Histology of Blood PDF

HISTOLOGY OF BLOOD













UMANAH, S. J. PhD Learning Objectives Identify the component cells of a typical blood smear Distinguish the different classes of white blood cells and the conditions under which each would be expecte...

HISTOLOGY OF BLOOD













UMANAH, S. J. PhD Learning Objectives Identify the component cells of a typical blood smear Distinguish the different classes of white blood cells and the conditions under which each would be expected to dominate Know some key pathological examples related to bone marrow and peripheral blood Blood is a fluid connective tissue ---- the mesenchymal origin of its cells ----- a low ratio of cells to liquid intercellular substance, the blood plasma. In human adults about 5 liter of blood contribute 7-8 % to the body weight of the individual. The contribution of red blood cells (erythrocytes) to the total volume of the blood (haematocrit) is about 43%. The formed elements of blood include; Erythrocytes(99%), Leucocytes ,platelets Erythrocytes and blood platelets perform their functions exclusively in the blood stream.  leukocytes reside only temporarily in the blood.  Leukocytes can leave the blood stream through the walls of capillaries , venules and enter either connective or lymphoid tissues. Erythrocytes or Red blood cells are the most numerous type of cell found in blood. One microlitre of blood contains around 5 million cells. They are essential for transport of carbon dioxide and oxygen around the body. They are 'born' and mature in the bone marrow ( haemopoesis). When they mature, they make haemoglobin, the protein that binds oxygen. Haemoglobin can also bind carbon dioxide, but at a different site to that for oxygen. Eventually around 90% of the dry weight of the cell is made up of this protein. The nucleus is lost from the cell, phagocytosed by macrophages, and the DNA broken down.  The red blood cells can then enter the circulation.  EM shows the biconcave shape of red blood cells.  Partly due to the arrangement of the actin cytoskeleton that lies just underneath the plasma membrane,  Partly because the nucleus is absent. This means that the cells can deform easily,  Have a high surface area/volume ratio - good for exchange. These cells only live for about 120 days  The iron in the haemoglobin is extracted from the erythrocytes by the liver and spleen, The remaining heme is excreted by the liver as bile pigments. Around 3 million RBCs die and are scavenged by the liver each second.  WHITE BLOOD CELLS ( LEUCOCYTES) White blood cells are much less common than red blood cells. There are five types of white blood cell (leucocyte). These are divided into two main classes Granulocytes (includes Neutrophils, Eosinophils and Basophils) Agranulocytes (includes Lymphocytes and Monocytes). In healthy individuals the relative numbers of circulating leukocyte types are quite stable. A differential leukocyte count would typically produce the following cell frequencies reported in different texts): ~ 60% neutrophils (50% - 70%) ~ 3% eosinophils (>0% - 5%), ~ 0.5% basophils (>0% - 2%) ~ 5% monocytes (1% - 9%) ~ 30% lymphocytes(20% - 40%) Classification depends on whether granules can be distinguished in their cytoplasm using a light microscope and conventional staining methods). All the white blood cells are able to move like an amoeba, and can migrate out of blood vessels into the surrounding tissues. To identify them, look for the shape of the nucleus, and compare their size, relative to that of a red blood cell.  NEUTROPHILS Below is a neutrophil in a blood smear. The neutrophils are 12-14 µm diameter, and so look bigger than the surrounding red blood cells. There is a single nucleus, which is multilobed, and can have between 2 and 5 lobes. The chromatin in the nucleus is condensed. This means that there isn't protein synthesis. There are few organelles in the cytoplasm. Neutrophils are the commonest type of white blood cell found in a blood smear. They make up 60-70% of the total amount of white blood cells.  have a very characteristic nucleus. Which is divided into 3-5 lobes connected by thin strands of chromatin.  The number of lobes increases with cell age. Up to 7 lobes can be found in very old neutrophils (hypersegmented cells). Neutrophils contain all the organelles that make up a typical cell. In addition they also contain two types of granules. Primary granules (or A granules) contain lysosomal enzymes and are likely to be primary lysosomes, although they are larger (0.4 µm) than the "ordinary" primary lysosome. Secondary granules (or B granules), the specific granules of the neutrophils, contain enzymes with strong bactericidal actions. The specific granules of neutrophils stain only weakly if they are at all visible - they are "neutral", hence the term neutrophil.  FUNCTIONS Neutrophils play a central role in inflammatory processes.  Large numbers invade sites of infection in response to factors (e.g. cytokines) released by cells which reside at an infection site.  Neutrophils are the first wave of cells invading infection sires. Receptors in their plama membrane allow them to recognise foreign bodies, e.g. bacteria, and tissue debris, which they begin to phagocytose and destroy. The phagocytotic activity of neurophils is further stimulated if invading microorganisms are "tagged" with antibodies (or opsonised). Neutrophils cannot replenish their store of granules. The cells die once their supply of granules has been exhausted. Dead neutrophils and tissue debris are the major components of pus. Their lifespan is only Lost neutrophils are quickly replenished from a reserve population in the bone marrow. Because they are younger, their nuclei have fewer lobes than the "average" neutrophil. A high proportion of neutrophils, with few nuclear lobes indicates a recent surge in their release from the bone marrow. EOSINOPHIL GRANULOCYTES (EOSINOPHILS) Their nucleus usually has only two lobes. Almost all of the cytoplasm appears filled with the specific granules of the eosinophils. As the term "eosinophil" indicates, these granules are not neutral but stain red or pink when stained with eosin. Eosinophils contain some large rounded vesicles (up to 1 µm in diameter) in their cytoplasm. These granules correspond to the eosinophilic grains.  These specific granules contain, in addition to enzymes that otherwise are found in lysosomes, an electron-dense, proteinaceous crystal. This crystal is composed of major basic protein (MBP) This picture above shows an eosinophil in a blood smear. These cells are 12 - 17 µm in diameter - larger than neutrophils, and about 3 times the size of a red blood cell. These cells have large acidophilic specific granules - stain bright red, or reddish-purple. These granules contain proteins that are 'destructive' and toxic. Functions The presence of antibody-antigen complexes stimulates the immune system. Eosinophils phagocytose the antibody-antigen complexes, andprevent the immune system from "overreacting". Their granules also contain the enzymes histaminase and arylsufatase. These enzymes break down histamine and leukotrienes,  may dampen the effects of their release by basophils or mast cells accompanied by an increase in the number of eosinophils. MBP, which can also function as a cytotoxin, Its release by eosinophils may be involved in the response of the body against parasitic infections.  BASOPHILS Basophilic granulocytes have a 2 or 3 lobed nucleus. The lobes are usually not as well defined as in neutrophilic granulocytes and the nucleus may appear S-shaped. The specific granules of basophils are stained deeply bluish or reddish-violet They contain heparin, histamine, lysosomal enzymes and leukotrienes. Function

