M2: PLATELETS IN HEMOSTASIS

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Megakaryopoiesis

- translated as the development and maturation of megakaryocytes from the progenitor cell.

CFU-GEMM

- common myeloid progenitor where megakaryocytes arise from

GATA-1

- transcription gene that influences the rise of megakaryocytes from CFU-GEMM

FOG1

- cofactor that regulates GATA-1

MYB

- transcription gene product that suppresses megakaryocyte differentiation

GATA-1 and MYB

- act in opposition to balance megakaryopoiesis in one arm with differentiation to the red blood cell line in another arm.

1. burst-forming unit (BFU-MEG)
2. colony-forming unit (CFU-MEG)
3. light-density CFU (LD-CFU-Meg)

- three progenitor stages from least mature to more mature

Lymphocytes

- what do the three progenitor stages resemble making it not be distinguished by Wright-stain microscopy

BFU-Meg and CFU-Meg

- diploid and participate in normal mitosis, maintaining a viable pool of megakaryocyte progenitors.

LD-CFU-Meg

- does not have the capacity to divide but retains its DNA replication and cytoplasmic
maturation, endomitosis

Endomitosis

- partially characterized form of mitosis unique to megakaryocytes

Endomitosis

- a form of mitosis that lacks telophase and cytokinesis (separation into daughter cells).

RUNX1

- transcription factor that mediates mediates the switch from mitosis to endomitosis by suppressing the Rho/ROCK signaling pathway
- this is activated when GATA-1 and FOG1

Rho/ROCK signaling pathway

- suppresses the assembly of the actin cytoskeleton.

Actin and Myosin

- these are assembled inadequately in cytoplasmic constrictions, preventing cytokinesis

NF-E2

- transcription factor that influences production of ploidy in DNA replication

1. 8N
2. 16N
3. 32N

- ploidy with duplicate chromosome sets influenced by NF-E2

128N

- considered an unusual level of ploidy and may signal hematologic disease.

Terminal Differentiation

- As endomitosis proceeds, megakaryocyte progenitors leave the proliferative
phase and enter ________

MK-I stage or megakaryoblast

- least differentiated precursor of platelets

False

True

T or F

Megakaryoblasts resemble lymphocytes.

Megakaryoblasts resemble myeloblasts or
pronormoblasts.

plasma membrane blebs

- distinguishing feature of megakaryoblast from myeloblasts or pronormoblasts

plasma membrane blebs

- blunt projections from the margin that resemble platelets.

1. procoagulant-laden alpha granules
2. dense granules (dense bodies)
3. demarcation system (DMS)

- cytoplasmic ultrastructures of a developing megakaryoblast

DMS

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

DMS

- biologically identical to the megakaryocyte plasma membrane and ultimately delineates the individual platelets during thrombopoiesis.

Nuclear lobularity

- becomes apparent as an indentation at the 4N
replication stage, which makes the cell identifiable by light microscopy as the MK-II stage, or promegakaryocyte.

Promegakaryocyte

- features nuclear lobularity
- promegakaryocyte reaches its full ploidy level by the end of the MK-II stage.

MK-III stage

10x

30- to 50-um

- At the most abundant______, the megakaryocyte is easily recognized at ____ magnification because of its ______ diameter.

MK-I: 20
MK-II: 25
MK-III: 55

-% of precursors in MK-I, MK-II, MK-III

MK-I: 14-18 um
MK-II: 15-40 um
MK-III: 30-50 um

- diameter of MK-I, MK-II, MK-III

MK-I: round
MK-II: indented
MK-III: multilobed

-nucleus of MK-I, MK-II, MK-III

MK-I: 2-6
MK-II: variable
MK-III: not visible

- nucleoli of MK-I, MK-II, MK-III

MK-I: homogenous
MK-II: mod condensed
MK-III: deeply and variably condensed

- chromatin of MK-I, MK-II, MK-III

MK-I: 3:1
MK-II: 1:2
MK-III: 1:4

- nucleus to cytoplasm ratio of MK-I, MK-II, MK-IIIq

MK-I: absent
MK-II: absent
MK-III: absent

- mitosis in MK-I, MK-II, MK-III

MK-I: present
MK-II: ends
MK-III: absent

- endomitosis in MK-I, MK-II, MK-III

MK-I: basophilic
MK-II: basophilic and granular
MK-III: azurophilic and granular

- cytoplasm of MK-I, MK-II, MK-III

MK-I: present
MK-II: present
MK-III: present

- alpha granules, dense granules, and demarcation system of MK-I, MK-II, MK-III

Thrombopoiesis

- platelet shedding at full maturation

naked megakaryocyte nuclei

- what does thrombopoiesis leave behind that is consumed by marrow macrophages

2000-4000

- number of platelets a megakaryocyte can produce through thrombopoiesis

10^8 megakaryocytes
10^11 platelets

- number of megakaryocytes, platelets produced on an average

8-9 days

- total turnover rate of megakaryocytes to platelets

abluminal

sinusoid-lining endothelial cell

proplatelet

- Megakayocyte is adjacent to the _____ membrane of the ____ and extends a ______ process through or between the endothelial cells into the vascular sinus.

1. TPO
2. IL-3
3. IL-6
4. IL-11

- cytokine/hormone involved in platelet production (megakaryopoiesis and thrombopoiesis)

1. Differentiation to Progenitors
2. Differentiation to Megakaryocytes
3. Late Maturation

- function of TPO in platelet production

1. Differentiation to Progenitors
2. Differentiation to Megakaryocytes

- function of IL-3 in platelet production

1. Late Maturation
2. Thrombopoiesis

- function of IL-6 in platelet production

1. Differentiation to Megakaryocytes
2. Late Maturation
3. Thrombopoiesis

- function of IL-11 in platelet production

Proplatelet Process

- sheds the platelets which are cells consisting of
granular cytoplasm with a membrane but no nuclear material, into the venous sinus of the bone marrow.

