blood, formation, composition and types of cells

blood

1. Blood (Components and function of Blood and its components, hematopoiesis)

2. Blood: integrates tissues and organs and provide a special means of communications. complex mixture of cells, water, proteins and sugars transports nutrients, oxygen, and hormones to all parts of the body helps regulate body temperature helps maintain stability of the body’s fluid volume transports waste products away from body cells An average 70 kg man has almost 5L blood (5.5 kg). Blood circulates throughout the body and supports the functions of all other body tissues.

3. PHYSICAL PROPERTIES OF BLOOD Specific gravity: - Whole blood: - Plasma: Viscosity: Mass: Blood volume: 1.055 - 1.065 1.024 - 1.028 5-6 times that of water. 6-8% of the body weight. ~ 8% of body weight. ~ 86% ml/kg body weight. 5-6L in adults pH—the normal pH range of blood is 7.35 to 7.45, which is slightly alkaline. Venous blood normally has a lower pH than does arterial blood because of the presence of more carbon dioxide. Osmotic pressure: 7-8 atmosphere at body temperature.

4. Composition of Blood Formed Elements (45%), i- Red blood cells ii- White blood cells iii- Platelets (erythrocytes). (leukocytes). (thrombocytes) Fluid medium i.e. the plasma (55%).

5. BLOOD CELLS There are three kinds of blood cells: red blood cells, white blood cells, and platelets. Blood cells are produced from stem cells in hemopoietic tissue . After birth this is primarily the red bone marrow , found in flat and irregular bones such as the sternum, hip bone, and vertebrae. Lymphocytes mature and divide in lymphatic tissue , found in the spleen, lymph nodes, and thymus gland. The thymus contains stem cells that produce T lymphocytes, and the stem cells in other lymphatic tissue also produce lymphocytes.

6. BLOOD CELLS (Red blood cells) (White blood cells) Erythrocytes Leukocytes Granulocytes Neutrophils Basophils Eosinophils Monocytes Lymphocytes T B Megacaryocyte (Platelets)

7. NORMAL RANGES RBC 4.6 – 6.2 x 10 12 /l. 4.2 – 5.4 x 10 12 /l Men Women Total number of red cells in circulation = 2.5x10 13 WBC Men and women 5-7 x 10 9 /l. Platelets Men and women 250 x 10 9 /l Hb Men W omen 14 – 16 g/l 12 – 16 g/l PCV (Haematocrit) Men W omen 0.42 – 0.52 l/l 0.37 – 0.47 l/l

8. Plasma Contains 91-95% water. Solutes in plasma range from 5-9% Proteins are the major solute in the plasma and their level ranges from 6-8 gm %. The solvent ability of water enables the plasma to transport many types of substances. Nutrients absorbed in the digestive tract, such as glucose, amino acids, and minerals, are circulated to all body tissues. Waste products of the tissues, such as urea and creatinine , circulate through the kidneys and are excreted in urine. Hormones produced by endocrine glands are carried in the plasma to their target organs , and antibodies are also transported in plasma . Most of the carbon dioxide produced by cells is carried in the plasma in the form of bicarbonate ions (HCO3–). When the blood reaches the lungs, the CO2 is re-formed, diffuses into the alveoli, and is exhaled.

9. Also in the plasma are the plasma proteins . The clotting factors prothrombin , fibrinogen , and others are synthesized by the liver and circulate until activated to form a clot in a ruptured or damaged blood vessel . Albumin is the most abundant plasma protein. It too is synthesized by the liver. Albumin contributes to the colloid osmotic pressure of blood, which pulls tissue fluid into capillaries. This is important to maintain normal blood volume and blood pressure. Other plasma proteins are called globulins . Alpha and beta globulins are synthesized by the liver and act as carriers for molecules such as fats. The gamma globulins are antibodies produced by lymphocytes. Antibodies initiate the destruction of pathogens and provide us with immunity.

10. Functions of Plasma proteins Transport : e.g. Transferrin transports iron. Ceruloplasmin transports copper. Albumin transports fatty acids, bilirubin calcium, many drugs etc. Transcortin transports cortisol and corticosterone Retinol binding protein transports retinol. Lipoproteins transport lipids. Haptoglobin transports free haemoglobin. Thyroxin binding globulin transports thyroxin.

