Hematopoiesis

Question 1

describe early stages of hematopoiesis

Answer 1

• yolk - sac phase: hematopoiesis begins in the 3rd week of gestation, formation of “blood islands” (3-8 weeks)
• hepatic phase - major blood forming organ in 2nd trimester
• bone marrow phase - begins during 2nd trimester

Question 2

contrasts myeloid and lymphoid stem cells

Answer 2

• myeloid stem cells give rise to GEMM:
  
  • granulocytes (BEN)
  • erythrocyte
  • monocyte
  • megakarocytes
    
    • thrombocyte
• lymphoid stem cell
  
  • T cell
  • B cell

Question 3

describe the endosteal niche

Answer 3

• microenvironment in which HSCs are housed and maintained by allowing self renewal in the absence of differentiation
• functions of stem cell niche:
  
  • storage of quiescent stem cells
  • self renewal
  • inhibition of differentiation
• found in the epiphysial region of bones

Question 4

describe the vascular niche

Answer 4

• quiescent HSCs detach from the endosteal niche and migrate towards the center of the bone marrow to the vascular zone from where they establish hematopoiesis

Question 5

describe the structure of bone marrow

Answer 5

• structure: reticular fibers, veins, arteries, sinusoids (type of capillaries), islands of cells
• red marrow: active hematopoiesis
• yellow marrow: fat, capillaries, reticular cells, inactive hematopoeisis

Question 6

name the hematopoietic growth factors

Answer 6

• erythropoietin (EPO)
  
  • stimulate production of erthyrocytes
• thrombopoietin (TPO)
  
  • stimulate production of megakaryocytes –> thrombocytes
• GM-CSF
  
  • granulocytes
• interleukin-7
  
  • lymphocytes (B and T cells)

Question 7

describe erthropoiesis

Answer 7

main stimulus is hypoxia, 3-5 days to complete development

erythropoietin - produced by kidney

• proerythroblast: no hemoglobin, large nucleus, basophilic
• basophilic erythroblast: some hemoglobin, condensing nucleus
• polychromatophilic erythroblast:
• orthochromatophilic erythroblast: increased hemoglobin
• reticulocyte: no nucleus, some ribosomes
• erythrocyte: only hemoglobin, no ribosomes

Question 8

describe granulocytopoiesis

Answer 8

• myeloblast
  
  • precursor cells
• promyelocyte
  
  • Golgi appears, granules form
• myelocyte
  
  • specific granules are produced, where cell type is determined
• metamyelocyte

Question 9

describe monocytopoiesis

Answer 9

• monoblasts
• promonocytes
• monocytes: kidney shaped nucleus
• enter the circulation, proceed to tissue spaces, differentiate into macrophages

Question 10

describe thrombopoiesis

Answer 10

• controlled by thrombopoietin (TPO) produced by liver
• megakaryoblast:
• megakaryocyte: large multi-lobed nucleus
  
  • platelets are formed from fragments of megakaryocyte

Question 11

describe lymphopoiesis

Answer 11

• lymphoblasts: large, undifferentiated cells
• prolymphocytes: medium-sized cells, condensing chromatin, no cell surface antigens
• some migrate from bone marrow to thymus, divide and differentiate into T-cells
• others remain in bone marrow, differentiate to B-cells, migrate to lymph tissues