocytic cells but may also be consumed at sites of minor endothelial cell injury, thereby activating intravascular coagulation.

Lymphocytes. These cells are long-lived, and their life span may be measured in months or years.

The primary function of red blood cells is transport of oxygen.

Erythrocytes, or red blood cells (RBCs), are highly specialized cells that do not have nuclei. In contrast to RBCs in the peripheral blood, their precursors in the bone marrow are nucleated. As the erythroid precursors mature, their nuclei become smaller and smaller and are finally extruded from the cytoplasm. The anucleated RBCs that still have residual RNA and a few cytoplasmic organelles may be also released into the circulation. These cells, called reticulocytes, account for less than 1% of all RBCs under normal circumstances.

The primary function of RBCs is transport of oxygen from the lungs into the peripheral tissues, but they also can bind other gases and also act as buffers. These functions are accomplished primarily through Hb, which accounts for 98% of their total weight.

Hemoglobin is a complex protein composed of globin and heme (Fig. 6-9). Globin is a tetramer composed of two α chains and two β chains or their equivalents γ and δ. The most abundant form of Hb is Hb A composed of two α and two β chains (α₂β₂). During fetal life the most predominant Hb is fetal hemoglobin (Hb F), which contains two α chains and two γ chains (α₂γ₂), but its level drops precipitously after birth, and by the age of 6 months it accounts for only 1% of the total Hb. Adult blood also contains Hb A₂, which is composed of two α and two δ chains (α₂δ₂), accounting for less than 3.5% of total Hb. The concentration of Hb in the blood is lower in females than in males and also lower in adults than in newborns (Table 6-1).

Figure 6-9 Hemoglobin structure.

(Reproduced with permission from Damjanov I: Pathology for Health Professions, 3rd ed. Philadelphia, Elsevier, 2005.)

Table 6-1 Normal Values of Hemoglobin (Hb) in Blood

Affinity of hemoglobin for oxygen varies and depends on several factors.

Hemoglobin has a high affinity for oxygen, but it also binds carbon dioxide and carbon monoxide. The affinity of Hb for oxygen depends on the pH of the blood, temperature, and concentration of 2,3-biphosphoglycerate (2,3-BPG), and the presence of variant hemoglobins that have a higher affinity for oxygen. The affinity for oxygen can be determined by measuring the oxygen pressure needed to achieve 50% saturation of Hb (P₅₀) (Fig. 6-10). In a normal person at pH 7.35 this saturation can be achieved at a PO₂ of 27 mm Hg. The oxygen saturation curve can be left-shifted, meaning that a P₅₀ can be achieved at lower partial pressure by changing the following variables:

Figure 6-10 Oxygen dissociation curve. Alkalization of the blood, low temperature, low partial pressure of CO₂ (PCO₂), and a low concentration or availability of 2,3-biphosphoglycerate (2,3-BPG) cause a left shift, whereas acidity of the blood, high temperature, and a high 2,3-BPG cause a right shift. Fetal hemoglobin (Hb F) has a higher affinity for oxygen than adult hemoglobin, thus causing a left shift. Hb S found in sickle cell disease shifts the curve to the right.

Variant and abnormal hemoglobins influence oxygen binding and its release in tissues as follows:

Granulocytes participate in the defense against infection.

Granulocytes respond to infection by entering the circulation and moving toward the site of infection (Fig. 6-11). At the site of infection the neutrophils marginate, adhere to the endothelial cells of capillaries and venules, and undergo activation. In addition to surface changes and changes in motility, activated neutrophils secrete biologically active substances that act on other cells in the tissues, most notably endothelial cells, macrophages, and lymphocytes. In concert with the activated neutrophils these cells secrete cytokines and growth factors, such as interleukins, tumor necrosis factor (TNF), platelet-activating factor (PAF), and many others. These substances act on the bone marrow, stimulating it to release new neutrophils into circulation and to proliferate stem cells and the precursors of granulocytes. All these events lead to neutrophilia—an increase in the number of neutrophils in circulation.

Figure 6-11 Reaction of neutrophils to infection. The neutrophils become marginated, adhere to the endothelium, and are activated in that process. Neutrophils respond to chemotactic stimuli and migrate toward the bacteria in the tissue. Cytokines and other biologically active substances produced at the site of inflammation act on the bone marrow, causing a release of neutrophils into the circulation (neutrophilia) and increased production of neutrophils in the bone marrow.

Neutrophils phagocytose and kill bacteria.

