Histology Week 3

Question 1

What is a stem cell?

Answer 1

A cell that has the ability to differentiate into any other kind of cell. Characterized by: - High self-renewal - Low mitotic index

Question 2

Describe the four types of stem cells

Answer 2

Totipotent cells - unlimited cell potential Pluripotent cells - multi-organ potential Progenitor cells - limited stem potential Precursor cells - cells committed to differentiate

Question 3

How are stem cells identified?

Answer 3

Stem cells can be identified by looking for combinations of surface proteins which are stage specific

Question 4

Give two examples of stem cell markers

Answer 4

CD34 - Positive marker HLA-Dr - Negative marker

Question 5

Provide three points when stem cells may be collected and describe the stem cells collected

Answer 5

Blastocyst: Embryonic stem cells (essentially totipotent) Fetus: Embryonic germ cells (not as totipotent) Teratocarcinoma (within a tumor in the gonads): Embryonal carcinoma (more like a progenitor cell)

Question 6

Relate how the renewal of cell populations relies on stem cell populations and how this impacts on the potential medical uses of stem cells

Answer 6

Because of the cell life of a RBC (120 days) approximately 1% of all RBCs have to be replaced daily. Labile tissue requires a high turnover of cells which requires the constant production by stem cells. The plasticity of stem cells, or the ability to give rise to progeny outside the tissue type where it resides (i.e. use a muscle stem cell to develop liver tissue), can have a big impact in the future (multiple clinical trials).

Question 7

List hematopoietic growth factors

Answer 7

Multi-potential - usually early acting Erythropoietic - EPO Granulopoietic - G-CSF Granulopoietic Macrophage - GM-CSF Magakaryocytopoietic - TPO (T = Thrombopoietic) Lymphopoietic - interleukins (CSF = Colony stimulating factor)

Question 8

Describe the basic structure of bone marrow

Answer 8

Bone marrow is the niche where hematopoietic stem cells reside and differentiate the blood cell lineage forming red and white blood cells and platelets. Like all niches it consists of stem cells and their products, adventitial cells, and endothelium forming blood vessels.

Question 9

Describe the interaction between the cellular environments (extracellular matrix and soluble molecules), the supporting cells (stroma) and the developing stem cells.

Answer 9

The niche, or stem cell microenvironment, helps to protect and define stem cells and how they develop and differentiate. The extracellular matrix consists of different proteins which help cell differentiation. Stroma or adventitial cells establish the matrix by secreting extracellular matrix molecules, and help cell differentiation by presenting cell surface molecules and growth factors. Endothelial cells support differentiation by permitting targeting, adhesion, and transmigration of cells.

Question 10

List the extracellular matrix proteins that influence hematopoietic stem cells

Answer 10

Fibronectin - interacts with integrins on cell surface Thrombospondin - binds soluble cytokines present to cells Hemonectin - binds myeloid cells Heparan sulfate and chondroitin sulfate - bind various ligands

Question 11

Outline at a basic molecular level how the interaction of ligands and receptors mediates the differentiation of cells

Answer 11

Very small concentrations of mediators (soluble effector molecules: interleukins, cytokines, chemokines, growth factors) control cell differentiation because the synergy between certain combinations of mediators can have a powerful effect. The presentation of molecules on extracellular molecules or neighboring cells leads to bound molecules which have a higher “effective” concentration. Binding of the stem cells to different matrix proteins may alter the responsiveness of the stem cells to regulatory factors.

Question 12

Distinguish between embryonic and adult stem cells

Answer 12

Embryonic stem cells are derived from blastocyst cell masses or embryonic germ cells. They tend to be pluripotent and tumorigenic (harder to control, with a higher potential for unregulated growth into a tumor). Adult stem cells are found in hematopoietic, mesenchymal-derived, hepatic and neural niches. Compared to embryonic stem cells they are not as primitive (i.e. more diverse, more specific) but possibly less tumorigenic.

