Chapter 12: Blood

Blood is a specialized connective tissue consisting of cells and fluid extracellular material called plasma. Propelled mainly by rhythmic contractions of the heart, about 5 L of blood in an average adult moves unidirectionally within the closed circulatory system. The so-called formed elements circulating in the plasma are erythrocytes (red blood cells), leukocytes (white blood cells [WBCs]), and platelets.

When blood leaves the circulatory system, either in a test tube or in the extracellular matrix (ECM) surrounding blood vessels, plasma proteins react with one another to produce a clot, which includes formed elements and a pale yellow liquid called serum. Serum contains growth factors and other proteins released from platelets during clot formation, which confer biological properties very different from those of plasma.

Collected blood in which clotting is prevented by the addition of anticoagulants (eg, heparin or citrate) can be separated by centrifugation into layers that reflect its heterogeneity (Figure 12–1). Erythrocytes comprise the sedimented material, and their volume, normally about 44% of the total blood volume in healthy adults, is called the hematocrit.

The straw-colored, translucent, slightly viscous supernatant comprising 55% at the top half of the centrifugation tube is the plasma. A thin gray-white layer called the buffy coat between the plasma and the hematocrit, about 1% of the volume, consists of leukocytes and platelets, both less dense than erythrocytes.

Blood is a distributing vehicle, transporting O₂, CO₂, metabolites, hormones, and other substances to cells throughout the body. Most O₂ is bound to hemoglobin in erythrocytes and is much more abundant in arterial than venous blood (Figure 12–2), while CO₂ is carried in solution as CO₂ or HCO₃^–, in addition to being hemoglobin-bound. Nutrients are distributed from their sites of synthesis or absorption in the gut, while metabolic residues are collected from cells throughout the body and removed from the blood by the excretory organs. Hormone distribution in blood permits the exchange of chemical messages between distant organs regulating normal organ function. Blood also participates in heat distribution, the regulation of body temperature, and the maintenance of acid-base and osmotic balance.

Leukocytes have diverse functions and are one of the body’s chief defenses against infection. These cells are generally spherical and inactive while suspended in circulating blood, but when called to sites of infection or inflammation, they cross the wall of venules, become motile and migrate into the tissues, and display their defensive capabilities.

Plasma is an aqueous solution, pH 7.4, containing substances of low or high molecular weight that make up 7% of its volume. As summarized in Table 12–1, the dissolved components are mostly plasma proteins, but they also include nutrients, respiratory gases, nitrogenous waste products, hormones, and inorganic ions collectively called electrolytes. Through the capillary walls, the low-molecular-weight components of plasma are in equilibrium with the interstitial fluid of the tissues. The composition of plasma is usually an indicator of the mean composition of the extracellular fluids in tissues.

The major plasma proteins include the following:

• Albumin, the most abundant plasma protein, is made in the liver and serves primarily to maintain the osmotic pressure of the blood.

• Globulins (α- and β-globulins), made by the liver and other cells, include transferrin and other transport factors; fibronectin; prothrombin and other coagulation factors; lipoproteins and other proteins entering blood from tissues.

• Immunoglobulins (antibodies or γ-globulins) secreted by plasma cells in many locations.

• Fibrinogen, the largest plasma protein (340 kD), also made in the liver, which, during clotting, polymerizes as insoluble, cross-linked fibers of fibrin that block blood loss from small vessels.

• Complement proteins, which comprise a defensive system important in inflammation and destruction of microorganisms.

Blood cells can be studied histologically in smears prepared by spreading a drop of blood in a thin layer on a microscope slide (Figure 12–3). In such films the cells are clearly visible and distinct from one another, facilitating observation of their nuclei and cytoplasmic characteristics. Blood smears are routinely stained with mixtures of acidic (eosin) and basic (methylene blue) dyes. These mixtures may also contain dyes called azures that are more useful in staining cytoplasmic granules containing charged proteins and proteoglycans. Azurophilic granules produce metachromasia in stained leukocytes like that seen with mast cells in connective tissue. Some of these special stains, such as Giemsa and Wright stain, are named after hematologists who introduced their own modifications into the original mixtures.