11. Functions of Plasma proteins Osmotic regulation : Plasma proteins are colloidal and non-diffusable and exert a colloidal osmotic pressure which helps to maintain a normal blood volume and a normal water content in the interstitial fluid and the tissues. Decrease in albumin level results in loss of water from blood and its entry into interstitial fluids causing edema. Catalytic function (enzymes ): e.g lipases for removal of lipids from the blood . Buffering capacity : Proteins in plasma help to maintain acid-base balance

12. Functions of Plasma proteins Protective function : Immunoglobulins combine with foreign antigens and remove them. -Complement system removes cellular antigens. Blood clotting : Many factors are involved in clotting mechanism and prevent loss of excessive amount of blood. e.g. clotting factors IX, VIII, thrombin, fibrinogen etc. An excess of deficiency leads to a disease. e.g hemophilia, thrombus formation. Anticoagulant activity (thrombolysis ): Plasmin breaks down thrombin and dissolves the clot

13. Biconcave disks: Diameter Thickness: Volume; Highly specialized 6 - 9 µm 1 - 2 µm ~ 88 fl. Deformable - i.e. can change shape to transverse smallest blood vessels. Contain haemoglobin (~ 33%). No nucleus or mitochondria. Function: Transport of O 2 and CO 2 . Normal Range: - 5.5  1.0 x 10 12 /L - 4.8  1.0 x 10 12 /L Red blood cells (Erythrocytes)

14. Red blood cells (Erythrocytes) Deliver oxygen to tissues and CO 2 from tissues to lungs. Synthesis is increased by erythropoietin Red cell life span is 120 days. Senescent red cells (old) are destroyed by spleen and replaced by juvenile cells released by bone marrow. An average 70 Kg adult male produces 2.3x10 6 red cell/sec.

15. ERYTHROCYTE STRUCTURE Biconcave shape. Spherical. Simple structure: Membrane surrounding cytoplasm. Almos t 95 % o f solute s i n cytosa l is haemoglobin. No intracellular organnels Non-nucleated Has a cytoskeleton, which plays an important role in determining shape. Has deformability due to special structure of cytoskeleton

16. HAEMOGLOBIN IN THE RED CELLS Haemoglobin Major solute in red cells. Globular protein Conjugated protein: globin + haem . Made of 4 subunits (Quarternary structure) 4 globins + 4 haems  haemoglobin . Binds O 2 to haem group to form oxyhaemoglobin Hb + 4 O 2  Hb (O 2 )4.

17. PLATELETS (Thrombocytes) Discoid, anucleated cells with agranular cytoplasm. Diameter T hickness Volume = 3 µ m = 1 µ m = 7 fl. 250x10 9 platelets/litre . Synthesis increased by thrombopoietin . Synthesised from megakaryocytes.

18. PLATELETS (Thrombocytes) Survival in circulation 10-12 days. Primary role: - in haemostasis : stick to the edges of wounds and form a plug to arrest blood loss. Platelets also involved in development of atherosclerosis and hence can lead to thrombosis.

19. PLATELETS Involved in coagulation of blood PLATELETS

20. White Blood Cells (Leucocytes) ----------Play a role in protecting the the body against infection by phagocytosis. Two Main Groups: The Phagocytes a- Granulocytes: Neutrophils E osinophils Basophils b- Monocytes ii. The Lymphocytes (immunocytes)--Function in protecting. a- B-Lymphocytes-----------------Provide humoral immunity. b- T- Lymphocytes----------------Provide cellular immunity. Total leucocytes: 4.00-11.0x 10 6 /l

21. GRANULOCYTES Have numerous lysosomes and granules (secretory vesicles). Also known as polymorphonuclear leukocytes (PMN) as they have multilobular nuclei Types of granulocytes: Neutrophils, basophils and eosinophils are distinguished by their morphology and staining properties of their granules.