Question 13

Consider the effect of using embryonic or adult stem cells from a scientific and ethical standpoint

Answer 13

Because the collection of embryonic stem cells requires the destruction of an embryo and the higher tumorigenic characteristics the trend is to use adult stem cells. Methods have also been derived to develop induced pluripotent stem cells by using a patient’s own cells, transfecting those cells with transcription factors, reverse differentiation and guide redifferentiation into desired cell types and infuse into patient.

Question 14

Organize the cellular components of the hematopoietic system (platelets and cells) in the context of hemostasis and the coagulation process.

Answer 14

CFU-MK-E pluripotent cells → CFU-MK → Megakaryoblast undergoes endomitosis (DNA synthesis without cell division, multiple copies of nucleus) → Megakaryocyte, a large polyploid cell in bone marrow which gives off threads of platelets. Platelets cause clotting. Endothelial cells also have a role in hemostasis and the coagulation process mediating platelet adhesion.

Question 15

Describe the cellular structure of platelets with sufficient detail to understand cellular mechanisms.

Answer 15

Platelets are enclosed by a cell membrane which is ringed by a set a microtubules. They contain mitochondria, many granules, and a very high concentration of receptors on the cell membrane (leading to a high sensitivity).

Question 16

Describe platelet activation

Answer 16

Examples of platelet interactions with soluble molecules include thrombin, ADP, epinephrine, serotonin. The thrombin receptor is a protease activated receptor, or PAR, consisting of 7 transmembrane loops and an activation site whereby ligand binding causes cleavage of the receptor and resulting conformational change which induces signal transduction. Platelet signal transduction: - Receptor couple to G-proteins - Activate phospholipases - Hydrolyze phosphotidylinositol (PIP) - IP3 and diacylglycerol (DAG) as messengers cause morphology changes and secretion

Question 17

Describe the biochemical mechanisms for activation and down-regulation of platelet responses during coagulation and clot formation.

Answer 17

Platelet interactions with filamentous molecules: fibrinogen-fibrin, collagen, von Willebrand factor, provoke clotting. Platelets promote clotting in opposition to endothelium which promotes blood flow and down regulation of platelet aggregation. When necessary, endothelial cells secrete von Willebrand factor which promotes clotting.

Question 18

Apply the principles of stem cell differentiation to the specific case of hematopoiesis and identify the developmental stages of blood cell formation. This should be in conjunction with the following laboratory exercise.

Answer 18

• Multipotential hematopoietic stem cell (Hemocytoblast)
  
  • Common lymphoid progenitor
    
    • Lymphocytes
    • Natural Killer cells
  • Common myeloid progenitor
    
    • Platelets
    • Red Blood cells
    • White Blood cells

Question 19

Provide red blood cell lineage

Answer 19

• Proerythroblast
• Basophilic Erythroblast
• Polychromatic Erythroblast
• Orthrochromatophilic Erythroblast
• Reticulocyte
• Erythrocyte

Question 20

Provide myeloid differentiation lineage

Answer 20

Myeloblast

• Promyelocyte
  
  • Early neutrophilic myelocyte
    
    • Late neutrophilic myelocyte
    • Neutrophilic metamyelocyte
    • Band cell
    • Mature neutrophil
  • Early eosinophilic myelocyte
    
    • Late eosinophilic myelocyte
    • Eosinophilic myelocyte
    • Mature eosinophil
  • Early basophilic myelocyte
    
    • Late basophilic myelocyte
      
      • Mature basophil

Question 21

Describe the initiation of vasculogensis in the developing embryo:

Answer 21

Blood Islands in the yolk sac of the embryo:

• Development of blood cell formation begins at the 3rd week.
• Blood vessels begin to form by the end of the 3rd week
• Blood cells develop from the early endothelium
• Heart starts to beat at day 21-22

Endothelial cells can develop into vessels or hematopoietic cells- adult (angiogensis) and fetal (vasculogensis and angiogensis) processes different