Erythrocytes

Erythrocytes (red blood cells [RBCs]) are terminally differentiated structures lacking nuclei and completely filled with the O₂-carrying protein hemoglobin. RBCs are the only blood cells whose function does not require them to leave the vasculature.

Human erythrocytes suspended in an isotonic medium are flexible biconcave discs (Figure 12–4). They are approximately 7.5 μm in diameter, 2.6-μm thick at the rim, but only 0.75-μm thick in the center. Because of their uniform dimensions and their presence in most tissue sections, RBCs can often be used by histologists as an internal standard to estimate the size of other nearby cells or structures.

The biconcave shape provides a large surface-to-volume ratio and facilitates gas exchange. The normal concentration of erythrocytes in blood is approximately 3.9-5.5 million per microliter (μL, or mm³) in women and 4.1-6.0 million/μL in men.

Erythrocytes are normally quite flexible, which permits them to bend and adapt to the small diameters and irregular turns of capillaries. Observations in vivo show that at the angles of capillary bifurcations, erythrocytes with normal adult hemoglobin frequently assume a cuplike shape. In larger blood vessels RBCs may adhere to one another loosely in stacks called rouleaux (Figure 12–4c).

The erythrocyte plasmalemma, because of its ready availability, is the best-known membrane of any cell. It consists of about 40% lipid, 10% carbohydrate, and 50% protein. Most of the latter are integral membrane proteins (see Chapter 2), including ion channels, the anion transporter called band 3 protein, and glycophorin A. The glycosylated extracellular domains of the latter proteins include antigenic sites that form the basis for the ABO blood typing system. Several peripheral proteins are associated with the inner surface of the membrane, including spectrin, dimers of which form a lattice bound to underlying actin filaments, and ankyrin, which anchors the spectrin lattice to the glycophorins and band 3 proteins. This submembranous meshwork stabilizes the membrane, maintains the cell shape, and provides the cell elasticity required for passage through capillaries.

Erythrocyte cytoplasm lacks all organelles but is densely filled with hemoglobin, the tetrameric O₂-carrying protein that accounts for the cells’ uniform acidophilia. When combined with O₂ or CO₂, hemoglobin forms oxyhemoglobin or carbaminohemoglobin, respectively. The reversibility of these combinations is the basis for the protein’s gas-transporting capacity.

Erythrocytes undergo terminal differentiation (discussed in Chapter 13) which includes loss of the nucleus and organelles shortly before the cells are released by bone marrow into the circulation. Lacking mitochondria, erythrocytes rely on anaerobic glycolysis for their minimal energy needs. Lacking nuclei, they cannot replace defective proteins.

Human erythrocytes normally survive in the circulation for about 120 days. By this time defects in the membrane’s cytoskeletal lattice or ion transport systems begin to produce swelling or other shape abnormalities, as well as changes in the cells’ surface oligosaccharide complexes. Senescent or worn-out RBCs displaying such changes are recognized and removed from circulation, mainly by macrophages of the spleen, liver, and bone marrow.

Leukocytes

Leukocytes (WBCs) leave the blood and migrate to the tissues where they become functional and perform various activities related to immunity. Leukocytes are divided into two major groups, granulocytes and agranulocytes, based on the density of their cytoplasmic granules (Table 12–2). All are rather spherical while suspended in blood plasma, but they become amoeboid and motile after leaving the blood vessels and invading the tissues. Their estimated sizes mentioned here refer to observations in blood smears in which the cells are spread and appear slightly larger than they are in the circulation.

Granulocytes possess two major types of abundant cytoplasmic granules: lysosomes (often called azurophilic granules in blood cells) and specific granules that bind neutral, basic, or acidic stains and have specific functions.