2.5 um

- average diameter of a platelet

8-10 fL

- MPV of platelets

1. Circular to Irregular
2. Lavender
3. Granular

- appearance of platelets on a Wright-stained wedge preparation blood film

150-450x10^9/L

- normal platelet count in peripheral blood

Spleen

- sequesters the additional 1/3 of platelets

Relative Thrombocytopenia

- what is the possible condition if there is hypersplenism or splenomegaly

Platelets become irregular and sticky, extending pseudopods, and adhering to neighboring structures or aggregating with one another.

- how do platelets respond to conditions of hemostatic need through cellular and humoral immunity

Reticulated Platelets (stress platelets)

- appear in compensation for thrombocytopenia
- markedly larger than ordinary mature circulating platelets

6 um

- diameter of reticulated platelets

12-14 fL

- MPV of reticulated platelets

False (baliktad)

T or F

Stress platelets round up in citrated whole blood.

Stress platelets become cylindrical and beaded in EDTA.

True

True

T or F

Stress platelets resemble the fragments of megakaryocyte proplatelet processes.

This is observed in citrated whole blood.

True

T or F

Under healthy steady-state conditions, the rate of platelet release from the
megakaryocytes is equivalent to the rate of platelet removal from the circulation.

35,000 +/- 4300 platelets/mL/day

- net rate of production is expressed as platelet turnover

2-9 days

- lifespan of a platelet

Electron Microscopy

- where is the detailed anatomy of a platelet best elaborated

1. plasma membrane
2. submembrane
3. cytoskeleton
4. organelles

- four major areas of the platelet structure

plasma membrane

- part of platelet which consists of the platelet's outer membrane and related structures

submembrane

- part of platelet which links the membrane and the inner cell body

cytoskeleton (sol-gel zone)

- part of platelet which constitutes the matrix or muscle and skeletal portion of the platelet

1. granules
2. dense bodies
3. lysosomes
4. mitochondria

- platelet organelles consists of ___

7.5 nm

- thickness of the platelet plasma membrane

trilaminar

- unit structure of platelet plasma membrane

Glycocalyx

- thicker and denser than that of most blood cells, ranging in thickness from 10 to 50 nm.

Ia
Ib
Ic
IIa
IIb
III
IV
V
IX

- glycoproteins in the glycocalyx of platelets

Glycocalyx

- provides a surface to which some coagulation factors may adhere during
secondary hemostasis.

Plasma membrane

- underlying the glycocalyx which serves as the physical and chemical barrier between the intracellular and extracellular constituents of the platelet.

Sodium/Potassium ATPase Pump

- maintains a transmembrane ionic gradient

1. phosphatidylserine
2. phosphatidylcholine
3. phosphatidylinositol

- phospholipid constituents in the plasma membrane of platelets

platelet factor VIII (von Willebrand factor)

- an important constituent for the acceleration of blood coagulation and for platelet adhesion, is also located in the lipoprotein-rich plasma membrane of platelets

submembrane

- this area lies beneath the plasma membrane.

submembrane

- separates the organelles within the inner matrix of unaltered platelets and the cell wall.

submembrane

- contains an organized system of filaments which are physiologically comparable to the
microfilaments and submicrofilaments of the microtubule system.

submembrane filaments

- contribute to the regulation of the normal platelet discoid shape, act as a base for pseudopod formation, and interact with other contractile proteins to
modulate platelet adhesion and clot retraction after activation.

cytoskeleton

- underlies the submembrane area.

cytoskeleton

- represents the matrix of the platelet cytoplasm and consists of a circumferential microtubule system and randomly arranged microfilaments that form an intraplatelet matrix that support the discoid shape of the platelet.

Microfilaments in cytoskeleton

- provide the contractile force after activation that directs the organelles toward the center of the cell with control and direction from the microtubules.

Cytoskeleton

- serves as stabilizing component to regulate the arrangement of the internal organelles and microtubular system within the resting platelet body.

Cytoskeleton

- facilitates communication between the organelles and the platelet's external surroundings.

Microtubule System in Cytoskeleton

plays an important role in
the contractile response of platelets to stimulus.

Microtubule System in Cytoskeleton

- It acts to mediate and influence the extent of the platelet contractile response.

platelet activation

1. ADP, ATP
2. GDP, GTP
3. Calcium
4. Magnesium
5. Serotonin

- granule content of dense bodies of platelets

1. Platelet Specific Proteins
2. Multimerin
3. Adhesive glycoproteins
4. Coagulation Factors
5. Mitogenic Factors
6. Fibrinolytic Inhibitors
7. Membrane-associated proteins

- granule content of alpha-granules

1. Platelet Factor 4 (PF4)
2. Beta-Thromboglobulin family

- platelet specific proteins

Fibrinogen
Von Willebrand Factor
Fibronectin
Thrombospondin
Vitronectin

- adhesive glycoproteins

Factor V
Factor XI
Protein S

- coagulation factors

Platelet-derived growth factor (PDGF)
Transforming growth factor-B
Endothelial cell growth factor

- mitogenic factors

Alpha2-Plasmin Inhibitor
Plasminogen activator inhibitor-1

- fibrinolytic inhibitors