Hematopoietic Tissue

The hematopoietic system refers to the whole system of blood cell production sites in the body including hematopoietic organs and hematopoietic cells, as well as the blood cells themselves. Blood cell production sites (hematopoietic sites) vary during various stages of fetal development and postnatally. During fetal life the hematopoietic sites include the yolk sac (which is the initial site of hematopoiesis), liver, spleen, thymus, lymph node and finally the bone marrow. Postnatally, new blood cell formation (hematopoiesis) is primarily carried out in the bone marrow. Thus, in normal adults, the hematopoietic system consists of the bone marrow and the cells it produces (leukocytes, erythrocytes, and blood platelets).

The hematopoietic system is one the largest systems of the body. Hematopoiesis, which is the process of blood cell production, starts during fetal life and continues throughout the life of the individual. This is because blood cells are short-lived and must be replaced by new ones. Thus, new blood cells are produced steadily by hematopoietic tissues to replace the old and lost ones; more than 100 billion blood cells are lost and replaced daily. Thus, the hematopoietic system is a self-renewing system.

The earliest indication of blood cell production is the appearance of blood island in the splanchnopleure of the embryonic yolk sac. Splanchnopleure comprises the endoderm and the splanchnic mesoderm. Blood islands of the splanchnopleure gradually differentiate into blood cells and endothelial cells of blood vessels.

Fig. L1. The figure shows developing red blood cells within a blood island in the splanchnic mesoderm of the yolk sac.

The Bone Marrow

In late fetal period the bone marrow becomes the primary hemopoietic tissue and continues to be so after birth. Bone marrow occupies marrow spaces within bones; these include the medullary cavity of long bone and the marrow spaces of spongy (cancellous) bone. There are two types of bone marrow:

1.      Red marrow, which is haematopoietically active.

2.      Yellow marrow, which is inactive haematopoietically.

In newborns, all bone marrow is red marrow. During childhood, yellow marrow gradually replaces red marrow in many locations. Ultimately the red marrow becomes confined to certain adult bones that include the sternum, vertebrae, ribs, clavicles, scapula, pelvis, cranium and epiphysis of long bones.

Fig, L2. The illustration shows sites of red bone marrow in the Skeleton of adults. Red bone marrow is confined to locations shown black in this illustration.   

RED BONE MARROW

Red bone marrow takes its name from its macroscopic appearance. It is present in all bones during intrauterine life and early childhood, occupying all marrow cavities of spongy bones and the medullary cavity of long bones. At about the age of 6, the red bone marrow starts to transform into yellow narrow gradually. In adults, it is confined to flat bones such as the pelvis, cranial bones, vertebrae and ribs and in epiphysis and metaphysis of long bones. It appears red to the naked eye because it contains numerous RBCs. It is a gelatinous or spongy tissue where blood cells are continuously produced. By doing so, it maintains adequate numbers and percentages of blood cells. It also participates in eliminating damaged ones. Red bone marrow has three main components, which are the

1.      Sinusoids

2.      Reticular connective tissue (stroma)

3.      Hemopoietic cords (free cells)

Sinusoids of the Bone Marrow

Bone marrow sinusoids are also known as sinuses, though technically they are sinusoids. They are wide blood vessels lined with endothelium only. They are supplied by the nutrient artery that supplies the bone and sends branches towards the marrow cavities. Arterioles originating from these branches supply the network of marrow sinusoids. The sinusoids drain into venules that leave the bone marrow. The endothelium lining the bone marrow sinusoids is a continuous endothelium without fenestrae, but rests on a discontinuous basal lamina. Accordingly, the bone marrow circulation is a closed circulation. However, there is a continuous exchange of cells and molecules between the bone marrow and blood circulation across the endothelium, i.e. a transendothelial exchange. It is a selective exchange controlled by the endothelial cells. The bone marrow is devoid of lymphatic vessels, thus new blood cells produced in the bone marrow can gain access to the circulation only through the marrow sinusoids.

Fig. L3. A histological section of the bone marrow showing a sinusoid within a hematopoietic cord. The sinusoid endothelium is a single layer of flat cells. 