Granulocytes also have polymorphic nuclei with two or more distinct (almost separated) lobes and include the neutrophils, eosinophils, and basophils (Figure 12–1 and Table 12–2). All granulocytes are also terminally differentiated cells with a life span of only a few days. Their Golgi complexes and rough ER are poorly developed, and with few mitochondria they depend largely on glycolysis for their energy needs. Most granulocytes undergo apoptosis in the connective tissue and billions of neutrophils alone die each day in adults. The resulting cellular debris is removed by macrophages and, like all apoptotic cell death, does not itself elicit an inflammatory response.

Agranulocytes lack specific granules, but do contain some azurophilic granules (lysosomes). The nucleus is spherical or indented but not lobulated. This group includes the lymphocytes and monocytes (Figure 12–1 and Table 12–2). The differential count (percentage of all leukocytes) for each type of leukocyte is also presented in Table 12–2.

All leukocytes are key players in the constant defense against invading microorganisms and in the repair of injured tissues, specifically leaving the microvasculature in injured or infected tissues. At such sites factors termed cytokines are released from various sources and these trigger loosening of intercellular junctions in the endothelial cells of local postcapillary venules (Figure 12–6). Simultaneously the cell adhesion protein P-selectin appears on the endothelial cells’ luminal surfaces following exocytosis from cytoplasmic Weibel-Palade bodies. The surfaces of neutrophils and other leukocytes display glycosylated ligands for P-selectin, and their interactions cause cells flowing through the affected venules to slow down, like rolling tennis balls arriving at a patch of velcro. Other cytokines stimulate the now slowly rolling leukocytes to express integrins and other adhesion factors that produce firm attachment to the endothelium (see Figure 11–21d). In a process called diapedesis (Gr. dia, through + pedesis, to leap), the leukocytes send extensions through the openings between the endothelial cells, migrate out of the venules into the surrounding tissue space, and head directly for the site of injury or invasion. The attraction of neutrophils to bacteria involves chemical mediators in a process of chemotaxis, which causes leukocytes to rapidly accumulate where their defensive actions are specifically needed.

The number of leukocytes in the blood varies according to age, sex, and physiologic conditions. Healthy adults have 4500-11,000 leukocytes per microliter of blood.

Neutrophils (Polymorphonuclear Leukocytes)

Mature neutrophils constitute 50%-70% of circulating leukocytes, a figure that includes slightly immature forms released to the circulation. Neutrophils are 12-15 μm in diameter in blood smears, with nuclei having two to five lobes linked by thin nuclear extensions (see Table 12–2; Figure 12–7). In females, the inactive X chromosome may appear as a drumstick-like appendage on one of the lobes of the nucleus (Figure 12–7c) although this characteristic is not always seen. Neutrophils are inactive and spherical while circulating but become amoeboid and highly active during diapedesis and upon adhering to ECM substrates such as collagen.

Neutrophils are usually the first leukocytes to arrive at sites of infection where they actively pursue bacterial cells using chemotaxis and remove the invaders or their debris by phagocytosis.

The cytoplasmic granules of neutrophils provide the cells’ functional activities and are of two main types (Figure 12–8). Azurophilic primary granules or lysosomes are large, dense vesicles with a major role in both killing and degrading engulfed microorganisms. They contain proteases and antibacterial proteins, including the following:

• Myeloperoxidase (MPO), which generates hypochlorite and other agents toxic to bacteria

• Lysozyme, which degrades components of bacterial cell walls

• Defensins, small cysteine-rich proteins that bind and disrupt the cell membranes of many types of bacteria and other microorganisms.

Specific secondary granules are smaller and less dense, stain faintly pink, and have diverse functions, including secretion of various ECM-degrading enzymes such as collagenases, delivery of additional bactericidal proteins to phagolysosomes, and insertion of new cell membrane components.

