Hematopoiesis - Lecture Notes PDF

Hematopoiesis Raquel M. Fernandez, RMT, MSPH  Mesoblastic Phase begins at the 19th day of embryonic development after fertilization cells arising from the aorta-gonad mesonephros region give rise to HSC’s Mesodermal cells give rise to primitive erythroblasts Angioblasts su...

Hematopoiesis Raquel M. Fernandez, RMT, MSPH  Mesoblastic Phase begins at the 19th day of embryonic development after fertilization cells arising from the aorta-gonad mesonephros region give rise to HSC’s Mesodermal cells give rise to primitive erythroblasts Angioblasts surrounding the cavity of the yolk sac form the future blood vessels  Hepatic Phase Begins at 4-5 gestational weeks Characterized by clusters of developing erythroblasts, granulocytes and monocytes Decline in primitive hematopoiesis of the yolk sac Lymphoid cells begin to appear Liver hematopoiesis peaks at the 3rd month of fetal life Thymus becomes the major site of T-cell production; kidney and spleen produce B-cells  Hepatic Phase Production of megakaryocytes also begins here Spleen gradually decreases granulocytic production and involves itself in lymphopoiesis Detectable levels of HbF, HbA and HbA2 may be present  Medullary Phase begins at the 5th month in the inner part of the bone marrow (medulla) Myeloid activity is apparent Myeloid to erythroid ratio approaches the adult level of 3:1 by 21 weeks of gestation Becomes the primary site of hematopoiesis by the end of the 6th month Measurable levels of erythropoietin, G-CSF,GM- CS, fetal Hgb, HbA2 and adult Hgb can be detected  Adult Hematopoietic Tissue Primary lymphoid tissue – Bone marrow – Thymus Secondary lymphoid tissue – Spleen – Lymph nodes – Gut-associated lymphoid tissue  Bone Marrow the tissue located within the cavities of the cortical bone Cavities consist of the trabecular bone, resembling a honey comb 2 types of marrow – Red – Yellow  Bone Marrow Red Marrow Yellow Marrow Hematopoietically active Inactive marrow marrow Composed primarily of Found in the sternum, adipocytes (fat cells) skull, scapulae, vertebrae, ribs, pelvic bones, proximal ends of long bones  Adult Hematopoietic Tissue During infancy and early childhood the bone consists primarily of red active marrow 5-7 y.o. – retrogression eventually results in restriction of the active marrow to the flat bones, sternum, vertebrae, pelvis, ribs, skull and proximal portion of the long bones Yellow marrow scattered throughout active red marrow can revert back to active marrow in cases of increased demand  Red Marrow Composed of extravascular cords that contain all of the developing blood cell lineages, stem cells, progenitor cells, adventitial cells and macrophages Cords are separated from the lumen of sinusoids by endothelial and adventitial cells located bet the trabeculae of spongy bone Hematopoietic cells develop in specific niches within the cords  Red Marrow Normoblasts develop in small clusters adjacent to the outer surfaces of the vascular sinuses Megakaryocytes are located close to the vascular walls of the sinuses which facilitates the release of platelets into the lumen of the sinusoids Myeloid cells are located deep within the cords  Liver In severe haemolytic anemias and rbc dysplasias, the conjugation of bilirubin and the storage of iron are increased Sequesters membrane damaged rbc’s and removes them from circulation Capable of extramedullary hematopoietic production in case of bone marrow shutdown  Spleen Largest lymphoid organ in the body located beneath the diaphragm behind the fundus of the stomach Vital but not essential for life and functions as an indiscriminate filter of the circulating blood Synthesizes IgM in the germinal centers Serves as storage site for platelets Methods of removing senescent rbc’s from the circulation Culling – the cells are phagocytosed with subsequent degradation of cell organelles Pitting – splenic macrophages remove inclusions or damaged surface membrane from the circulating rbc’s  Lymph Nodes Organs of the lymphatic system located along the lymphatic capillaries that parallel but are not part of the circulatory system Bean-shaped structures (1-5 mm in diameter) that occur in groups or chains at various intervals along lymphatic vessels. They maybe superficial (inguinal, axillary, cervical, supratrochlear) or deep ( mesenteric or retroperitoneal)  Lymph Nodes Lymph is the fluid portion of the blood that escapes into the connective tissue characterized by low protein concentration and the absence of rbc’s. 3 main functions: – They play a role in the formation of new lymphocytes from the germinal centers – Involved in the processing of specific Ig – They filter particulate matter, debris, and bacteria entering the lymph node via the lymph  Thymus Originates from endodermal and mesenchymal tissue. Initially populated by lymphocytes from the yolk sac and the liver. The cortex and the medulla are populated by lymphocytes, mesenchymal cells, reticular cells, macrophages The cortex seems to function as a waiting zone densely populated with progenitor cells from the bone marrow  Thymus The progenitor lymphoid cells give rise to T cells that later express surface antigens T cells later leave the thymus to populate specific regions of the lymphoid tissue such as the T-cell dependent areas of the spleen, lymph nodes etc.  