Hematopoiesis Definition Hemopoiesis is a continuous regulated process

Hematopoiesis Definition : Hemopoiesis is a continuous, regulated process of blood cell production. q A process by which blood cells are formed by proliferation and differentiation of stem cells. Ø When ( Prenatal ---Postnatal) Where , How Prenatal hematopoiesis q The embryo requires red cells for the transport of maternal oxygen to permit its growth and development. • During embryogenesis, hemopoiesis occur in temporally distinct sites including yolk sac, the fetal liver and the preterm bone marrow The process has been divided into three(3) main phases; q Mesoblastic phase (yolk sac) q Hepatic phase q Medullary phase.

. Prenatal hematopoiesis Yolk sac Liver& spleen Bone marrow

Mesoblastic phase of hematopoiesis q generally is considered to begin on the 19 th day of embryologic development, after fertilization. q Progenitor cells of mesenchymal origin migrate from the aorta-gonad -mesonephros to the yolk sac. q The cells arising from the aorta-gonad-mesonephros region give rise to hematopoietic stem cells [HSC’s] q erythroblasts found in the yolk sac which arising from mesodermal cells, initially line the cavity of the yolk sac. q These primitive cells migrate from periphery into the central cavity of the yolk sac where they develop into primitive erythroblasts. q The remaining cells surrounding the cavity of the yolk sac are called ANGIOBLASTS and form the future blood vessels. q This phase of hemopoiesis occurs intravascularly, or within a developing blood vessel q Yolk sac erythroblasts remain nucleated as they circulate.

1 - Yolk Sac Hematopoiesis (blood islands) 2 -8 weeks: In the yolk sac, mesenchymal cells differentiate to clusters of hemangioblast cells. 1 -Peripheral hemangioblasts further differentiate into endothelial cells 2 -Central hemangioblasts give rise to nucleated red blood cells no leukocytes are formed in this phase. 4

Hepatic phase of hematopoiesis q The liver serves as a primary source of red cells from 9 th to the 24 th weeks of gestation , Hemopoietic function of the organ reaches its peak during the first 2 -4 postnatal days. q in this phase hemopoiesis occurs extravascularly. q Hemopoietic cells of the fetal liver exist in a specific microenvironment that controls their proliferation and differentiation. q characterised by recognizable clusters of developing erythroblasts, granulocytes and monocytes, in addition, lymphoid cells begin to appear. q Erythrocytes still have nuclei, leukocytes begin to appear. All blood cell types (except T cells) can differentiate in the fetal liver & spleen. q **extra-medullary hematopoiesis q Along with the development of the hemtopoietic system, the liver acquires the characteristics of the metabolic organ (that it is shows an increase in size).

Medullary (MYELOID) phase q It begins in 5 th month of development and occurs in medulla of bone marrow q Mesenchymal cells migrate into bone derived from the fetal liver &spleen and differentiate into skeletal and hemopoietic cells. q There are two types of marrow which are, the red bone marrow and the yellow bone marrow. q red are found in the sternum, skull, scapulae, ribs, vertebrae, etc In newborns, all marrow is red but most later becomes yellow. q The yellow marrow is hemopoietically inactive and composed of adipocytes q All blood cell types (except T cells) can differentiate in the bone marrow.

Postnatal Hematopoiesis Prior to puberty: skull, ribs, sternum, vertebrae, clavicles, pelvis, and long bones. After puberty: same bones except no more shafts of long bones. Extra-medullary hemopoiesis: liver and spleen continue to produce blood cells even after birth.

Hematopoiesis begins during embryogenesis in the yolk sac and aorta– gonad–mesonephros (AGM) region; hematopoietic progenitor cells then migrate to the fetal liver. Migration to the bone marrow occurs late in fetal development, and this is a site of hematopoiesis postnatally and in adult life.