Stroma of the Bone Marrow

Bone marrow is supported by a loose connective tissue called the bone marrow stroma.  It is a loose reticular connective tissue which forms a 3-dimentional supportive network. It consists of reticular fibers and reticular cells, which are also known as stromal cells. Reticular fibers cannot be seen in routinely stained histological sections; they can be seen in histological sections stained by special methods such as the Gordon & Sweet method, or with the electron microscope. Stromal cells are usually located in the vicinity of sinusoids. They have long branching processes passing between developing blood cells providing support. Stromal cells secrete colony stimulating factors (CSFs) that promote hematopoiesis. The ground substance of the stroma contains cytokines and cell-binding substances such as proteoglycans, fibronectin, laminin and hemonectin.

Fig. L4. The section on the left shows a reticular cell characterized by long processes passing in different directions between the hematopoietic cells. The section on the right shows slender black branching thread-like reticular fibers forming a network. Both sections are stained with the Gordon and Sweet silver staining technique.

Hematopoietic Cords of the Bone Marrow

Hemopoietic cords of the bone marrow are made of red and white blood cells at different stages of development, in addition to stem cells, megakaryocytes, macrophages, mast cells and a few fat cells. Careful examination reveals that the different types of blood cells develop in separate clusters forming cords or nests. Erythrocytic nests and megakaryocytes are often present close to the sinusoids, whereas granulocyte nests are located further away from the sinusoids.

Fig. L5.  An illustration of the red bone marrow showing white and red blood cells at different stages of development. It also shows a megakaryoblast, a macrophage, and part of an adipocyte.

Yellow Bone Marrow

After birth the red bone marrow starts to be replaced by yellow bone marrow. By the age of seven, about half the red bone marrow is already replaced by yellow bone marrow. After puberty red bone marrow is reduced greatly and becomes confined to flat and irregular bones, such as the pelvis, sternum, skull, ribs, vertebrae, and shoulder blades. Red marrow is also present in the metaphysis and epiphysis of long bones. Replacement of red marrow by yellow marrow is accomplished by the degradation and disappearance of the hematopoietic cords and the concurrent appearance of adipocytes in large numbers eventually forming adipose tissue. The source of adipocytes are the adipocyte stem cells  and the mesenchymal stem cells present the bone marrow stroma. Yellow marrow increases with age and is hematopoietically inactive but may be reactivated in cases of urgent need for blood formed elements. In addition to mature adipocytes, the yellow bone marrow contains hematopoietic stem cells and adipocyte stem cells. Red and yellow bone marrow together constitute about 5% of the body weight, thus constituting double the size of the liver.

Fig. L6. An H&E-stained histological section of the yellow bone marrow. Numerous adipocytes form an adipose tissue that fills the marrow spaces. 

HEMATOPOIESIS

Hematopoiesis (formerly called hemopoiesis) is the process whereby blood cells and platelets are produced. About 150 billion blood cells are formed each day. White blood cells, red blood cells and blood platelets are collectively known as the blood formed elements. All formed elements of blood are derived from a common multipotent mother cell called the hematopoietic stem cell, hemocytoblast, the spleen colony forming units (CFU-s). The hematopoietic stem cell (HSC) is a rare cell constituting only about 0.01% of the nucleated cells of hemopoietic tissues. Most HSCs are quiescent cells that remain in the G₀ phase of the cell cycle. A few HSCs undergo mitosis continuously; these are known as cycling UFU-Ss. They produce two types of daughter cells:

1.      One daughter cell is identical to the cycling mother cell and replaces the mother cell as a cycling CFU-S for self-renewal.

2.      The other daughter cell transforms into a committed progenitor cell that proliferates and enters several differentiation steps giving rise to large numbers of blood cells of different types and platelets.

Both daughter cells are CD34+. The committed progenitor daughter is the mother cell of all RBCs, WBCs and platelets.

Fig. L7.  The illustration shows self-renewal of the cycling hemocytoblast (CFU-s) and the concurrent development of the committed daughter stem cell into different types of blood formed elements.