Stages of Hematopoiesis Primitive Hematopoiesis – occurs during the megaloblastic stage of development Definitive Hematopoiesis – begins during the fetal liver stage and continues through adult life Hematopoietic Progenitor Cells Two major types: Non-committed or undifferentiated stem cells Multi-potential committed progenitor cells Theories in the Origin of Progenitor Cells Monophyletic Theory: suggests that all blood cells are derived from a single progenitor stem cell called a pluripotential stem cell Polyphyletic Theory: suggests that each of the blood cell lineages is derived from its own unique stem cell.  Characteristics of Stem Cells They are capable of self-renewal They give rise to differentiated progeny They are able to reconstitute the hematopoietic system of a lethally irradiated host  Lineage-progenitor cells Consists of: Common lymphoid progenitor – proliferates and differentiates into lymphocytes of T and B and natural killer lineages Common myeloid progenitor – proliferates and differentiates into individual granulocytic, erythrocytic, monocytic, and megakaryotic lineages Theories on the fate of Stem Cells Stochastic Model: hematopoiesis is a random process whereby HSCs randomly commits to self-renewal or differentiation Instructive Model: microenvironment in the bone marrow determines whether the stem cells will self-renew or differentiate Multi-lineage priming Model: HSC’s receive signals from the hematopoietic inductive microenvironment to amplify or repress genes associated with commitment to multiple lineages that are expressed only at low levels  Features of Maturation Over-all decrease in cell size and decrease in the ratio of nucleus to cytoplasm Changes in the nucleus: – Loss of nucleoli – Decrease in the size of the nucleus – Condensation of chromatin – Shape change in the nucleus – Loss of the nucleus  Features of Maturation Changes in the cytoplasm: – Decrease in basophilia – Increase in the proportion of cytoplasm – Possible appearance of granules  Stem Cell Cycle Kinetics Bone marrow is estimated to be capable of producing approx. 3B erythrocytes, 2.5B platelets, and 1.5B granulocytes/kg body weight daily. The determining factor for the rate of production is the physiologic need. Stem cell exist in 1: 1000 nucleated blood cells  HEMATOPOIETIC STEM CELLS Differentiate into multiple cell lines. Proliferation is under influence of hematopoietic growth factors present in reticuloendothelial system. Morphologically they resemble large immature lymphocytes cell membrane phenotyping with monoclonal antibodies has identified them by presence of surface markers.  ERYTHROPOIESIS In normal state, the balance of production and destruction is maintained at remarkably constant rate Both endocrine and exocrine hormones make important contributions to this dynamic well balanced mechanism The earliest recognizable erythroid precursor seen in the bone marrow is large basophilic staining cell,15-20 um Contains a single large well defined, rounded nucleus,ribosomes, mitochondria and golgi apparatus  ERYTHROPOIESIS As the early precursor cell matures, its nucleus increases in size. As maturation goes on cell becomes smaller and more eosinophilic indicating hemoglobin. During intermediate stages of maturation, cytoplasm becomes polychromatic indicating mixture of basophilic proteins and eosinophilic hemoglobin.  ERYTHROPOIESIS Further maturation, hemoglobin synthesis continue and cytoplasm becomes entirely eosinophillic. Late stages of maturation, hemoglobin is abundant. Few mitochondria and ribosomes are present., nucleus is small dense and well circumscribed.  ERYTHROKINETICS Number is constant normally as their life span is 120 days approximately. 1-2 days of further maturation in systemic circulation and spleen reticulocytes loose membrane coated transferrin. Differentiation and maturation from a basophillic erythroblast occurs in 5 to 7 days. 10-15% of erythroid precursors never mature and are destroyed.  GRANULOPOIESIS Committed myeloid stem cells differentiate into three types of cells, neutrophils, Basophils and eosinophils FORMATION OF NEUTROPHILLS 1. Myeloblast, an early precursor cell, diameter 15-20µm, lower nuclear cytoplasmic ratio, no cytoplasmic granules.  