PLATELETS

Megakaryocytopoeisis

Nonnucleated blood cells

Megakaryocytopoeisis

Main function:

Trigger primary hemostasis upon exposure to subendothelial collagen

150-400 X10^9/L

RV of Megakaryocytopoeisis

ave. diameter of 2.5um

PBS of Megakaryocytopoeisis

7-21 cells/field

Megakaryocytes is distributed around a monolayer of RBCs at

8-10fl

Mean Platelet volume of megakaryocytes

30–50 um

PLT is the largest cells in the bone marrow. How many um is it?

0.5%

Megakaryocytes is less than _____ of all BM cells

polyploid

Megakaryocyte is polyploid or haploid?

Thrombopoieitin (TPO)

Megakaryocytes respond to the growth factor:

Common Myeloid Progenitors (CMP)

Megakaryocyte is recruited from

• Burst Forming Units Megakaryocyte (BFU-Meg)

• Colony Forming Units Megakaryocyte (CFU-Meg)

• Light Density colony forming unit Meg (LD-CFUMeg)

3 Megakaryocyte Progenitors

Colony Forming Units Megakaryocyte (CFU-Meg)

• Resembles a lymphocyte

• Diploid

• Participates in mitosis

Light Density colony forming unit Meg (LD

CFU-Meg)

• Resembles a lymphocyte

• Low mitotic capacity

• Retains DNA replication and cytoplasmic maturation

• More mature progenitor

endo-reduplication

Endomitosis is also known as ____

Mitosis without telophase and cytokinesis

2000 to 4000 platelets

Single megarkaryocyte can give rise to _________ (thrombocytopoiesis)

BFU- Meg

clones hundreds of daughter cells through mitosis.

CFU-Meg

clones dozens of daughter cells through mitosis.

LD-CFU-Meg

undergoes the first stage of endomitosis.

1. Proliferative Phase

2. Terminal Differentiation

2 developmental compartments

BFU-Meg

CFU-Meg

LD-CFU-Meg

3 Proliferative Phase

Terminal Differentiation

series of stage in which microscopists can distinguish/recognize progenitors from each other.

• MK-I: Megakaryoblast

• MK-II: Promegakaryocyte

• MK-III: Megakaryocyte

Terminal Differentiation

• Goes through 3 different stages

20

% of precursors of MK-I

25

% of precursors of MK-II

55

% of precursors of MK-III

14-18 um

Diameter of MK-I

15-40 um

Diameter of MK-II

30-50 um

Diameter of MK-III

round

Nucleus of MK-I

Indented

Nucleus of MK-II

Multilobed

Nucleus of MK-III

2-6

Nucleoli of MK-I

Variable

Nucleoli of MK-II

Not visible

Nucleoli of MK-III

Homogenous

Chromatin of MK-I

Moderately Condensed

Chromatin of MK-II

Deep and Variably condensed

Chromatin of MK-III

3:1

Nucleus to Cytoplasm Ratio MK-I

1:2

Nucleus to Cytoplasm Ratio MK-II

1:4

Nucleus to Cytoplasm Ratio MK-III

Mitosis

Mitosis or Endomitosis?
Absent in all MK-I; MK-II; MK-III

Present

Endomitosis in MK-I

Ends

Endomitosis in MK-II

Absent

Endomitosis in MK-III

Basophilic

Cytoplasm of MK-I

Basophilic and Granulophilic

Cytoplasm of MK-II

Azurophilic and granulophilic

Cytoplasm of MK-III

Megakaryoblast

blebs (blunt projections)

Promegakaryocyte

nuclear lobularity

Megakaryocyte

cytoplasm is azurophilic (lavender), granular

CD34

CMP

CD41

GP IIb/IIIa

CD62

P-selectin (an integrin that facilitates

platelet binding to endothelial cells,

leukocytes and to one another)

CD62: P-selectin

an integrin that facilitates platelet binding to endothelial cells, leukocytes and to one another

Thrombocytopoiesis

Platelet shedding

8-9 days

Plt life span of Thrombocytopoiesis

demarcation system

series of membrane-lined channels that invade from the plasma membrane and grow inward to subdivide the entire cytoplasm

Thrombocytopoiesis

Longitudinal bundles of tubules are formed.

Thrombocytopoiesis

Transverse constrictions appear.

Thrombocytopoiesis

Proplatelet process extends.

Thrombocytopoiesis

Leaves behind naked MEG nuclei to be consumed by narrow macrophages

Proplatelet process

projections that resemble strings of beads

Proplatelet process

• pierce through the sinusoidal lining of endothelial cells

• extend into the venous blood

• shed platelets

70; 23%

Thrombopoietin has ____ KD molecule with ____ homology to EPO

Liver

Primary source of Thrombopoietin

kidney, stromal cells, and smooth muscle cells

Other sources of thrombopoietin

Thrombopoietin

Concentration is inversely proportional to the platelet and megakaryocyte mass.

Thrombopoietin

Induces stem cell differentiation, proliferation, maturation, and release

Thrombopoietin

In vitro studies show that it increases the amount of platelet in the bone marrow in healthy subjects.

Interleukin 3

Interleukin 6 & 11

Interleukin 11 (Neumega)

Cell derived stimulators of megakaryopoiesis

Interleukin 3

acts in synergy with TPO; induces early differentiation of stem cells

Interleukin 6 & 11

act in the presence of TPO; enhance endomitosis, megakaryocyte maturation, and thrombocytopoiesis

Interleukin 11 (Neumega)

stimulates platelet production in chemotherapy induced thrombocytopenia

• PF 4

• Β – thromboglobulin

• Neutrophil-activating peptide 2

• IL-8

four inhibit megakaryocyte development from progenitors.

Platelet

Granular cytoplasm, anucleate

Platelet

With cell membrane but without a clear nuclear material

2.5um

Diameter of Plt

8 –10 fL

MPV of Plt

Biconvex

Circulating, resting platelets

spleen (1/3)

Plt is Sequestered in the ________

Reticulated platelets

“stress” platelets

Reticulated platelets

Appear in compensation with thrombocytopenia

Reticulated platelets

Larger than ordinary platelets

Reticulated platelets

Has free ribosomes and fragments of

RER

> 6um

Diameter of Reticulated Plt

12 – 14 fL

MPV of Reticulated Plt

1. Peripheral zone

2. Sol-gel zone

3. Organelle zone

4. Membranous system

Four Areas of Platelets

Peripheral Zone

• Membranes

• Responsible for platelet adhesion & aggregation

a. glycocalyx

b. plasma membrane

c. sub-membranous area

3 parts of Peripheral Zone

Glycocalyx

• Fuzzy coating

Glycocalyx

• w/ glycoproteins, coagulation factors V,

VIII & fibrinogen

Glycocalyx

• Important in platelet reactions w/ thrombin, von Willebrand factor, & fibrinogen

Glycocalyx

• Absorbs albumin, fibrinogen, other plasma

proteins

Plasma membrane

• Lies directly beneath the glycocalyx

Plasma membrane

• Selectively permeable

Plasma membrane

• Bilayer lipid and proteins

Plasma membrane

•Glycoprotein Ib/IX/V

Glycoprotein Ib/IX/V

receptor for von Willebrand factor