22. FUNCTIONS OF GRANULOCYTES Neutrophils: Basophils: Eosinophils: Phagocytose bacteria and play a major role in accurate information. Resemble mast cells and contain histamine and heparin – play a major role in immunologic hypersensitivity reaction. Involved in certain allergic reactions and parasitic infection.

23. Granulocytes Neutrophils Eosinophils Basophils

24. NEUTROPHILS Increase vascular permeability Cause entry of activated neutrophils into tissues Cause Activation of Platelets Responsible for acute inflammatory response Spontaneous subsidence (resolution) of invading organism that have been dealt with successfully By: Platelet, activating factor (PAF) Eicosanoids (various prostaglandins and leukotriens)

25. FUNCTIONS OF MONOCYTES Monocytes are precursors of macrophages, which are actively involved in phagocytosis.

26. FUNCTIONS OF LYMPHOCYTES B-Lymphocytes: Synthesize and secrete antibodies (humoral immunity) T-Lymphocytes: Involved in cellular immune mechanism e.g killing virally infected cells and some cancer cells. activate B cells to make antibodies. Lymphocytes

27. Haemopoisis The process of formation of blood. Erythropoisis Formation of Erythrocytes Thrombopoisis Formation of Thrombocytes Leucopoisis Formation of Leucocytes

28. Process Product Erythropoesis Leucopoiesis Granulopoiesis Lymphopoiesis Megakaryocytes RBC WBC Granulocytes L ymphocytes Platelets

29. Site of Haemopoisis Fetal Life 1-2 m 2-6 m 1-9 m from 4 m At Birth Adult life Yolk Sac Spleen Liver Bone marrow Bone marrow Bone marrow

30. ,. He m at op oiesis i s t h e c o n ti n u o u s , r eg ul a t e d p r o c e s s o f r e n e w a l , p r o l i f e r a t i o n, dif f e r e n t ia t i o n , and m a tu r a t i o n of all bl o o d c e ll lin e s. These processes result in the formation, development, and specialization of all functional b lo od c ells t h a t a r e r e l e a s e d f r o m the b o ne mar r o w i nt o the c i r c u l a t i o n. Mature blood cells have a limited lifespan (e.g., 120 days for red blood cells [RBCs]) and a cell p o p u l a t i o n c a p a b l e of sel f- r e n ew a l t h a t s u s t a i n s t h e s y s t e m . A hematopoietic stem cell (HSC) is capable of self-renewal (i.e., replenishment) and di r e c t e d di f f e r e n ti a ti o n i n t o a ll r e qui r e d c e ll l in e a g e s . Thus, the hematopoietic system serves as a functional model to study stem cell biology, proliferation, and maturation and their contribution to disease and tissue repair. Hematopoiesis

31. 32 Haematopoietic stem cell Stem cells have two essential properties self renewal potency. Self renewal of course means that they can proliferate, indefinitely in the case of some. Potency means thay can generate a range (one or many) differentiated cell types. Stem cells themselves are undifferentiated i.e. have no specific functions other than division.

32. 33 The haematopoietic stem cell Mature blood cells in healthy individuals mostly have short lifetimes (exception: lymphocytes) and are constantly regenerated in the bone marrow. we make 5 x 10 11 blood cells daily This is accelerated when there is haematological stress e.g. infection, need more leukocytes e.g. high altitude, need more red cells.

33. Haematopoietic stem cell Stem cells occupy a special niche in tissues which may help to define their “stem-ness”. At division, one cell leaves the niche and becomes a transit cell, the other stays put asymmetric division in space as well as in kind niche containing stem cell niche containing dividing stem cell niche containing stem cell and transit cell outside niche

34. Haematopoietic stem cell The niche is a complex organisation of stromal (i.e. non-haematopoietic cells) interacting via adhesion molecules with the stem cell. There may be more than one kind of niche in a tissue . The role of the transit cell is to divide rapidly but a limited number of times so amplifying cell numbers and its progeny differentiate to form the functional end cell of that particular lineage .