Activated neutrophils at infected or injured sites also have important roles in the inflammatory response, which begins the process of restoring the normal tissue microenvironment. They release many polypeptide chemokines that attract other leukocytes and cytokines, which direct activities of these and local cells of the tissue. Important lipid mediators of inflammation are also released from neutrophils.

Neutrophils contain glycogen, which is broken down into glucose to yield energy via the glycolytic pathway. The citric acid cycle is less important, as might be expected in view of the paucity of mitochondria in these cells. The ability of neutrophils to survive in an anaerobic environment is highly advantageous, because they can kill bacteria and help clean up debris in poorly oxygenated regions, for example, damaged or necrotic tissue lacking normal microvasculature.

Neutrophils are short-lived cells with a half-life of 6-8 hours in blood and a life span of 1-4 days in connective tissues before dying by apoptosis.

Eosinophils

Eosinophils are far less numerous than neutrophils, constituting only 1%-4% of leukocytes. In blood smears, this cell is about the same size as a neutrophil or slightly larger, but with a characteristic bilobed nucleus (Table 12–2; Figure 12–9). The main identifying characteristic is the abundance of large, acidophilic specific granules typically staining pink or red.

Ultrastructurally the eosinophilic-specific granules are seen to be oval in shape, with flattened crystalloid cores (Figure 12–9c) containing major basic proteins (MBP), an arginine-rich factor that accounts for the granule’s acidophilia and constitutes up to 50% of the total granule protein. MBPs, along with eosinophilic peroxidase, other enzymes and toxins, act to kill parasitic worms or helminths. Eosinophils also modulate inflammatory responses by releasing chemokines, cytokines, and lipid mediators, with an important role in the inflammatory response triggered by allergies. The number of circulating eosinophils increases during helminthic infections and allergic reactions. These leukocytes also remove antigen-antibody complexes from interstitial fluid by phagocytosis.

Eosinophils are particularly abundant in connective tissue of the intestinal lining and at sites of chronic inflammation, such as lung tissues of asthma patients.

Basophils

Basophils are also 12-15 μm in diameter but make up less than 1% of circulating leukocytes and are therefore difficult to find in normal blood smears. The nucleus is divided into two irregular lobes, but the large specific granules overlying the nucleus usually obscure its shape.

The specific granules (0.5 μm in diameter) typically stain purple with the basic dye of blood smear stains and are fewer, larger, and more irregularly shaped than the granules of other granulocytes (see Table 12–2; Figure 12–10). The strong basophilia of the granules is due to the presence of heparin and other sulfated GAGs. Basophilic-specific granules also contain much histamine and various other mediators of inflammation, including platelet activating factor, eosinophil chemotactic factor, and the enzyme phospholipase A that catalyzes an initial step in producing lipid-derived proinflammatory factors called leukotrienes.

By migrating into connective tissues, basophils appear to supplement the functions of mast cells, which are described in Chapter 5. Both basophils and mast cells have metachromatic granules containing heparin and histamine, have surface receptors for immunoglobulin E (IgE), and secrete their granular components in response to certain antigens and allergens.

Lymphocytes

By far the most numerous type of agranulocyte in normal blood smears, lymphocytes constitute a family of leukocytes with spherical nuclei (Table 12–2; Figure 12–11). Lymphocytes are typically the smallest leukocytes and constitute approximately one-third of these cells. Although they are morphologically similar, mature lymphocytes can be subdivided into functional groups by distinctive surface molecules (called “cluster of differentiation” or CD markers) that can be distinguished using antibodies with immunocytochemistry or flow cytometry. Major classes include B lymphocytes, helper and cytotoxic T lymphocytes (CD4⁺ and CD8⁺, respectively), and natural killer (NK) cells. These and other types of lymphocytes have diverse roles in immune defenses against invading microorganisms and certain parasites or abnormal cells. T lymphocytes, unlike B cells and all other circulating leukocytes, differentiate outside the bone marrow in the thymus. Functions and formation of lymphocytes are discussed with the immune system in Chapter 14.