Erythropoiesis

Erythropoiesis is the process of formation of red blood cells within blood forming tissues of the body. In adults, erythropoiesis takes place exclusively in the bone marrow. The process lasts about 1-2 weeks and yields about 2.5 million RBCs / second. The mother cell of RBCs and all other blood formed elements are the hemopoietic stem cells (HSCs), which are undifferentiated pluripotential cells. They are characterized by a large vesicular nucleus surrounded by a thin rim of basophilic cytoplasm. A nucleolus is often present. HSCs undergo slow mitotic divisions; some of their daughter cells gradually lose pluripotency and become erythroid burst forming units (BFU-Es). The BFU-Es proliferate rapidly forming erythroid colony forming units (CFU-Es). Erythropoiesis takes place prenatally and postnatally in the following sites in a chronological order:

• In blood islands of the yolk sac in embryos 2-5mm long.

• In the liver and the spleen in embryos 5-7mm long.

• In the bone marrow in fetuses 20 weeks old and more.

• In the medullary marrow of all bones in children before puberty persons.

• In the pelvis, ribs, sternum, vertebrae, cranium and the epiphysis of long bones in adults. Elsewhere, red marrow is replaced by yellow bone marrow.

However, erythropoiesis may resume in certain sites such as the liver and the spleen in certain diseases and chronic stressful conditions.

Erythropoiesis is a sequence of events that comprise the development of CFU-Es into basophilic erythroblasts, then into polychromatic erythroblast, then into normoblasts, then into reticulocytes and finally into mature erythrocytes.

Basophilic Erythroblasts

The CFU-Es, also known as proerythroblasts, are unipotential stem cells; they produce only one type of mature cells which are mature erythrocytes. CFU-Es start to accumulate ribosomes, an essential prerequisite for the synthesis of hemoglobin. The cytoplasm gradually becomes packed with free ribosomes that impart a basophilic (bluish) appearance to the cytoplasm. In this way the CFU-Es differentiate into basophilic erythroblasts, which are smaller than the CFU-Es. Basophilic erythroblasts have a dense nucleus that lacks a nucleolus. Disappearance of the nucleolus indicates cessation of synthesis of ribosomes. The cytoplasm appears basophilic because it contains large amounts of acid (ribonucleic acid of ribosomes).

Polychromatic Erythroblast

Newly synthesized hemoglobin starts to form eosinophilic (acidophilic) islands in the basophilic cytoplasm. Hence, the cytoplasm becomes heterochromatic, parts of it appearing pinkish and other appearing bluish. At this stage, the cell is called polychromatic erythroblast. Gradually more as more hemoglobin is laid the cytoplasm gradually becomes homogenously acidophilic.

Fig. L8. The figure shows three stages in the development of erythrocytes. The basophilic erythroblast has a basophilic large nucleus surrounded by a rim of basophilic bluish cytoplasm. The polychromatic erythroblast is smaller and has more cytoplasm that appears bluish pinkish cytoplasm. In the orthochromic erythroblast, the cytoplasm is acidophilic pinkish, and the nucleus is condensed and pyknotic.

Orthochromic Erythroblast

Orthochromic erythroblasts are more commonly known as normoblasts. The nucleus of the normoblasts gradually condenses and is then extruded. The resulting non-nucleated cell is called mature erythrocyte or red blood corpuscle (RBC). There is yet a clinically important stage in the development of erythrocytes called the reticulocyte stage.

Fig. L9. The micrograph shows the process of the extrusion of the nucleus of the normoblast. The nucleus pyknotic. After extruding its nucleus, the normoblast becomes an anucleate mature erythrocyte (RBC).

Reticulocytes

Reticulocytes are immature red blood cells released into circulation. They usually stay 3 days in the bone marrow and one day in blood circulation before they transform into typical mature erythrocytes. They are slightly larger than mature RBCs. During formation of reticulocytes organelles including the Golgi apparatus, lysosomes and mitochondria break down and remnants are expelled. Ribosomes decrease in amount, but a small amount remains and can be seen in blood films using special techniques such as supravital staining with brilliant cresyl blue. They continue synthesizing hemoglobin while circulating. When reticulocytes are released into circulation about 20% of their hemoglobin is yet to be synthesized; hemoglobin synthesis is completed within 24-48hrs. Reticulocytes constitute about 1% of blood erythrocytes. The percentage of reticulocytes in peripheral blood reflects the rate of RBC formation and loss. Severe RBC loss, as in hemolytic anemia and hemorrhage, causes an increase in the amount of reticulocytes in blood (reticulocytosis).