35. long-lived stem cell in special niche slowly cycles: one daughter is a new stem cell, the other is a transit cell which leaves the niche the transit cell amplifies the cell number, going through a limited number of divisions the amplified cells finally form functional end cells, which do not divide. Haematopoietic stem cell

36. 37 Haematopoiesis In haematopoiesis there is an intervening step called a “progenitor cell” this is like a stem cell in that it is multipotent ( tho ’ less so than the HSC) it is unlike the stem cell in that it does not divide indefinitely (a controversial issue ). There are 2 progenitor cells in haematopoiesis: Common lymphoid progenitors (CLPs) which give rise to all lymphoid cells. Common myeloid progenitors (CMPs) which give rise to all other blood cells including erythrocytes and platelets.

37. 38 The haematopoietic stem cell CLP HSC CMP Lymphocytes Granulocytes erythrocytes thrombocytes Transit cells, dividing and differentiating End cells, not dividing, functional Progenitor cells, dividing and committed

38. Hemangioblast and Angioblast The hemangioblast, a common precursor for hematopoietic and vascular lineages, was proposed nearly a century ago based on the close proximity of cells in the yolk sac that give rise to both blood cells and blood vessels. It was Murray who in 1932 coined the term “hemangioblast” to indicate the thickenings of the mesoderm in the chick yolk sac, the mesodermal “masses” located at the sites where later the blood islands emerge. Angioblast is one of the extraembryonic mesenchyme cells that differentiate into the endothelium of the embryonic blood vessels. Angioblasts form capillary channels by vasculogenesis (de novo capillary formation) and by angiogenesis (the formation of new capillaries from existing ones).

39. Site of Hematopoiesis Hematopoiesis in the developing human can be characterized as a select distribution of embryonic cells in specific sites that rapidly changes during development. In humans, hematopoiesis, the formation and development of red and white blood cells, begins in the embryonic yolk sac during the first weeks of development. Here, yolk-sac stem cells differentiate into primitive erythroid cells that contain embryonic hemoglobin. In the third month of gestation, hematopoietic stem cells migrate from the yolk sac to the fetal liver and then to the spleen; these two organs have major roles in hematopoiesis from the third to the seventh months of gestation. After that, the differentiation of HSCs in the bone marrow becomes the major factor in hematopoiesis, and by birth there is little or no hematopoiesis in the liver and spleen. There are three phases. During fetal development , the restricted, sequential distribution of cells is initiated in the yolk sac and then progresses in the aorta- gonad- mesonephros (AGM) region ( mesoblastic phase), then to the fetal liver (hepatic phase), and finally resides in the bone marrow (medullary phase). Because of the different locations and resulting microenvironmental conditions (i.e., niches) encountered, each of these locations has distinct but related populations of cells.

40. Rodak's Hematology (Sixth Edition), 2020

41. Mesoblastic phase Hematopoiesis is considered to begin around the nineteenth day of embryonic development after fertilization. Early in embryonic development, cells from the mesoderm migrate to the yolk sac. Some of these cells form primitive erythroblasts in the central cavity of the yolk sac, and others (angioblasts ) surround the cavity of the yolk sac and eventually form blood vessels. These primitive but transient yolk sac erythroblasts are important in early embryogenesis to produce hemoglobin (Gower-1, Gower-2, and Portland) needed for delivery of oxygen to rapidly developing embryonic tissues. Yolk sac hematopoiesis differs from hematopoiesis that occurs later in the fetus and adult in that it occurs intravascularly (or within developing blood vessels). Cells of mesodermal origin migrate to the AGM region and give rise to HSCs for definitive or permanent adult hematopoiesis. The AGM region was previously considered to be the only site of definitive hematopoiesis during embryonic development. However, subsequent studies clearly demonstrated that the yolk sac was the major site of adult blood formation in the embryo .