Neutrophils (also known as polymorphonuclear leukocytes—PMNs) form the first line of defense against bacterial infection. PMNs are so efficient at fighting bacteria because of the following properties:

Figure 6-12 Phagocytosis of bacteria. Several phases can be recognized. (1) The bacterium acts chemotactically, attracting the neutrophil. (2) Adherence of bacteria to the surface of leukocytes is facilitated by opsonins such as complement fragments (C3) or IgG. (3) The bacterium is engulfed in the pseudopodia formed by the elongation of the cytoplasm of the neutrophil. (4) Phagocytosis of the bacterium includes formation of a phagosome from the phagocytic vacuoles and lysosomes. (5) Phagocytosed bacteria are killed by oxygen-dependent and oxygen-independent mechanisms. (6) Killed bacteria are digested. (7) During the interaction with the bacterium the leukocyte may be killed, releasing its cytoplasmic contents into the extracellular space. The exudate composed of dead and dying leukocytes and lyzed tissue components is called pus.

Loss of phagocytic cells, such as occurs in agranulocytosis caused by cytotoxic drugs and various disorders affecting the basic functions of the neutrophils, results in increased susceptibility to bacterial infections. These disorders can be congenital or acquired. The most important examples of deficient function of neutrophils are listed in Table 6-2.

Table 6-2 Congenital Disorders of Leukocyte Function

Neutrophil dysfunction may be induced by drugs and alcohol and is typically found in many metabolic diseases, such as diabetes or end-stage kidney disease. Autoimmune diseases and HIV infection also cause neutrophil dysfunctions.

Eosinophils play a role in defending the body against parasites.

Eosinophils participate in the body’s defense against bacteria. However, eosinophils are less numerous and migrate much slower than neutrophils, and thus they contribute significantly less in the fight against bacteria than PMNs. If the infection is long-lasting eosinophils become more prominent. Eosinophils are most prominently involved in the reaction to parasites. Eosinophilia is also found in patients who have allergies, autoimmune diseases, and skin diseases and is present in response to certain malignancies, such as Hodgkin’s disease.

Lymphocytes are involved in the immune reactions.

Like all other WBCs and RBCs, lymphocytes originate from a common hematopoietic stem cell, which gives rise to developmentally restricted stem cells, populating the bone marrow and the peripheral lymphoid tissues. Three cell lines develop from this common lymphocytic precursor: T lymphocytes, B lymphocytes, and natural killer (NK) cells. In the circulating blood approximately 70% to 80% lymphocytes are B cells, 10% to 15% T cells, and 10% to 15% NK cells.

The differentiation of lymphocytes occurs gradually and is characterized by the appearance and disappearance of cell surface molecules known as clusters of differentiation (CD).

Platelets are involved in coagulation of the blood.

Platelets are derived from the fragmentation of the cytoplasm of megakaryocytes in the bone marrow. This process is primarily regulated by thrombopoietin, a cytokine produced by the liver. The platelets released from the bone marrow are carried to the spleen, which serves as their primary reservoir. From the spleen the platelets periodically enter the circulation and are dispatched to areas of endothelial injury.

Platelets are small membrane-bounded particles measuring 2 to 4 μm. They do not have nuclei and are barely visible under light microscopy. The cytoplasm of platelets contains several components that are essential for their function (Fig. 6-13). Thus, it contains the following organelles:

Figure 6-13 Ultrastructure of the platelet. The external plasma membrane is studded with receptors and adhesion molecules (cannot be seen by electron microscopy). The plasma membrane invaginates, forming a canaliculus. Cytoskeletal fibers (microtubules and microfilaments) are important for the changes in the shape of platelets and the excretion of preformed substances sorted in various granules (alpha granules, dense granules, dense tubular system, lysosomes). Glycogen forms pools and is an important source of energy, which is produced primarily by the mitochondria.

Circulating platelets readily adhere to damaged endothelial cells and fill the endothelial defects in disrupted or damaged blood vessels. The hemostatic platelet plug that forms under such conditions is a hallmark of primary hemostasis (Fig. 6-14). It leads to the activation of the coagulation cascade and formation of the fibrin clot known as secondary hemostasis (Fig. 6-15).

Figure 6-14 Formation of the platelet plug. A, Adhesion of platelets to the collagen in the vessel wall denuded of endothelial cells is mediated by von Willebrand’s factor (vWF), which binds to collagen and the platelet adhesion molecule GPIb. B, Adhesion is followed by a change in the shape of platelets accompanied by a release of various procoagulants, most notably adenosine diphosphate (ADP) and thromboxane A₂ (TXA₂), and fibrinogen. C, The procoagulants released from the activated platelets recruit additional platelets to the site of endothelial injury and also promote their aggregation, leading to the formation of the primary hemostatic plug.