Fig. L10. The light micrograph (left) taken from a blood film shows the characteristic features of reticulocyte.  The cytoplasm is eosinophilic and contains large ribosomal aggregations displayed as basophilic purplish clumps (arrows). The illustration in the middle demonstrates the presence of ribosomal remnants in reticulocytes, whereas the mature RBC on the right is devoid of any remnants.  

Erythrocytes

Erythrocytes or red blood corpuscles (RBCs) are small (average diameter 7.2um) biconcave corpuscles full of hemoglobin. They are homogenously acidophilic, yet their central part appears paler because they are biconcave. Biconcavity increases the surface area of RBCs by about 25%, thus greatly enhancing their efficiency in gaseous exchange. RBCs are supported by a cytoskeleton made of a fibrous protein called spectrin. The life span of RBCs is about 120 days. Ailing RBCs are got rid of mostly in the spleen.

Fig. L11. The illustration shows the stages of erythropoiesis.

Erythropoietin

Erythropoietin is the key factor in promoting the process of red blood cell formation (erythropoiesis). It is produced by the kidney and taken by blood to the red bone marrow where it promotes survival of erythrocyte progenitor cells and enhances development of erythrocytes. Erythropoietin (EPO) is recognized by erythropoietin receptors (EPO-R) present on cell membranes of proerythrocytes. 

Fig. L12. The illustration shows the relationship between arterial oxygen tension, erythropoietin production & erythropoiesis.

Granulopoiesis

Granulopoiesis is the process whereby pluripotential hematopoietic stem cell differentiates to give rise to granulocytes (neutrophils, eosinophils and basophils). In adults, it takes place in the bone marrow (red bone marrow). Granulopoiesis involves a succession of cells divisions and differentiation whereby stem cells give rise to myeloid progenitor cells that give rise to myeloblasts which develop into promyelocytes, to myelocytes, to metamyelocytes, to mature granulocytes.

Fig. L13. The illustration shows the steps of granulopoiesis whereby basophils, neutrophils and eosinophils develop from the hemocytoblast (CFU-S)

Myeloblasts

The common myeloid progenitor cell proliferates rapidly and differentiates into myeloblasts. Myeloblasts are 15-20 um in diameter and have a large nucleus containing 2-3 nucleoli. The cytoplasm is basophilic (darker in the peripheries) and devoid of granules. It is larger than the erythroblast. In adult humans, myeloblasts give rise to about 4 x 108 granulocytes per hour (as compared to 10¹⁰ erythrocytes/hour originating from erythroblasts).

Fig. L14. The illustration shows the origin and microscopic features of myeloblasts.

Promyelocytes

These are large cells (16-24um) with a round or indented nucleus showing coarse chromatin with or without nucleoli. Mitotic activity is high (causing reduction in cell size). Cytoplasmic basophilic and acidophilic non-specific (primary, azurophilic) granules begin to appear. These non-specific granules are lysosomes. Their number gradually decreases during the subsequent stages of granulopoiesis concomitant with a decrease in Golgi complexes and rER. Up to this stage, the process of development of all three types of granulocytes is essentially the same.

Fig. L15. The illustration shows the appearance of the primary azurophilic non-specific granules during development of myeloblasts into promyelocytes.

Myelocytes

Myelocytes have smaller and denser nuclei, and a cytoplasm that is less basophilic than that of promyelocytes. The nucleus does not contain nucleoli. The cytoplasm contains both non-specific (primary) and specific (secondary) granules of various size and shape. The specific granules could be basophilic, eosinophilic or neutrophilic. At this stage the three different types of myelocytes can be identified by the characteristics of their specific granules.

Fig. L16. The illustration shows the appearance of the secondary (specific) granules in the cytoplasm promyelocytes as they develop into myelocytes.