42. Hepatic phase The hepatic phase of hematopoiesis begins at 5 to 7 gestational weeks and is characterized by recognizable clusters of developing erythroblasts, granulocytes, and monocytes colonizing the fetal liver, thymus, spleen, placenta, and ultimately the bone marrow space in the final medullary phase. These varied niches support development of HSCs that migrate to them. Developing erythroblasts signal the beginning of definitive hematopoiesis with a decline in primitive hematopoiesis of the yolk sac. In addition, lymphoid cells begin to appear. Hematopoiesis during this phase occurs extravascularly , with the liver remaining the major site of hematopoiesis during the second trimester of fetal life. Hematopoiesis in the AGM region and the yolk sac disappear during this stage. Hematopoiesis in the fetal liver reaches its peak by the third month of fetal development, then gradually declines after the sixth month , retaining minimal activity until 1 to 2 weeks after birth. The developing spleen, kidney, thymus, and lymph nodes contribute to the hematopoietic process during this phase. The thymus, the first fully developed organ in the fetus, becomes the major site of T cell production, whereas the kidney and spleen produce B cells. Production of megakaryocytes begins during the hepatic phase. The spleen gradually decreases granulocytic production and subsequently contributes solely to lymphopoiesis . During the hepatic phase, fetal hemoglobin (Hb F) is the predominant hemoglobin, but detectable levels of adult hemoglobin (Hb A) may be present.

43. Medullary (myeloid) phase Hematopoiesis in the bone marrow (termed medullary hematopoiesis because it occurs in the medulla or inner part of the bone cavity ) begins between the fourth and fifth month of fetal development. During the myeloid phase, HSCs and mesenchymal cells migrate into the core of the bone. Mesenchymal cells , a type of embryonic tissue, differentiate into structural elements (e.g., stromal cells such as endothelial cells and reticular adventitial cells) that support developing hematopoietic elements. Hematopoietic activity, especially myeloid activity, is apparent during this stage of development, and the myeloid-to-erythroid ratio gradually approaches 3:1 to 4:1 (normal adult levels). By the end of 24 weeks’ gestation, the bone marrow becomes the primary site of hematopoiesis. Measurable levels of erythropoietin (EPO), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and hemoglobins F and A can be detected. In addition, cells at various stages of maturation can be seen in all blood cell lineages.

44. Hematopoiesis Early in hematopoiesis, a multipotent stem cell differentiates along one of two pathways, giving rise to either a common lymphoid progenitor cell or a common myeloid progenitor cell. During the development of the lymphoid and myeloid lineages, stem cells differentiate into progenitor cells, which have lost the capacity for self-renewal and are committed to a particular cell lineage. Common lymphoid progenitor cells give rise to B, T, and NK (natural killer) cells and some dendritic cells. Myeloid stem cells generate progenitors of red blood cells (erythrocytes), many of the various white blood cells (neutrophils, eosinophils, basophils, monocytes, mast cells, dendritic cells), and platelets. When the appropriate factors and cytokines are present , progenitor cells proliferate and differentiate into the corresponding cell type, either a mature erythrocyte, a particular type of leukocyte, or a platelet-generating cell (the megakaryocyte ).

45. Red and white blood cells pass into bonemarrow channels, from which they enter the circulation. In bone marrow, hematopoietic cells grow and mature on a meshwork of stromal cells, which are nonhematopoietic cells that support the growth and differentiation of hematopoietic cells. Stromal cells include fat cells, endothelial cells, fibroblasts, and macrophages. Stromal cells influence the differentiation of hematopoietic stem cells by providing a hematopoietic-inducing microenvironment (HIM) consisting of a cellular matrix and factors that promote growth and differentiation. Many of these hematopoietic growth factors are soluble agents that arrive at their target cells by diffusion, others are membrane-bound molecules on the surface of stromal cells that require cell- to-cell contact between the responding cells and the stromal cells.

46. Self-renewing hematopoietic stem cells give rise to lymphoid and myeloid progenitors Subhadipa 2020

47. Important cytokines Hematopoietic cytokines are large family of extracellular ligands that stimulate hematopoietic cells to differentiate into eight principle types of blood cells. Numerous cytokines are involved in the regulation of hematopoiesis within a complex network of positive and negative regulators. Some cytokines have very narrow lineage specificities of their actions, while many others have rather broad and overlapping specificity ranges. This includes GM-CSF, G-CSF, M-CSF, interleukins, EPO and TPO. There are a number of other cytokines that exert profound effects on the formation and maturation of hematopoietic cells, which include stem cell factor (SCF), flt-3/flk - 2 ligand (FL) and leukemia inhibitory factor (LIF). Other cytokines or ligands such as jagged-1, transforming growth factor-β (TGF- β) and tumor necrosis factor-α (TNF-α) also play significant roles in modulating hematopoiesis. Acidic glycoproteins, the colony-stimulating factors (CSFs), named for their ability to induce the formation of distinct hematopoietic cell lines. Glycoprotein erythropoietin (EPO). Produced by the kidney, this cytokine induces the terminal development of erythrocytes and regulates the production of red blood cells.