Figure 6-15 Primary and secondary hemostasis. As shown in Figure 6-14, vessel injury leads to the formation of the platelet adhesion, followed by the release of procoagulants and platelet aggregation. These events, leading to the formation of the primary hemostatic plug, are known as primary hemostasis. Substances released from platelets also cause vasoconstriction (thereby reducing blood flow to the damaged vessels) and activate the coagulation cascade and the formation of thrombin. Thrombin acts on platelets, promoting platelet aggregation, consolidation of the primary hemostatic plug, and its retraction. It also promotes polymerization of fibrin and thus contributes to the formation of a stable fibrin clot. These events are called secondary hemostasis. The fibrin clot is the substrate for several anticoagulants and fibrinolytic proteins, the most important of which is plasmin. ADP, adenosine diphosphate.

Defective primary hemostasis manifests as bleeding from capillaries and venules. Clinically skin bruises (petechiae and ecchymoses) or bleeding from mucosae of the mouth (gingival bleeding), nose (epistaxis), or intestine (hematochezia) may be evident. Bleeding usually results from vascular defects or platelet abnormalities, such as thrombocytopenia. The bleeding time is prolonged.

Defective secondary hemostasis is associated with bleeding from small arteries and arterioles and is often related to trauma, surgical procedures, or tooth extraction. It may result in the formation of large intramuscular or retroperitoneal hematomas, or hemarthrosis. Bleeding is usually related to a congenital deficiency of clotting factors, as occurs in hemophilia, or acquired abnormalities of the coagulation pathway, as occurs in chronic liver disease. Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are prolonged.

The typical features of defective primary and secondary hemostasis are compared in Table 6-3.

Table 6-3 Disturbances of Primary and Secondary Hemostasis

Family and personal history may provide important diagnostic clues.

Family and personal history can point to some important risk factors that play a role in the pathogenesis of hematologic diseases. A few examples of such links and associations are given in Table 6-4.

Table 6-4 Risk Factors for Hematologic Diseases

Physical examination is important for evaluating hematologic disorders.

Symptoms and signs of hematologic diseases usually result from a loss or disturbance in the formation and function of RBCs, WBCs, or platelets and coagulation proteins. In some patients the hematologic problems dominate the clinical picture, whereas in others they are of secondary importance. Most manifestations of hematologic disorders are nonspecific and linked to a specific disturbance of blood-forming organs only with the judicious use of laboratory examinations. The following list includes some signs and symptoms that illustrate how these findings could point to a hematologic disease.

Table 6-5 Causes of Lymphadenopathy

Table 6-6 Causes of Splenomegaly

Laboratory studies are essential for the exact diagnosis of hematologic disorders.

Laboratory studies used in the work-up of hematologic disorders range from routine tests, such as complete blood count (CBC), to specialized tests for assessing the function of WBCs or platelets, to cytogenetic analysis of bone marrow cells and molecular analysis of specific genes known to be mutated in certain diseases. These tests are essential for the proper diagnosis of all hematologic diseases.

We discuss only the most commonly performed tests, however, as they are used for evaluating RBC, WBC, and coagulation disorders. The most important hematologic values used in general clinical practice are listed in Tables 6-7 to 6-9.

Table 6-7 Normal Red Blood Cell Indices in Adults

Hemoglobin	Male: 14.0–17.5 g/dL Female: 12.3–15.3 g/dL
Hematocrit	Male: 41.5–50.4% Female: 35.9–44.6%
Red blood cell (RBC) count	Male: 4.5–6.5 × 106/μL Female: 4.1–5.1 × 106/μL
Mean corpuscular hemoglobin	27–33 pg/cell
Mean corpuscular hemoglobin concentration	33–35%
Mean corpuscular volume	80–96 m3 (fL)
Red cell distribution width	11.5–14.5%
Reticulocyte count	0.5–2.5% of all RBCs
Iron	Male: 65–175 μg/dL Female: 50–170 μg/dL Only gold members can continue reading. Log In or Register to continue Share this: Click to share on Twitter (Opens in new window) Click to share on Facebook (Opens in new window) Like this: Like Loading... Related Related posts: THE EXOCRINE PANCREAS LABORATORY MEDICINE CARDIOVASCULAR DISEASES THE KIDNEYS Stay updated, free articles. Join our Telegram channel Join Tags: Pathophysiology With STUDENT CONSULT Oct 29, 2016 | Posted by admin in PATHOLOGY & LABORATORY MEDICINE | Comments Off on BLOOD, BONE MARROW, AND THE LYMPHOID SYSTEM Full access? Get Clinical Tree Get Clinical Tree app for offline access Get Clinical Tree app for offline access %d