Metamyelocytes

Metamyelocytes are smaller than myelocytes. As in myelocytes, the cytoplasm contains both specific and non-specific (primary) granules, but the specific ones are more numerous. Metamyelocytes are characterized by bean-shaped or horseshoe-shaped nuclei. Neutrophilic metamyelocytes (juvenile) are the most numerous. They have small specific granules in addition to primary ones (lysosomes). Eosinophilic metamyelocytes have a slightly basophilic cytoplasm with eosinophilic granules. Basophilic metamyelocytes are rare, and their specific granules are metachromatic and are water soluble. They are smaller and have paler nuclei than those myelocytes.

Fig. L17. The illustration shows transformation of myelocytes into metamyelocytes. Cytoplasmic granules remain the same, but the nucleus has become horseshoe shaped.

Fig. L18. A transmission electron micrograph showing electron microscopic features of two metamyelocytes. Both have horseshoe-shaped nuclei and a cytoplasm containing large moderately electron-dense granules.

Mature Granulocytes

Mature granulocytes are characterized by lobulated or bean shaped nuclei and a cytoplasm that contains two types of granules: specific and nonspecific granules. According to the staining characteristics of the specific (secondary) granules mature granulocytes are classified into neutrophils, eosinophils and basophils.

Mature Neutrophils (Polymorphs)

Neutrophils are the most abundant type of white blood cells constituting 50-80% of WBCs. They are 10-12um in diameter in dry smears. They are terminally differentiated cells characterized by a well-lobulated nucleus comprising 3-5 lobes interconnected by narrow strands. Neutrophils take about 10 days to develop and mature. Young neutrophils are known as band neutrophils, which constitute about 1-3% of the WBC count. They are characterized by a horse-shoe shaped nucleus. As they become older their nuclei become bilobed then multilobulated (more in older cells). Because they have nuclei of different shapes they are known as polymorphonuclear leukocytes or polys. The proportion of band (young) cells in peripheral blood is an index of entry of new neutrophils into the circulation. A drumstick structure formed by condensed X chromosome is a Barr body characteristic of female neutrophils. Neutrophils are phagocytic cells which constitute the first line of defense in the body.  They are short-lived cells, with a lifespan of 5-8 days.

The nucleus is dense and devoid of nucleoli. The cytoplasm contains azurophilic primary granules and numerous specific secondary granules. Both are small but can be differentiated histochemically.  Azurophilic (primary) granules are lysosomes that contain myeloperoxidase, acid phosphatase, defensins and glucuronidase. Specific (secondary) granules are more numerous; they contain a wide array of bactericidal and bacteriostatic substances (lysozyme, cathelicidin, lactoferrin, phagocytins). Tertiary granules (gelatinase) / secretory vesicles are small; they contain tissue degrading enzymes. Neutrophils are highly phagocytic (first line of defense). They have a short life span (a few days).

Band cells are not yet fully mature neutrophils released from bone marrow into circulation. They differ from fully mature neutrophils in that their nucleus is not lobulated. A fully mature neutrophil with a multilobed nucleus is called a segmented neutrophil. Band cells differ from segmented neutrophils in that the nucleus is U or S shaped, without lobulation. 

Fig. L19. A light microscopic picture (left) and an illustration (right) showing features of the nucleus and the cytoplasm of neutrophils. The nucleus has several lobes, and the cytoplasm contains fine granules. The three main types of cytoplasmic granules of neutrophils and the contents of each type are shown. 

Eosinophils

Eosinophils constitute about 1-6% of blood leukocytes. They are terminally differentiated cells that often present in loose connective tissues. They participate in specific immune reactions of tissues including allergic reactions. They are larger than neutrophils (about 12-17um) in size. The nucleus is often bilobed and shows no nucleoli. The cytoplasm contains coarse pink granules; these are the eosinophil’s specific (secondary) granules. Under the EM these granules appear ellipsoid and are characterized by a crystalline core. In addition, the cytoplasm contains smaller nonspecific (primary) granules.