Although generally small, circulating lymphocytes have a wider range of sizes than most leukocytes. Small, newly released lymphocytes have diameters similar to those of RBCs; medium and large lymphocytes are 9-18 μm in diameter, with the latter representing activated lymphocytes or NK cells. The small lymphocytes are characterized by spherical nuclei with highly condensed chromatin and only a thin surrounding rim of scant cytoplasm, making them easily distinguishable from granulocytes. Larger lymphocytes have larger, slightly indented nuclei and more cytoplasm that is slightly basophilic, with a few azurophilic granules, mitochondria, free polysomes, and other organelles (Figure 12–11d).

Lymphocytes vary in life span according to their specific functions; some live only a few days and others survive in the circulating blood or other tissues for many years.

Monocytes

Monocytes are precursor cells of macrophages, osteoclasts, microglia, and other cells of the mononuclear phagocyte system in connective tissue of nearly all organs (see Chapter 5). Circulating monocytes have diameters of 12-15 μm and have nuclei that are large and usually distinctly indented or C-shaped (Figure 12–12). The chromatin is less condensed than in lymphocytes and typically stains lighter than that of large lymphocytes. Cells of the mononuclear phagocyte system arise in developing organs from monocytes formed in the embryonic yolk sac and are supplemented throughout life by monocytes from the bone marrow. All monocyte-derived cells are antigen-presenting cells with important roles in immune defense as well as tissue repair.

The cytoplasm of the monocyte is basophilic and contains many small lysosomal azurophilic granules, some of which are at the limit of the light microscope’s resolution. These granules are distributed through the cytoplasm, giving it a bluish-gray color in stained smears. Mitochondria and small areas of rough ER are present, along with a Golgi apparatus involved in the formation of lysosomes (Figure 12–12e).

Platelets

Blood platelets (or thrombocytes) are very small non-nucleated, membrane-bound cell fragments only 2-4 μm in diameter (Figure 12–13a). As described in Chapter 13, platelets originate by separation from the ends of cytoplasmic processes extending from giant polyploid bone marrow cells called megakaryocytes. Platelets promote blood clotting and help repair minor tears or leaks in the walls of small blood vessels, preventing loss of blood from the microvasculature. Normal platelet counts range from 150,000 to 400,000/μL (mm³) of blood. Circulating platelets have a life span of about 10 days.

In stained blood smears, platelets often appear in clumps. Each individual platelet is generally discoid, with a very lightly stained peripheral zone, the hyalomere, and a darker-staining central zone rich in granules, called the granulomere. A sparse glycocalyx surrounding the platelet plasmalemma is involved in adhesion and activation during blood coagulation.

Ultrastructural analysis (Figure 12–13b) reveals a peripheral marginal bundle of microtubules and microfilaments, which helps to maintain the platelet’s shape. Also in the hyalomere are two systems of membrane channels. An open canalicular system of vesicles is connected to invaginations of the plasma membrane, which may facilitate platelets’ uptake of factors from plasma. A much less prominent set of irregular tubular vesicles comprising the dense tubular system is derived from the ER and stores Ca²⁺ ions. Together, these two membranous systems facilitate the extremely rapid exocytosis of proteins from platelets (degranulation) upon adhesion to collagen or other substrates outside the vascular endothelium.

Besides specific granules, the central granulomere has a sparse population of mitochondria and glycogen particles (Figure 12–13b). Electron-dense delta granules (δG), 250-300 nm in diameter, contain ADP, ATP, and serotonin (5-hydroxytryptamine) taken up from plasma. Alpha granules (αG) are larger (300-500 nm in diameter) and contain platelet-derived growth factor (PDGF), platelet factor 4, and several other platelet-specific proteins. Most of the stained granules seen in platelets with the light microscope are alpha granules.