These cells are involved in immune responses to parasites. They have IgE receptors and the granules are released when the cells bind IgE. These cells also accumulate at sites of infection, and the release of prostaglandins, serotonin and histamine help to increase blood flow to the area of damage, as part of the inflammatory response. Heparin and histamine are vasoactive substances. They dilate the blood vessels, make vessel walls more permeable and prevent blood coagulation. They facilitate the access of other lymphocytes and of plasma-borne substances of importance for the immune response NON-GRANULAR LEUKOCYTES MONOCYTES These cells can be slightly larger than granulocytes (~ 12-18 µm in diameter). Their cytoplasm stains usually somewhat stronger than that of granulocytes, Does not contain any structures which would be visible in the light microscope using most traditional stains (a few very fine bluish gains may be visible in some monocytes) Monocyte has a C-shaped nucleus. Monocytes contain granules (visible in the EM) In appearance and content correspond to the primary granules of neutrophils, i.e. the granules correspond to lysosomes. FunctionS:
Monocytes in the circulation are precursors of tissue macrophages that are actively phagocytic. Monocytes circulate in the blood for 1-3 days, then migrate into body tissues, where they transform into macrophages. They will phagocytose dead cells and bacteria. Some monocytes can also transform into osteoclasts. Monocytes are important in the inflammatory response.  Lymphocyte These are the second most common white blood cell (20-50%), and are easy to find in blood smears. Although the cells look similar there are two main types, B-cells and T-cells. B-cells develop in the bone marrow. T cells are born in the bone marrow, but are matured in the Thymus. Functions
The B-cells develop into plasma cells which make antibodies, The T-cells attack viruses, cancer cells, and transplants. BLOOD PLATELETS (OR THROMBOCYTES) Blood platelets do not contain a nucleus.  Blood platelets are fragments of the cytoplasm of very large thrombocyte precursor cells, the megakaryocytes. Like other cells involved in the formation in blood cells,  megakaryocytes are found in the bone marrow. Platelets are about 3 µm long. Their cytoplasm is divided into two zones: an outer hyalomere, which hardly stains,  an inner granulomere, which contains bluish staining granules. These granules are usually not individually visible with the highest magnification and  the granulomere appears more or less homogeneously blue In the thrombocytes are present different types of vesicles (i.e. the granules),  mitochondria, ribosomes, lysosomes and  a little ER are present in the thrombocyte granulomere. Different types of vesicles contain either serotonin (electron-dense delta granules; few)  compounds important for blood coagulation They also contain platelet-derived growth factor (PDGF) and play a role in the repair of damaged tissue). The hyalomere contains cytoskeletal fibres, which include actin and myosin. Functions Platelets assist in haemostasis, the arrest of bleeding. Serotonin is a potent vasoconstrictor. The release of serotonin from thrombocytes, which adhere to the walls of a damaged vessels, is sufficient to close even small arteries. Platelets, which come into contact with collagenous fibers in the walls of the vessel which are not usually exposed to the blood stream, swell, become "sticky“ Activate other platelets to undergo the same transformation. This cascade of events results in the formation of a platelet plug (or platelet thrombus).  Finally, activating substances are released from the damaged vessel walls and from the platelets. These substances mediate the conversion of the plasma protein prothrombin into thrombin.  Thrombin catalyzes the conversion of fibrinogen into fibrin, which polymerizes into Platelets captured in the fibrin net contract leading to clot retraction, which further assists in haemostasis Blood coagulation is a fairly complex process, which involves a large number of other proteins and messenger substances. Deficiencies in any one of them, either inherited or acquired, will lead to an impairment of haemostasis.  Haemopoiesis Haemopoiesis is the process by which mature blood cells develop from precursor cells. It continues continuously throughout embryonic and adult life As a result new cells formed in the haemopoietic regions constantly replace mature blood cells in the circulation.
 During embyonic and foetal development haemopoiesis commences in wall of the yolk sac. After the second month of foetal development, the liver, slightly later, the spleen, become the dominant sites of haemopoiesis From the 6th month, and dominating from the 7th month onwards, the formation of blood cells occurs in bone marrow, -- the major site of In the adult, erythrocytes, granulocytes, monocytes and platelets are formed in the bone marrow The lymphocytes are formed mainly in the lymph nodes, spleen, thymus and lymphatic nodules of the gastrointestinal tract.  However, the lymphocytes present in these organs originate directly or indirectly from the bone marrow. Haemopoiesis occurs in red bone marrow, found between the trabeculae of spongy bone in the epiphysis of adult long bones. Both age and demands on haemopoiesis may effect the relative amounts of red and yellow bone marrow.  Haemopoietic cells surround the vascular sinusoids and are supported by reticular connective tissue. Haemopoietic Cells Haemopoiesis begins with a small population of self-replicating stem cells, which ultimately can generate all types of blood cells. Their progeny may develop into either lymphocytic stem cells or pluripotent haemal stem cells (colony-forming unit - stem cell - CFU- S). The latter type gives rise to stem cells which can form the major groups of blood cells other than lymphocytes. Depending on their progeny it is possible to differentiate; burst-forming unit of the erythroid line (BFU-E), colony-forming unit - granulocytes and macrophages (CFU-G/M), and colony-forming unit - megakaryocytes (CFU- Mk). FORMATION OF ERYTHROCYTES The first identifiable stage of erythropoiesis is the proerythroblast – a large, slightly basophilic cell, which contains a large, lightly stained nulceus. Proerythroblasts proliferate to generate a sequence of cells with decreased size and condensed chromatin.  Named after changes in the staining characteristic of their cytoplasm (basophilic erythroblast, polychromatophilic and orthochromic normoblasts). The nucleus is finally extruded from the normoblast.  The cell enters circulation as a reticulocyte, which still contains some organelles and stain slightly basophilic.  Reticulocytes remain for a few days in either the bone marrow or the spleen to mature to erythrocytes.
 FORMATION OF GRANULOCYTES Myeloblast appear similar to proerythroblast. They proliferate to generate promyelocytes. Promyelocytes begin to accumulate nonspecific granules, but they are still able to divide. The maturation of their progeny,myelocytes,is characterised by the accumulation of specific granules and changes in nuclear morphology. Metamyelocytes have a C-shaped nucleus.