48. Regulation of hematopoiesis by cytokines

49. Hematopoietic cytokines stimulate hematopoietic cells to differentiate into principle types of blood cells

50. 53 erythrocytes & platelets The early stages for production of both these cells pass through a common pathway generating a “megakaryocyte/ erthrocyte precursor” (MEP) from the CMP: they then split. erythropoietin ( EPO) produced by kidneys specifically stimulates erythropoiesis EPO production stimulated by hypoxia EPO binds to erythrocytes so haemorrhage or anaemia results in increased levels of EPO these lead to stimulation of erythropoiesis. (Other non-specific GFs/cytokines needed.)

51. 54 MEP pro-erythroblast erythroblast* reticulocyte** EP erythrocyte dividing cells committed to erythroid lineage: requires EPO (etc) non-dividing cells Simplified erythropoiesis *The suffix “-blast” indicates a large, proliferating cell. There are several different kinds of erythroblast. **The reticulocyte nucleus is condensed and inactive. Hb synthesis EP EP EP bone marrow circulation

52. 55 erythrocytes & platelets Erythrocyte clearance: removed by liver and spleen after 120 days as the erythrocyte ages surface proteins particularly “band 3” are progressively oxidised this provides a target for phagocytosis by macrophages lining liver & spleen sinuses.

53. 56 erythrocytes & platelets Generation of platelets ( thrombopoiesis ): stimulated by thrombopoietin (TPO) & other non-specific growth factors. TPO produced constitutively mostly by liver. Inflammation can double production by liver via cytokine IL-6. In thrombocytopenia (reduced platelets) BM stromal cells also produce TPO. Platelets have TPO receptors and so remove TPO from circulation (- ve feedback ). Platelet clearance: removed by liver and spleen after ~ 7 days .

54. 57 Platelets (1,000s) bud off surface of megakaryocyte. MEP megakaryocyte platelets bone marrow circulation requires TPO (etc) Thrombopoiesis

55. 58 granulocytes and monocytes Granulocytes (basophils, mast cells, eosinophils & neutrophils) and monocytes share early steps in differentiation via “granulocyte/monocyte precursor” (GMP) derived from the CMP. Formation of GMP from CMP is driven by granulocyte/monocyte colony stimulating factor (GM-CSF) (etc).

56. 59 granulocytes and monocytes Formation of specific cell types is driven by other growth factors, all of which are produced constitutively and are induced e.g. by inflammation.

57. 60 GMP mast cell macrophage neutrophil basophil eosinophil monocyte M-CSF G-CSF IL-5 SCF* *SCF = Stem cell factor. Note that some cell types differentiate further in the tissues. neutrophil eosinophil bone marrow circulation tissues Granulopoiesis

58. 61 granulocytes and monocytes Most of these cells are relatively short lived and are lost by apoptosis the fragments of the dead cells are phagocytosed by cells lining sinuses of spleen & BM or, in the tissues, by epithelial cells.

59. 62 lymphocytes Two pathways: T cells B cells and NK cells Both from CLP Initial stages in BM: B & NK cells complete maturation in BM, immature T cells migrtae to thymus and mature there.

60. 63 CLP DN DP bone marrow circulation & 2ry lymphoid tissue mature T (SP) CD4/8 +ve α chain re-arrangement β chain re-arrangement IL-7R T lymphopoiesis thymus CD25 +ve

61. 64 CLP pro-B pre-B bone marrow circulation & 2ry lymphoid tissue mature B CD19 +ve IgM +ve H chain re-arrangement L chain re-arrangement IL-7R B lymphopoiesis