Histology: Blood Cells & Hematopoiesis

Question 1

Blood

Answer 1

• Specialized CT consisting of cells and fluid ECM (plasma)
• Adult blood volume = 5-6 L
• Components of Blood:
  
  • ​55% Plasma (water, proteins, other solutes)
  • 45% Cellular Elements
    
    • 44% erythrocytes
    • 1% Buffy Coat (leukocytes and platelets)

Question 2

Erythrocytes

Answer 2

• Red Blood Cells
• Biconcave Disks that can readily deform
  
  • No nucleus, few organelles
  • Shape maximizes surface area
• 120 day lifespan
  
  • Old RBCs phagocytosed in spleen

Question 3

Hemoglobin

Answer 3

• Acidophilic Staining in RBC cytoplasm
• Tatrameric Protein:
  
  • 4 globin chains
  • 4 iron-containing heme groups
• Binds and transports O2 & CO2
• Sickle Cell Anemia:
  
  • Mutation in Hb causing RBCs to have sickle-shape

Question 4

Erythrocyte Membrane

Answer 4

• Very deformable
• Continuously rearranged to squeeze through vasculature
  
  • Unique composition and organization of PM and cytoskeletal proteins

Question 5

Anemia

Answer 5

• Reduction in hemoglobin, hematocrit (packed RBCs), RBC count
• Caused by: blood loss, nutritional deficits, hemolysis
• Visualized via Peripheral Smear
  
  • Helps visualize problems with RBCs, platelets, and WBCs

Question 6

Platelets

Answer 6

• Small, anucleate, cell fragments
  
  • Originate from cytoplasmic processes of megakaryocytes in bone marrow
• Important role in hemostasis (stopping blood flow)
  
  • Survey blood vessel walls for damage
  • Aggregate at injury sites
  • Promote formation of blood clots
• Thrombocytopenia: low platelet count
  
  • Easy bruising, petechiae, prolonged bleeding from cuts

Question 7

Leukocytes

Answer 7

• Immune cells: protect from pathogenic agents, foreign agents
• 1% total blood volume

Granulocytes:

• Prominent, specific granules
• Neutrophils, Basophils and Eosinophils

Agranulocytes:

• Lack specific granules
• Lymphocytes and Monocytes

Question 8

Neutrophils

Answer 8

• “PMN”
• Most common leukocyte (50-70%)
• Multilobed nucleus (3-5 lobes)
• Short lifespan: 8 hours in blood, 1-4 days in CT (eventually replaced by macrophages)
• First cells to arrive at acute bacterial infection
  
  • Attack via phagocytosis
• Can survive in anaerobic envrironments

Question 9

Neutrophil Granules

Answer 9

3 Types of Granules

1. Azurophilic (Primary)
   
   • ​​Lysosomes (proteases and antibacterial proteins)
2. Specific (Secondary) Granules
   
   • ​​Chemokines and cytokines involved in inflammatory response
3. Tertiary Granules
   
   • Contain metalloproteinases (facilitate migration into tissues)

Question 10

Diapedesis

Answer 10

1. Neutrophils express adhesion molecules on cell surface which bind to selectins on endothelial cells
   
   • First bind loosely, then tightly and extend pseudopodia
   • Allows migration through vessel wall

Question 11

Eosinophils

Answer 11

• Bi-lobed nucleus
• Lifespan: 1-2 weeks
• Kill parasites
• Modulate inflammatory responses
  
  • Sites of chronic inflammation
  • Limit duration and intensit of allergic reactions (inhibits mast cell degranulation)

Question 12

Basophils

Answer 12

• Least numerous leukocyte
• Bi-lobed or S-shaped nucleus (obscured by granules)
• IgE binds basophils that trigger degranulation of specific granules that contain vasoactive agents
  
  • Mediate allergic reactions, hypersensitivity and anaphylaxis

Question 13

Monocytes

Answer 13

• Large cells with kidney shaped nucleus
• Azurophilic granules (lysosomes)
  
  • No specific granules

Question 14

Lymphocytes

Answer 14

• Major functional cells of lymphatic system
• B Cells, T Cells and NK Cells
  
  • B/T Cells express unique CD (clusters of differentiation) on cell surfaces
    
    • Detected via IHC or Flow Cytometry
• Large, basophilic nucleus

Question 15

B Lymphocytes

Answer 15

• Mediate humoral immunity (produce antibodies)
  
  • Proliferate and differentiate into plasma cells that produce antibodies

Question 16

T Lymphocytes

Answer 16

• Mediate cellular immunity
  
  • Helper T Cells: activate other cells via cytokines
  • Cytotoxic T Cells: secrete cytotoxic substances to kill virally infected cells

Question 17

Natural Killer Cells

Answer 17

• Programmed to kill virus-infected cells and some cancer cells

Question 18

Hematopoiesis

Answer 18

• Formation of blood cells
• Throughout embryonic and adult life:
  
  • Prenatal period: yolk sac > liver/spleen > bone marrow
  • Post-Natal: Inactive marrow replaced with fat

Question 19

Red Bone Marrow

Answer 19

• Taking part in hematopoiesis
• Reticular CT stroma
• All blood cell types differentiate in bone marrow (except T lymphocytes)

Question 20

Hematopoietic Stem Cells

Answer 20

Start: Pluripotent Hemopoietic Stem Cells

• Myeloid Line
  
  • ​RBCs (erythropoiesis), Megakaryoctes (thrombopoiesis) and Granulocytes/Monocytes (leukopoiesis)
• Lymphoid Line
  
  • ​NK Cells, B and T Lymphocytes

Question 21

Erythropoiesis

Answer 21

• Stimulated by erythropoietin (made by kidneys)
• Cells decrease in size
• Nucleus becomes hyperchromatic (darker and more condensed)
• Cytoplasm changes from basophilic to acidophilic
  
  • Gaining hemoglobin
• Nucleus ejected —> becomes Reticulocyte
• Reticulocyte mature in peripheral blood to erythrocyte

Question 22

Thrombopoiesis

Answer 22

Formation of platelets

• Originate in red marrow
• Megakaryocyte (multilobed nucleus with multiple sets up chromosomes up to 64n)