Eosinophil specific (secondary) granules contain four types of biologically active cationic proteins. These proteins are the: major basic protein (MBP), eosinophilic cationic protein (ECP), eosinophil derived neurotoxin (EDN) and eosinophil peroxidase (EPO). These proteins play a major role in defending against helminths. The specific granules also contain lysosomal hydrolytic enzymes and histamine (related allergic reactions). Eosinophils have phagocytic capabilities particularly to immune complexes. Nonspecific granules contain acid phosphatase and gelatinase. Eosinophils produce cytokines including GM-CSF, IL-3, IL-5 and TNF-α.

Fig. L20. The illustration shows features of the eosinophil nucleus and cytoplasm. The nucleus is bilobed and the cytoplasm contains primary nonspecific and secondary specific granules, each containing specific bioactive substances.

Fig. L21. The micrograph on the left shows the light microscopic features of eosinophils; namely, a bilobed nucleus and a cytoplasmic full of pink eosinophilic specific granules. The transmission electron micrograph on the right shows the cytoplasm of an eosinophils containing the specific granules containing distinctive crystalline cores. 

Basophils

Basophils constitute 0.5-1% of the leukocytes. They are slightly larger than neutrophils and have U- or S-shaped nuclei. The nuclear chromatin is less dense than that of eosinophils. The cytoplasm contains water soluble specific granules of variable size. The granules are metachromatic and can be vitally stained by neutral red. Basophils resemble mast cells of connective tissue proper in having metachromatic granules that contain histamine, heparin and anaphylaxis mediators such as the slow reacting substances of anaphylaxis (SRS-A). Both basophils and mast cells release the contents of their granule (histamine, heparin and anaphylaxis mediators) when exposed to antigens and histamine liberating drugs.

Fig. L22. The micrograph on the left shows the light microscopic appearance of basophils. The cytoplasm is full of deep purple basophilic granules. The illustration on the right shows the nuclear features and the specific and nonspecific granules of basophils.

Agranulocyte Development

Agranulocytes are white those blood cells that don’t show clearly visible cytoplasmic granules under the light microscope in histological preparations stained routinely with blood stains as the granulocytes do. Agranulocytes are of two types, monocytes and lymphocytes. The process of development of agranulocytes is known as a granulopoiesis.  Agranulocytes develop from the pluripotent hemopoietic stem cell (CFU-S) – which is a common mother cell to all blood formed elements (granulocytes, erythrocytes, agranulocytes and blood platelets). The general morphologic changes during formation of agranulaocytes include a decrease in the overall cell diameter, a decrease in the nuclear size and an increase in the nuclear heterochromatin content. The morphologic characteristics of the agranulocytes precursors at the various stages of agranulopoiesis are much less obvious and the process is more complicated than those of erythrocytes and granulocytes.

Monopoiesis

Monopoiesis is the process of formation of monocytes from the granulocyte-monocyte colony forming units (CFU-GM), which develop from the CFU-S. Initiation of monopoiesis requires the presence of the granulocyte-monocyte colony stimulating factors (G-MCSFs). The monocyte progenitor cells (CD34+) develop from the CFU-GM, undergo cell divisions and differentiate to give rise to monoblasts, then promonocytes and then mature monocytes in order. Monopoiesis involves reduction in the cell size and indentation of the nucleus. Mature monocytes gain access to marrow sinusoid and leave bone via blood. Within 3-4 they leave blood into various tissues of the body. In tissues monocytes develop into a wide array of phagocytic cells that include connective tissue histiocytes, dust cells (lung alveolar macrophages), microglial cells, Kupffer cells, osteoclasts and Langerhans cells. Some of these phagocytic cells in addition to being phagocytic have antigen presenting capabilities. Monocytes and their progeny together form a system known as the mononuclear phagocyte system; this system was formerly called the reticuloendothelial system.  Mature blood monocytes are the largest of the white blood cells with a diameter of 15-22um. They have a kidney-shaped nucleus and a cytoplasm that appears devoid of granules in routinely stained blood films.  They are CD14+, and less the neutrophils and lymphocytes constituting about 2-10% the white blood cells.

