Definition: 

• It is the term used to describe the formation and development of blood cells. 
• Hemangioblasts give rise to both circulatory system and blood cells. They are derived from mesoderm.  

Stages and sites: 

• Primitive hematopoiesis 
  • Starts in aorto-gonado-mesonephros region of embryo at 19^th day after fertilization. 
  • It is called “blood island”, which consists of hemocytoblasts and erythroblastoid cells surrounded by endothelial cells. 
  • This continues up to 7 weeks of gestation. 
  • Hemoglobins of embryonic variety- Gower 1, Gower 2 and Portland are produced. 
• Hepatic erythropoiesis 
  • Starts in 3^rd month and continues up to 6 months of gestation 
  • Between 7 and 15 weeks, 60% of liver cells are of hematopoietic origin. 
  • HbF is the type of hemoglobin produced during this phase. 
  • During this phase, to some extent, spleen, thymus and kidney and lymph nodes also show hematopoiesis. 
• Medullary (Bone marrow) hematopoiesis 
  • Starts at 3^rd trimester of gestation 
  • Produces HbA and HbA2 
  • At birth throughout the skeleton, there is hematopoiesis. 
  • In adults, hematopoiesis occurs, mostly in sternum, pelvis, skull, ends of long bones and vertebra. 

Stem cells: 

• They are undifferentiated cells, capable of dividing for indefinite periods, to self-renew and to generate functional progeny of highly specialized cells. 
• Hierarchy of stem cells 
  • Totipotent (Present in zygote) 
  • Pluripotent (Present in embryo) 
  • Multipotent (Adult tissues such as bone marrow) 
  • Oligopotent 
  • Unipotent 
• 10¹³ new cells are needed every day to maintain steady state of blood counts (i.e., 6 billion cells/kg/day). 
• There is one stem cell per 20 million nucleated cells. 
• Stem cell pool consists of 1-2x10⁶ cells 
• When a stem cell divides, one cell remains in the stem cell pool, while the other cell goes into maturation pathway. 
• Stem cell plasticity- It is the ability of the hematopoietic and stromal stem cells to produce cells associated with other tissues, such as liver, lung and muscle. This differentiation depends on the cytokines present in the micro-environment. 
• Stem cell trafficking- It is the movement of stem cells from bone marrow into circulation and “homing in” of circulating stem cells into the bone marrow. Normally very few of stem cells are released into peripheral blood and most of them return back and home in.  
• “Homing in” involves transendothelial migration from the blood stream into the marrow micro-environment. Chemokine- CXCL-12, the ligand for CXCR-4 expressed on stem cells, plays important role in homing in. 
• Mobilization- It is detachment of stem cells from the micro-environment and transendothelial migration in the reverse direction. Matrix metalloproteases and reactive oxygen species, which are produced during G-CSF administration, degrade the components of endothelial tight junctions and enhance stem cell egress. 
• Immunophenotyping of stem cells  
  • Positive- CD34, CD90, CD133, CD117, CXCR4, Scal-1, CD164, CD150, CD110 
  • Negative- CD33, CD38, HLA-DR, Lineage specific markers 
• Anti-CD 34 antibodies, tagged with magnetic beads can be used for isolation of stem cells. This process is called as cell sorting. 1% of human bone marrow cells express CD34.  
• Stem cells are resistant to chemotherapy induced cytotoxicity due to high level of expression of drug efflux pumps (verapimil sensitive) 

Erythropoiesis: 

 Figure 1.1.1. - Pronormoblast (Single arrow) and early normoblast (Double arrow) 

Figure 1.1.2. - Intermediate normoblast (Single arrow) and late normoblast (Double arrow)

• Within bone marrow, erythroblasts are organized into erythroblastic islands which contain central 1-2 macrophages, surrounded by layer of erythroblasts in different stages of development. 
• Hemoglobin content has a negative feedback on cell proliferation and differentiation 

• If hemoglobin content is less (as in iron deficiency), cells undergo extra division, yielding smaller hypochromatic cells. 
• If hemoglobin synthesis exceeds DNA synthesis (as in case of megaloblastic anemia), cells skip a division and nuclear extrusion occurs early, leading to macrocytosis. 