The role of platelets in controlling blood loss (hemorrhage) and in wound healing can be summarized as follows:

• Primary aggregation: Disruptions in the microvascular endothelium, which are very common, allow the platelet glycocalyx to adhere to collagen in the vascular basal lamina or wall. Thus, a platelet plug is formed as a first step to stop bleeding (Figure 12–14).

• Secondary aggregation: Platelets in the plug release a specific adhesive glycoprotein and ADP, which induce further platelet aggregation and increase the size of the platelet plug.

• Blood coagulation: During platelet aggregation, fibrinogen from plasma, von Willebrand factor and other proteins released from the damaged endothelium, and platelet factor 4 from platelet granules promote the sequential interaction (cascade) of plasma proteins, giving rise to a fibrin polymer that forms a three-dimensional network of fibers trapping RBCs and more platelets to form a blood clot, or thrombus (Figure 12–14). Platelet factor 4 is a chemokine for monocytes, neutrophils, and fibroblasts, and proliferation of the fibroblasts is stimulated by PDGF.

• Clot retraction: The clot initially bulges into the blood vessel lumen, but soon contracts slightly due to the activity of platelet-derived actin and myosin.

• Clot removal: Protected by the clot, the endothelium and surrounding tunic are restored by new tissue, and the clot is then removed, mainly dissolved by the proteolytic enzyme plasmin, which is formed continuously through the local action of plasminogen activators from the endothelium on plasminogen from plasma.

SUMMARY OF KEY POINTS

Blood is a tissue in which highly specialized cells are suspended in a fluid matrix called plasma. Blood serves to transport many diverse substances, from gases, nutrients, and wastes to larger and functionally more complex molecules such as hormones and antibodies. Histologically, blood is sometimes classified as a specialized form of connective tissue. In stained blood smears, one can quickly distinguish between the oxygen-bearing red blood cells or erythrocytes, lacking nuclei, and the white blood cells or leukocytes, which are basophilic and nucleated and perform diverse roles in bodily defense (Table 12–2). Recognition of the cell types, particularly the various types of leukocytes, and the significance of their relative numbers are frequently important in medical diagnoses.

Blood cells in adults are produced in bone marrow, and an examination of marrow reveals immature forms of the blood cells, which can identified as members of the various leukocytic lineages, as well as platelet-producing cells (Table 13–2).

LABORATORY SESSION LEARNING OBJECTIVES

• Understand that blood is often classified as a specialized connective tissue with “formed elements” (cells and platelets) in a fluid matrix (plasma).

• Understand the morphology and function of red blood cells or erythrocytes.

• Understand the relative numbers of the various types of white blood cells or leukocytes in the blood of normal adults and know the major functions of each type.

• Understand that all leukocytes display cell motility and function in secretion, phagocytosis, etc. primarily in the extracellular matrix of tissues after migrating across the blood vessel wall (diapedesis).

• Recognize and find neutrophils, eosinophils, basophils, monocytes, and platelets in a blood smear.

• Understand and recognize the compartments and tissues in blood marrow.

• Understand the developmental sequence and recognize the intermediate cells in the formation of red blood cells and the three types of granulocytes.

• Recognize megakaryocytes and understand how platelets are formed and released.

TERMS TO UNDERSTAND & USE

• Erythrocytes/red blood cells

• Leukocytes/white blood cells

  • Granulocytes

    • Neutrophils

    • Eosinophils

    • Basophils

  • Agranulocytes

    • Lymphocytes

    • Monocytes

• Thrombocytes/platelets

• Hemopoiesis

• Bone marrow

• Red marrow

• Stromal cells

• Sinusoids

• Hemopoietic cord

  • Proerythroblasts

  • Erythroblasts

  • Reticulocytes

  • Myelocytes

  • Metamyelocytes

  • Stab cells

  • Megakaryocyte

• Yellow marrow

SLIDES & MICROGRAPHS

Mature Blood Cells

Examine a human blood smear (slide 10 ), prepared as shown in Figure 12–3 with polychromatic Wright stain, which stains both basophilic and acidophilic cellular components. This is a typical sample of circulating blood, containing mostly mature erythrocytes, with a smaller number of leukocytes.