Blood Forming Organs

The Myeloid Cells 1 -The Pluripotent hemopoietic stem cells (PHSCs): • great ability to divide. • ½ Reserve other ½ becomes more differentiated. • Daughter cells of the PHSCs. • Histologically, larger • Cell divisions but are more differentiated ►► cell colonies ►► 2 CFUs

CFUs►►► daughter stem cells more differentiated q CFU- lymphocyte (CFU- Ly): Some migrate to the thymus, spleen and lymph nodes where lymphopoiesis is completed. q CFU- granulocyte / monocyte, erythrocyte, megakaryocyte ►►unipotent progenitors Ø CFU-Erythrocyte (CFU- E) Ø CFU-Megakaryocyte (CFU-Meg). Ø CFU- Granulocyte /monocyte (CFU-GM),

ERYTHROPOIESIS q takes about 7 days q Involves the following stages: 1. PHSCs. ► MHSCs. 2. CFU. GEMM 3. CFU-E. 4. Pro- Erythroblasts. 5. Basophilic Erythroblasts. 6. Polychromatophilic Erythroblasts. 7. Orthochromatophilic Erythroblasts. 8. Reticulocyte. 9. Mature erythrocyte.

Erythropoiesis Definition, Site, Stages UMC PHSC FU-E o-erythroblast basophil erythroblast olychromatophil erythroblast rthochromatophil erythroblast (normoblast) reticulocyte 9 -erythrocyte

Erythropoiesis 1 -UMC 2 -Pluripotential hemopoietic stem cells (hemocytoblast) 3 -Restricted erythrocyte progenitor (CFU-E)(Colony-forming unit erythrocytes 4 - Pro- erythroblast: • Large. • basophilic cytoplasm with abundant Ribosomes. • nucleus 5 - Basophilic Erythroblast: • Most active hemoglobin synthesis. • Ribosomes are ▲▲abundant. • Cytoplasm is strongly basophilic

6 - Polychromatophilic Erythroblast: • hemoglobin ▲▲. • Ribosomes are still ▲▲. • Cytoplasm shows eosinophilic areas alternating with basophilic spots. • Last stage in repeated cell divisions. • Only morphological maturation of the erythroblasts. 7 - Orthochromatophilic Erythroblast (Normoblast): • Synthesis of hemoglobin is completed. • Ribosomes ▼▼. • Nucleus: Ø is condensed and reduced in size. Ø Gradually pushed towards the periphery ►►completely extruded from the cell.

8 - Reticulocytes • immature RBCs • Nucleated -----non-nucleated • differ than mature RBCs Ø slightly larger (8 m). Ø Cytoplasm contains remnants of ribosomes. Ø On staining with cresyl blue form a reticulate pattern. • Reticulocytes represent 1% of all RBCs in normal blood film. • Clinical significance: An increase in this percentage indicates an Ø accelerated rate of erythropoiesis. Ø compensate for anemia or hemorrhage. 9. Mature RBCs

ERYTHROPOIESIS

Principles of erythropoiesis 2015 Prof. Dr. SALWA M.

THROMBOPOIESIS 1 -UMC. 2 -Pluripotential hemopoietic stem cells (hemocytoblasts). 3 -Restricted megakaryocyte progenitor (Colony-forming unit (CFU-Meg). 4 -Megakaryoblast: numerous nulceoli, multiplication of nuclear DNA (polyploidy), intense basophilic cytoplasm. 5. Promegakaryocyte: lobulated nucleus + multiple granules(clotting factors, lysosomes……. etc. ? ) 6 -Megakaryocyte : (sites)…. demarcation membranes →platelets. Megakaryoblast

Megakaryocyte q. LM • • Ø Ø Giant cell (150μ). Nucleus: Lobulated. Polyploid. Cytoplasm: Filled with many organelles. 3 types of granules. q EM • Lobulated nucleus +numerous cytoplasmic granules. • Membranous demarcation lines around !!. ▼▼ • Lines of cleavage.