GRANULOPOIESIS 2. Promyeloctes, is the next stage of maturation, similar in size and appearance to Myeloblast but has numerous azurophillic primary granules in cytoplasm, that contain variety of enzymes. (myeloperoxidase, acid phosphates, beta galactosidase, 5-nucleotidase)  GRANULOPOIESIS 3. Myelocyte Secondary granules become apparent. Increased size and and smaller primary granules. secondary granules have several bactericidal enzymes nucleus become indented,  GRANULOPOIESIS 4. Metamyelocyte: Next stage in myelopoiesis is a cell having more indented and smaller nucleus and having more granule 5. Mature neutrophils arise from stem cells in approx 10 days. remain viable in systemic circulation for 8-12 hrs.  THROMBOPOIESIS Megakaryocytes differentiate from myeloid stem cell and are responsible for production of platelets. THREE STAGES OF MATURATION OF MEGAKARYOCYTES 1. Basophilic stage, megakaryocyte is small, has diploid nucleus and abundant basophilic cytoplasm.  THROMBOPOIESIS 2.Granular stage, here the nucleus is more polypoid, cytoplasm is more eosinophilic and granular 3.Mature stage, megakaryocyte is very large, with approx 16-32 nuclei, abundance of granular cytoplasm. It undergoes shedding to form platelets.  LYMPHOPOIESIS Lymphocytes are derived from committed stem cells that originate from pluripotent stem cell. Early lymphoid cells further differentiates into B. & T.lymphocytes. B-LYMPHOCYTES. As they mature in specialized organ in birds called bursa of fabricius. They proliferate and mature into antibody forming cells.  LYMPHOPOIESIS Bone marrow or fetal liver may be the organs in humans for development of B-lymphocytes from uncommitted lymphocytes. Maturation culminates in migration of B.lymphocytes to other lymphoid organs and tissues throughout the body (e.g. spleen, gut, liver , tonsils, lymph nodes)  LYMPHOPOIESIS 5. Plasma cells Bone marrow, lymphoid organs, normally found circulating in blood and lymph. little capacity to undergo mitosis. ultimate stage for synthesis and secretion of antibodies or immunoglobulin. 6.Clones of plasma cells and B-cells can expand and contract under influence of many regulating factors.  LYMPHOPOIESIS T-LYMPHOCYTES. Depends on thymus for their maturation and specialized functions. 60-70% of circulating lymphos able to cycle from blood, through lymphoid tissue and then back to blood via lymphatics.  LYMPHOPOIESIS T-LYMPHOCYTES Secrete cytokines(LYMPHOKINES). Regulate proliferation and differentiation of other T.cells, B.cells,and macrophages. Main component of cell mediated imunity.  LYMPHOPOIESIS 3.Differentiation and maturation of uncommitted lymhocytes take place in thymus,these Thymocytes loose their antigenic surface molecules and finally mature into helper/ effector T lymphocytes and suppressor T lymphocytes. 4. The helper and suppressor cells can be differentiated by presence of specific cell membrane molecules and receptors  HEMATOPOIETIC GROWTH FACTORS They are heterogeneous group of cytokines that stimulate the progenitor cells and induce proliferation and maturation They are glycoproteins synthesized by variety of cells in marrow. They bind to specific receptors on the surface of various cells of the hematopoietic system Characteristic and properties 1. Naturally occurring hormones. 2. Low molecular weight glycoprotiens. 3. Variable degrees of species specificity. 4. Available in purified form by recombinant DNA technology. 5. Responsible for stimulation and release of other growth factors and cytokines. Hematopoietic Growth Factors 1.ERYHTROPOIETIN: Synthesized by peritubular cells of kidney in response to hypoxemia Present in minute amounts in urine Liver secretes 10% of endogenous erythropoietin. Responsible for low level erythroid activity. Half life of 6-9 hrs. in anemic patient  Hematopoietic Growth Factors Thrombopoietin is a glycoprotein hormone produced mainly by liver and kidney that regulates the production of platelets in bone marrow. It stimulates the production and differentiation of Megakaryocytes  Hematopoietic Growth Factors 3.GM-CSF: Produced by fibroblasts, stromal cells,T.lymphocytes and endothelial cells. Stimulate progenitors for granulocytes, monocytes and erythrocytes 4. G-CSF: LMW glycoprotein Stimulates proliferation and maturation of granulocyte precursors. Produced by stromal cells, monocytes, macrophages, and endothelial cells.  Hematopoietic Growth Factors 5.M-CSF Secreted by stromal cells, macrophages and fibroblasts. Heavily glycosylated glycoprotein Potent stimulator of macrophage function and activation as it increases the expression of MHC.II antigen on macrophages.