• FGF –> Mesenchyme cells
• VEGF –> Receptor 2
• VEGF –> Receptor 1

Question 22

Identify the different classes of blood structures

Answer 22

Categories of Large vessels:

Arteries -

1. small to medium arteries
2. muscular arteries
3. elastic arteries (named arteries ex femoral artery)

Veins

1. Small to medium veins
2. Large veins (named veins)
3. Contain valves; like lymphangions, also have valves to prevent backflow of blood and force blood back to the heart

Question 23

Compare arteries and veins:

Answer 23

Often found as adjacent to each other so it is usually easy to compare the structures

Both lined by endothelial cells (tunica intima), but the cells can have different characteristics (responses to serotonin, histamine and acetylcholine)

Tunica media is more robust in artery compared to the equivalent vein with the artery having more layers of smooth muscle; tunica media of arteries contains sheets of elastic fibers

Tunica adventitia may contain smooth muscle bundles in the larger veins (ex. vena cava)

A vein will often be collapsed or misshapen due to decreased volume in the venous system

Question 24

Compare Venules & Arterioles:

Answer 24

small vessels; arterioles generally smaller than venules, BUT have complete layer of smooth muscle

Question 25

Describe capillaries:

Answer 25

Capillaries: smallest blood vessels; sites of exchange

Question 26

Name each of the following structures:

Answer 26

See pic:

Question 27

Describe the three capillary types:

Answer 27

Three types of capillaries: smallest blood vessels; sites of exchange for gases, digestive products, other small molecules, and water

• Continuous
  
  • Complete, Continuous Epithelium
  • Tight Junctions
  • Continuous Basal Lamina
• Fenestrated (Fenestra: window (Italian) (endocrine organs: pituitary, kidney)
  
  • Diaphragms (act as filters across the pores)
  • Continuous basal laminae
  • Fenestrated endothelial cells
• Discontinuous (sinusoid) Capillary (spleen, liver, bone marrow)
  
  • Incomplete endothelial cells
  • Advantageous in bone marrow because it allows easy exit for blood cells
  • Discontinuous basal laminae

Cell components:

• Endothelial Cells
• Pericytes
• No smooth muscle cells

Question 28

Describe lymphatic vessels:

Answer 28

Lymphatic Vessels: begins as closed endothelial-lined capillaries that anastomose to form vessels of increasing size (no smooth muscle cells)

• Growth of lymphatics stimulated by platelet aggregation
• Excess fluid in the extracellular matrix is taken up by lymphatic capillaries
• Activated immune cells can migrate from sites of infection to lymph nodes
• Flow of fluid in lymphatics determined by valves (lyphangion = larger lymph vessel)

Question 29

The walls of blood vessels (larger than capillaries) are composed of three layers, name the layers:

(tunica = layer)

Answer 29

Tunica intima: endothelium
Tunica media: smooth muscle layer
Tunica adventitia: consists of connective tissue and sparse smooth muscle

Question 30

What are the endothelial cell functions?

Answer 30

Endothelial Cell functions:

-simple squamous epithelium lines all vessels

• Permeability- gases, nutrients, water, lipids
• Regulation of Coagulation (stimulatory & inhibitory)
• Regulation of the immune response
• Modulate blood pressures - secretion and binding of various factors (ex. NO, endothelin, angiotensin 1 –> 2
• Growth and Repair
• Involvement in lipid metabolism

Question 31

Describe pericytes and their function:

Answer 31

Pericytes: specialized cell type found on capillaries and venules; increased numbers on venules compared with capillaries

Inside the basal lamina

Stabilize and monitor the maturation of endothelial cells by means of direct communication between the cell membrane as well as through paracrine signaling capillaries and venules

Responsive to signals (ex. NO)

Give rise to muscle and endothelium during injury

Regulate capillary blood flow, the clearance and phagocytosis of cellular debris, and the permeability of the blood–brain barrier

Can be involved in disease processes