A. Identify red blood cells (RBCs) or erythrocytes (Figures 12–3 and 12–4), by far the most numerous cell type present. Note their uniform diameter of about 7 mm. Red blood cells can be found in capillaries of most organs examined in these laboratory sessions and their size can provide a measuring standard by which to estimate sizes of other tissue structures.

• 1. Why are erythrocytes uniformly acidophilic, and how is their structure adapted to carry out their primary function?

B. White blood cells or leukocytes are very distinct from erythrocytes and serve various defensive functions in the body.

Identify the white blood cells (leukocytes) with lobulated or deeply indented nuclei. Such cells are the white cells called granulocytes, and carefully focusing on such cells will reveal cytoplasm filled with faintly stained granules. Note the size of granulocytes compared to red blood cells. The most abundant granulocytes are neutrophils (Table 12–2 and Figure 12–7). Examine with higher magnification the strangely segmented nuclei (which give rise to the common name for these cells in clinical laboratories: polymorphonuclear neutrophils [PMNs]) and the very lightly stained cytoplasmic granules of several neutrophils. Be sure that you can recognize these cells, and then compare their structure with that shown in the electron micrograph of Figure 12–8.

• 2. How does the structure of neutrophils correspond with their major function?

Look for the other two types of granulocytes, which are much less numerous than neutrophils: eosinophils and basophils, named for the general staining properties of their cytoplasmic granules. These cells represent only 1%-4% and less than 1% of the circulating leukocytes (Table 12–2). Identify them by their appearance in Figures 12–9 and 12–10, respectively. In both these cell types, the nuclei are less heavily segmented than in neutrophils, with their nuclei typically have just two lobes. In basophils the nuclei may be hidden by the darkly stained cytoplasmic granules.

• 3. How do the functions of eosinophils and basophils differ from that of neutrophils?

Identify lymphocytes of various sizes and with various amounts of cytoplasm, noting their almost spherical nuclei (Figure 12–11). Lymphocytes are sometimes called agranulocytes due to their relative paucity of cytoplasmic granules compared to other leukocytes. They range in size from 6 to 15 mm in diameter, a larger size range than that of granulocytes. Lymphocytes are abundant leukocytes, being approximately as numerous as neutrophils.

• 4. How does the agranulocytic nature of lymphocytes correlate with their function?

Identify the second type of agranulocyte: the monocytes (Figure 12–12), noting the distinctively larger size and “C-shaped” nuclei that make these cells relatively easy to recognize.

• 5. When and where do monocytes become functional?

Identify platelets (thrombocytes) (Figure 12–13), noting their very small size, their lack of nuclei, their distribution (singly or in clusters), and their ultrastructural features.

• 6. How might the size and structural features of platelets aid their function?

Hematopoiesis

Hematopoiesis, the process of blood cell formation, occurs in adults primarily in the bone marrow (Figures 13–4 and 13–5). The mature blood cell types examined earlier today each emerge from their specific lineages shown in Figure 13–2. Important cells of these lineages and their supportive cells are the focus of the following.

A. Examine the marrow cavity of long bones decalcified and sectioned (slide 34 and slide 104 ). Identify adipocytes and determine whether this is red marrow or yellow marrow, based on the amount of adipose tissue present. Note the overall appearance and organization of marrow and identity sinusoids. Recall that these are vascular spaces bounded by very thin endothelial cells and containing circulating blood. In the stroma identify hemopoietic cords, which are dense masses of proliferating and differentiating blood cells. Cells in mitosis should be visible here. Identify the stromal cells, which are specialized types of fibroblasts, and scattered large macrophages.