Promyelocyte that becomes irreversibly committed to the neutrophilic cell line. This cell is large, with a large round nucleus, prominent nucleoli, and purple azurophilic granules. These granules are primary, nonspecific granules. Promyelocytes also give rise to eosinophils and basophils The myelocyte stage is characterized by the production of secondary, specific granules. The metamyelocyte has a flattened nucleus with condense chromatin. The band cell has a horseshoe-shaped nucleus that is "immature." it will mature into a segmented nucleus with multiple lobes. It will then be a mature neutrophil. Eosinophils and basophils undergo sequential stages of differentiation in a very similar manner to those of neutrophils. Their specific granules are also produced during the myelocyte stage.  FORMATION OF BLOOD PLATELETS (THROMBOCYTES) Platelets are fragments of the cytoplasm of megakaryocytes. Megakaryocytes are very large cells (up to 160 µm in diameter), which contain very large, highly lobulated,polyploid nuclei.  Megakaryocytes are in turn the product of the differentiation of basophilic megakaryoblasts.  CLINICAL CORRELATE Precursors of blood cells usually only found in the bone marrow may be found in peripheral blood in a variety of pathological conditions.
1. If a Rh-negative mother has been immunised by erythrocytes of a Rh-positive foetus, a condition called Erythroblastosis fetalis may develop during subsequent pregnancies. It would show itself in the foetus of the newborn by the presence of erythrocyte precursors in peripheral blood 2. Chronic myeloid leukemia - in this case showing itself by the presence of all types of granulocyte precursors in peripheral blood.  REVIEW QUESTIONS 1. What is the difference between blood, plasma, and serum? 2. A patient has been experiencing fevers and fatigue. His white cell count comes back and you see that his neutrophil levels are elevated. What type of infection do you suspect? 3. Write the name of the mature cell that is associated with the lineage including each of the following cells: megakaryocyte band cell normoblast promyelocyte reticulocyte promonocyte