Fig L23. Light and electron microscopic picture showing features of monocytes. In the light micrograph (left), the monocyte appears several times larger than the surrounding RBCs. It is characterized by a basophilic kidney-shaped nucleus and a cytoplasm devoid of granules. The electron micrograph (right) shows abundance of nuclear euchromatin and the presence of lysosomes (black spherical structures) in the cytoplasm of the monocyte.

Fig L24. The illustration shows a list of cells specialized phagocytic cells derived from blood monocytes and distributed throughout the body. Together they constitute the mononuclear phagocyte system.

Lymphopoiesis

Lymphopoiesis is the process of formation of lymphocytes. It differs from hematopoiesis of the other types of blood formed elements it involves hemopoietic and lymphoid organs. It starts with the differentiation of the hematopoietic stem cell (CFU-s) into unipotential lymphocytic stem cell. The unipotential lymphocytic stem cells undergo mitosis and differentiate into lymphoblasts. The lymphoblasts divide and differentiate into prolymphocytes that ultimately give rise to immature lymphocytes, which gain access to circulation. In the developing fetus, some immature lymphocytes reach the thymus and differentiate to form T-lymphocytes, while others remain in the bone marrow or reach Peyer’s patches to form B lymphocytes. Both types circulate in blood and lymph and populate lymphoid organs and propagate in lymphoid tissues. B-lymphocytes further differentiate in connective and lymphoid tissue into plasma cells which profusely synthesize and secrete immunoglobulins.

Lymphocytes

Lymphocytes are classified in different ways into different types. According to their size, they are classified into small, medium and large lymphocytes. Small lymphocytes are less than 10um in diameter (less than x1.5 of the diameter of RBCs), medium-size lymphocytes are x1.5-x2.0 the size of RBCs (about 10-14 um), whereas large lymphocytes are more than 14um in diameter.

Most circulating lymphocytes are small lymphocytes, mostly CD3+ T-lymphocytes. Small lymphocytes are characterized by a spherical dense basophilic nucleus surrounded by a thin rim of pale basophilic agranular cytoplasm. Larger lymphocytes have paler indented nuclei and more cytoplasm.

Lymphocytes are important cells of the immune system; B-lymphocytes in humoral immunity, T-lymphocytes in cell-mediated immunity and natural killer cells in innate immunity. More information will be given on lymphocytes when dealing with the immune system.

Fig. L25. The Illustration shows the stages of development of the different types of lymphocytes (lymphocytopoiesis) in the bone marrow, thymus and Peyer’s patches.

Fig. L26. The micrographs show the light and electron microscopic features of lymphocytes. The micrograph (left) shows a small lymphocyte (slightly larger than RBCs) with a dense spherical nucleus filling the cell; two larger lymphocytes with paler nuclei are also present. The electron micrograph (right) shows a small lymphocyte with a spherical nucleus almost filling the cell. The nucleus rich in heterochromatin, and the cytoplasm is poor in organelles.

Fig. L27. The illustration shows classification of lymphocytes based on their functions.

Thrombopoiesis

Thrombopoiesis is the process of blood platelet formation. Blood platelets are the smallest of the blood formed elements, with a diameter of about 2um. They are short lived non-nucleated cytoplasmic fragments with a life span of 8-10 days. Their normal blood count is 150,000-450,000/µl. They participate in hemostasis by preventing blood bleeding. They also participate in wound healing. Platelets develop in the bone marrow from giant cells, about 100um in diameter, known as megakaryocytes. Megakaryocytes in turn develop from the multipotent hemocytoblasts (CFU-S) by a process called megakaryopoiesis, which comprises four stages: 1) the stage of proliferation, 2) the stage of endomitosis, which leads to polyploid megakaryoblasts containing many nuclei, 3) the post-replicative stage where megakaryocyte cytoplasmic maturation takes place, 4) the stage of platelet formation. Megakaryocytes shed large quantities of platelets into the bloodstream. Microtubules present in the cytoplasm of megakaryocyte facilitate the process of platelet shedding. Each megakaryocyte releases about 1,000-3,000 platelets into the blood stream. Thrombopoietin which promotes thrombopoiesis is produced by hepatocytes, hepatic sinusoidal cells and cells the proximal tubule of the kidney. Interleukins IL-3, IL-6, and IL-11 also participate, along with thrombopoietin, in enhancing thrombopoiesis.