• 7. What is the structural relationship among the endosteum, stroma, hemopoietic cords, endothelial cells, and sinusoids in marrow?

If one is available, examine a section from a needle biopsy of normal bone marrow (slide 153 ) that retains much of the normal organization of the marrow in bone. Again identify adipocytes, blood sinusoids, hemopoietic cords, megakaryocytes, and stromal cells comprising the marrow.

B. Examine a smear made from a bone marrow aspirate (slide 127 ), focusing on a region where the smear is relatively thin with low cell density and good cell morphology. Note that most of the cells present are mature red and white blood cells like those you identified in the smear of circulating blood. Here you want to study these cells’ precursors.

First identify megakaryocytes (Figure 13–13) and note their characteristic very large, polyploid nuclei. These large cells are very distinctive and should be found easily. Examine their ultrastructure (Figure 13–14), noting the extensive system of membranes in the cytoplasm, which facilitate the rapid production of membrane-bound platelets in the marrow.

• 8. Give three ways in which megakaryocytes differ from the other types of leukocytes.

Now look for the various erythrocyte precursors (Figure 13–7), including proerythroblasts and erythroblasts of different stages (basophilic, polychromatophilic and orthochromatophilic). Note the significance of these polysyllabic names by reference to Figure 13–6, and understand that the names simply indicate the cell’s cytoplasmic staining properties due to its ratio of RNA-to-hemoglobin. Reticulocytes are nearly mature RBCs that have ejected their nuclei, but can only be distinguished from fully mature erythrocytes by using a special stain for their RNA content (Figure 13–7b).

Review again Figure 13–2, concentrating on the lineage of the granulocytic leukocytes and noting carefully the developmental sequence there. Among the leukocytes present in your slide of marrow smear, look for and try to identify the following granulocyte precursors, using Figure 13–10 as a reference guide: neutrophilic myelocytes, metamyelocytes, and stab (or band) cells. (You may not be required to identify leukocytic precursor cells as this topic is examined in more detail when studying hematology.)

• 9. In a bone marrow transplant, what cells are transplanted and what is the goal of the procedure?

ANSWERS TO STUDY QUESTIONS

1. Erythrocytes have lost their nuclei and all organelles and are densely filled with hemoglobin, which is acidophilic. Hemoglobin binds oxygen reversibly, providing the major function of these cells.

2. The segmented nuclei of neutrophils facilitates their rapid emergence from between endothelial cells of venules at sites of infection, their abundant microfilaments allow active motility and phagocytosis, and exocytosis of their numerous cytoplasmic granules releases antibacterial compounds.

3. Eosinophils and basophils do not kill and remove bacteria like neutrophils, but rather secrete many other compounds with various immunogenic and defensive properties.

4. Lymphocytes do not actively secrete material after their emergence from the bloodstream. Rather they gradually develop as B or T cells which then differentiate further.

5. Like other leukocytes, monocytes only become functional after emergence from venules into tissue spaces.

6. Platelets’ small size and “open canalicular system” of cytoplasmic membranes facilitate immediate and complete release of material in cytoplasmic granules upon contact with collagen, promoting very rapid blood clotting.

7. In bone marrow endosteum covers the bony trabeculae and stroma and adipose tissue fills the marrow cavity. Cells of this stroma physically and physiologically support the abundant hemopoietic cords, consisting of blood cell precursors. As the latter cells mature, they enter nearby sinusoids through openings between their endothelial cells.

8. Unlike other leukocytes, megakaryocytes are extremely large, have highly polyploid nuclei, and never leave the bone marrow.

9. In a bone marrow transplant ALL of the cells mentioned above are unavoidably part of the transplant. Generally performed following the elimination of a patient’s genetically defective, malignant leukocytic cells (and other dividing precursor cells) by prior systemic radiation or chemotherapeutic treatments, the goal of the transplant is to replenish the patient’s marrow with a population of normal leukocytic stem cells.