Megakaryocyterelease of platelets • Megakaryocytes are located near BM sinusoids. • Extend cytoplasmic processes: ▼▼ • release of platelets into blood stream.

LEUKOPOIESIS CFUs►►► daughter stem cells more differentiated q - CFU- lymphocyte (CFU- Ly): Some migrate to the thymus, spleen and lymph nodes gland where lymphopoiesis is completed. q - CFU- granulocyte/monocyte, erythrocyte, megakaryocyte (CFU-GEMM): ►►unipotent progenitors • CFU-Erythrocyte (CFU- E) • CFU-Megakaryocyte (CFU-Meg). • CFU- Granulocyte /monocyte (CFU-GM),

Granulopoiesis 1 -UMC 2 -Pluripotential hemopoietic stem cells (hemocytoblasts) 3 -Restricted granulocyte progenitor, that are called (Colony-forming unit granulocytes (CFU-G)) 4 -Myeloblast 5 -Promyelocyte : (nonspecific granules) 6 -Myelocyte : (specific granules N, E, B…. . ? ) 7 -Metamyelocyte: (specific granules N, E, B +indentation of nucleus)

8 -Band cell Smaller cells , curved band nuclei , cannot divide. May be present in peripheral blood. 9 -Mature cells: (Neutrophils , Eosinophils Basophils )

Monopoiesis 1 -UMC 2 -Pluripotential hemopoietic stem cells (hemocytoblasts) 3 -Restricted monocyte progenitor (CFU-M) or with (CFU GM ): 4 -Monoblast. 5 -Promonocyte indented nucleus + lysosomes. 6 -Mature monocyte : 8 hours in blood stream, then → tissue macrophages for several months. 26

LYMPHOPOIESIS Definition & sites. • Involves the following: 1. PHSCs. 2. MHSCs. 3. CFU-Ly: Some daughters migrate to thymus ►► CFU- LY T, where: v Reside in outer cortex ►► v T- lymphoblasts: Ø Repeated mitosis. Ø Azurophilic granules. Ø Express surface markers. v Mature T- lymphocyte---- ► ► peripheral lymphoid organs.

Lymphopoiesis 1 -UMC 2 -Pluripotential hemopoietic stem cells (hemocytoblasts) 3 -Restricted lymphocyte progenitor (Colony-forming unit (CFU-L) 4 -Lymphoblasts 5 - Prolymphocytes 6 -Lymphocytes…… (B, T (precursors) & Null cells? )

LYMPHOPOIESIS • • 1. 2. 3. Definition & sites. Involves the following: PHSCs. MHSCs. CFU-Ly: Some daughters remain in bone marrow ►► CFU- LY B, where: ►► v B- lymphoblasts: Ø Repeated mitosis. Ø Azurophilic granules. Ø Express surface markers. v Mature B- lymphocyte---- ► ► peripheral lymphoid organs.

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Lymphocytes re- circulation

REGULATION OF HEMOPOIESIS q Internal factors : • Bone marrow stromal cells are the major source of hemopoietic cytokines in the non-infectious state. • It is regulated by various cytokines. The combination of various cytokines stimulate the proliferation and/or differentiation of various hemopoietic cell types. • In the presence of infection, cytokines produced by activated macrophages induce hemopoietic activity. • Some hemopoietic growth factors such as the Granulocyte – Macrophage colony stimulating factor are given to stimulate white blood cell formation in cancer patients receiving chemotherapy which kills their red bone marrow cells as well as cancer cells.

q External factors : Ø Stimulated by: 1. Erythropoietin. 2. Thyroxin. 3. Growth hormone. 4. Testosterone. Ø Inhibited by: 1 - Estrogen. 2 - Nutritional deficiency. Erythropoietin (EPO ) has a central role in regulating the rate of proliferation and differentiation of erythroid precursors, as it is the most important cytokine regulator of mammalian erythropoiesis. Targeted disruption of EPO or its receptor leads to almost complete blockage of fetal liver erythropoiesis, resulting in fetal death.