Megakaryocytes are huge cells with abundant cytoplasm. The cytoplasm of megakaryocytes possesses three zones: the perinuclear zone, the intermediate zone and marginal zone. The perinuclear zone ribosomes, rER, sER, Golgi complexes and mitochondria. The intermediate zone contains all afore-mentioned organelles in addition to platelet granules. A characteristic feature of the intermediate zone is the large numbers of vesicles that originate from the smooth endoplasmic reticulum. These vesicles coalesce to form an extensive interconnected network of cisternae and tubules known as the demarcation membranes. The demarcation membranes are continuous with the cell membrane. The third zone is the marginal zone, which is almost devoid of organelles and inclusions. Shedding of platelets takes place from the marginal zone but by the extension of the demarcation membranes from the intermediate zone towards the cell membrane and the subsequent separation of the delineated parts from the rest of the intermediate zone cytoplasm forming blood platelets. Accordingly, blood platelets contain all organelles and inclusions present in the intermediate zone of megakaryocytes. In other words, blood platelets are pieces of the intermediate zone cytoplasm pinched off during shedding.

Fig. L28.  The illustration shows the different stages of thrombopoiesis. Note increase in size of the cell and number of nuclei during this process.

Platelets released by megakaryocytes stay in the bloodstream for about a week, and then perish. Platelets participate in hemostasis and thrombus formation. Vascular injury causes platelets to stick to the exposed connective tissue and compile forming a fibrin and platelet plug.

Fig L29. The micrograph on the left shows that megakaryocytes are multinucleated, the electron microscope in the middle numerous channels (demarcation membranes in the cytoplasm of megakaryocyte, whereas the illustration on the right demonstrates the process of shedding of platelets by a megakaryocyte.  

Thrombopoietin, which is the main regulator of thrombopoiesis, is produced constantly by the liver and the renal tubular epithelial cells. Platelets clear and destroy excess blood thrombopoietin to prevent over concentration of plasma thrombopoietin in feedback-like mechanism.

Platelets are non-nucleated, discoid-shaped blood formed elements that are essential for hemostasis, which serves to maintain the integrity of the vasculature upon injury. In addition to their role in hemostasis, platelets participate in inflammatory processes, innate immunity, angiogenesis and wound healing. Granule exocytosis is central to the platelet functions and activities. There are several kinds of platelet granules. Most prominent of these are the α-granules, dense granules, lysosomes and peroxisomes. Dense granules are the most abundant; they contain serotonin and histamine; α granules contain fibrinogen, fibronectin and calcitonin, whereas lysosomes contain hydrolytic enzymes.

Fig. L30. The light micrograph on the left shows two platelets appearing as tiny basophilic non-nucleated purplish structures, whereas the electron micrograph on the right shows several platelets as non-nucleated ovoid cytoplasmic fragments containing numerous granules. Part of an RBC (black) is seen in the bottom.

Blood

Blood is specialized connective tissue made of widely separated cells (the blood cells) and a fluid matrix, known as the blood plasma. Blood cells are classified into white blood cells which are nucleated and red blood cells which are nonnucleated. In addition, there are small cellular fragments called blood platelets. The plasma is equivalent to the connective tissue matrix and fibrinogens that form fibers during blood clotting are equivalent to the connective tissue fibers. After removal of fibrinogens, the blood plasma is called serum, which is made up of water and blood proteins except fibrinogens. It contains antibodies, antigens and hormones, in addition to electrolytes. Blood cells and platelets are short-lived and are continuously replaced by new cells produced in the bone marrow and lymphoid tissues. Each has its specific functions and normal counts.

Features and functions of blood cells and the platelets are summarized below.

Fig. L31. The illustration summarizes the main features and functions of erythrocytes, platelets and agranulocytes (monocytes and lymphocytes).

Fig. L32. The illustration summarizes the main features and functions of granulocytes (neutrophils, eosinophils and basophils).

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