• Rate of erythropoiesis can increase to 5-10 folds in response to appropriate erythropoietin stimulation, if sufficient iron store is present. 

Regulation of erythropoiesis 

Hypoxia 

↓ 

Lack of oxygen for ferrous iron propylhydroxylase enzyme 

↓ 

No hydroxylation of proline residues of HIF-1-alpha, hence no degradation of HIF-1-alpha (Normally this happens through ubiquitation by von Hippel Lindau protein) 

↓ 

HIF-1-alpha heterodimerizes with HIF-1-beta which is constitutively present 

↓ 

Activation of following hypoxia responsive genes, resulting in increased production of various factors such as 

Erythropoietin- Binding of EPO to EPO receptor induces conformational change in EPO receptor, which activates JAK-2 pathway, which stimulates proliferation of erythroid cells. 

Vascular endothelial growth factor 

Transferrin receptors 

Granulopoiesis: 

Figure1.1.3 - Myeloblast (Single arrow), Band form (Double arrow) 

Figure 1.1.4 - Promyelocyte (Single arrow), Myelocyte (double arrow), Metamyelocyte (Triple arrow) 

Morphology of each type of mature granulocyte is explained in WBC section. 

Monocytic series 

Figure1.1.5 - Monocyte (Arrow) 

Lymphopoiesis: 

Figure 1.1.6 - Lymphocyte 

Megakaryopoiesis: 

 Figure 1.1.7- Promegakryocyte 

Figure 1.1.8- Megakryocyte

• Megakaryocytes are 1micrometer away from BM sinus and shed platelets directly into the sinus. 
• Platelets are released in groups called proplatelets, which later on break to form platelets. This breaking down principally occurs in the lung. 
• 5 days are needed for formation of platelets from megakaryocytes. 
• Each day 10¹¹ platelets are produced. 
• Life span of platelet is 7-10 days 
• Each megakaryocyte gives rise to 1000 to 3000 platelets before the residual nuclear material is engulfed and eliminated by marrow macrophage. 
• Nuclear maturation takes place first and largely complete before cytoplasmic maturation starts. 
• Nuclear maturation consists of series of endomitosis, in which the DNA content of the cell is doubled but cell division does not take place. So, ploidy becomes 4N to 128N.  
• IL6, IL11 and thrombopoietin stimulate megakaryocyte proliferation and differentiation. 
• Thrombopoietin is produced in liver and it acts through c-Mpl receptor on megakaryocytes.  
• Platelets also have c-Mpl receptors, which bind and clear TPO from circulation. 
• Emperipolesis- In marrow sections, neutrophils or other marrow cells are seen transiting through the cytoplasm of mature megakaryocyte. This is called emperipolesis. There is no pathological significance for this. 

Cytokines and genes involved in hematopoiesis

(Upregulated genes are written in bracket)

Figure 1.1.9 - Normal trephine biopsy of a young person 

Bone marrow microenvironment: It consists of following elements 

• Marrow stroma 
  • Sinusoids lined by endothelial cells and surrounded by adventitial reticular cells. Latter cells are fibroblasts which are capable of transforming in to adipocytes. 
  • Innervation- Both myelinated and non-myelinated nerve fibers are present in the periarterial sheath of marrow. Non-myelinated nerve endings secrete neurohumors that affect hematopoiesis. 
  • Osteoblasts and osteocytes 
  • Osteoclasts 
  • Mesenchymal stem cells- Located in the perisinusoidal space. 
  • Macrophages 
  • Lymphocytes 
  • Plasma cells 
  • Extracellular matrix- Composed of proteoglycans (Heparin sulphate, chondroitin sulphate), fibronectin, tenacin, collagen, laminin, hemonectin, thrombospondin, vitronectin 
• Cell adhesion molecules 
  • Integrins 
  • Immunoglobulins 
  • Selectins 
  • Sialomucins 
  • Hyaluridin 
• Various cytokines such as G-CSF, GM-CSF, glucocorticoids etc.