thrombocytes and Thrombopoiesis

1. Thrombocytes and
Thrombopoiesis
DR. AMANY M. ELSHAMY
LECTURER OF BIOCHEMISTRY AND MOLECULAR DIAGNOSTICS
PH.D OF BIOCHEMISTRY AND MOLECULAR BIOLOGY
MLS, AUC, CAIRO

2. Outlines

6. Maturation
• In healthy intact bone marrow tissue, megakaryocytes, under the
influence of an array of stromal cell cytokines, cluster with
hematopoietic stem cells in vascular niches adjacent to venous
sinusoid endothelial cells.
• Responding to the growth factor thrombopoietin (TPO),
megakaryocyte progenitors are recruited from common myeloid
progenitors and subsequently differentiate through several
maturation stages.

8. Megakaryopoiesis
• The development process of
megakaryocytes and platelets in bone
marrow is known as megakaryopoiesis.
• It is divided into four stages.
• Mature megakaryocytes extend long and
slender cytoplasmic processes
(proplatelets) between endothelial cells
of sinusoids in the bone marrow and
platelets are released from
fragmentation of these processes.

10. Megakaryopoiesis
• Each megakaryocyte produces 1000 to 5000
platelets, leaving behind a ‘bare’ nucleus
which is removed by macrophages.
• A unique feature of thrombocytopoiesis is
endomitosis. This refers to nuclear division
with cytoplasmic maturation but without
cell division.
• As the cell matures from megakaryoblast to
the megakaryocyte, there is a gradual increase
in cell size, number of nuclear lobes, and
red-pink granules and gradual decrease in
cytoplasmic basophilia.

11. Megakaryocyte
• Upon complete maturation, megakaryocytes
extend pseudopods through the walls of the
marrow sinusoids and individual platelets
break off into the peripheral circulation.
• There is evidence that some of the
megakaryocytes are carried to the lungs where
platelets are released.
• A humoral factor, thrombopoietin, controls
the maturation of megakaryocytes.

12. Megakaryocyte
Megakaryocytes, the most
abundant cells of the platelet
series in the marrow, are large
and contain numerous nuclear
lobes with dense nuclear
chromatin, and small
aggregates of granules in the
cytoplasm.

13. Megakaryocyte
• Megakaryocyte is 30 to 50 mm in
diameter with a multilobulated
nucleus and abundant granular
cytoplasm.
• Megakaryocytes account for less than
0.5% of all bone marrow cells, and
on a normal Wright-stained bone
marrow aspirate smear the
microscopist may identify two to four
megakaryocytes per 10x low-power
field.
Megakaryocytes are the largest cells in the bone marrow

14. Thrombocytes
Thrombocytes= Mature
Platelets (PLTs)
•Origin: Myeloid progenitor cell
in response to growth factor
Thrombopoietin, gives rise to
Megakaryocytes.
•Mature platelets have no
nucleus.
•Reference range (SI units) is
150,000 - 450,000 µL.

15. Thrombocytes
Size: 2-4 µm
Appearing as purple blue
cells in stained blood film.
Life span: 8-12 days.

16. Platelets
• Platelets
• Platelets are derived from cytoplasmic fragmentation of bone
marrow cells called megakaryocytes.
• They measure 2 to 3 μ in diameter and do not contain a nucleus.
• It is round or oval, anucleate (for this reason some hematologists
prefer to call platelets “cell fragments”), and slightly granular.

17. Platelets
• Platelets remain viable in circulation for approximately 10 days.
• About one-third of the total platelets in the body are in the spleen and
the remainder in peripheral blood.
• Uncontrolled platelet and hemostatic activation are responsible for deep
vein thrombosis, pulmonary emboli, acute myocardial infarctions (heart
attacks), cerebrovascular accidents (strokes), peripheral artery disease,
and repeated spontaneous abortions (miscarriages).
• Elevated platelet counts, called thrombocytosis, signal inflammation or
trauma but convey modest intrinsic significance.

18. Ultrastructure of
Platelets

19. Ultrastructure of Platelets

20. Ultrastructure of Platelets

21. Ultrastructure of Platelets
• Ultrastructurally, the following three zones can be distinguished:
• (1) Peripheral zone: exterior coat (glycocalyx), cell membrane, open
canalicular system.
• (2) Sol-gel zone: microfilaments, circumferential microtubules, dense
tubular system.
• (3) Organelle zone: alpha granules, dense granules, mitochondria,
lysosomes.

23. Ultrastructure of Platelets

24. Platelets
• The central cytoplasm
is dominated by three
platelet granules:
• the δ (dense) granules
• α granules
• lysosomal granules.

26. Platelet Membrane

27. Platelet Membrane Glycoproteins
• The cell membrane contains integral membrane glycoproteins (Gp),
which play an important role in hemostasis.
• Important platelet membrane glycoproteins and their functions are as
follows:
• Gp Ib-IX-V: This is a constitutively active receptor that mediates
vWF-dependent adhesion of platelets to subendothelial collagen.

28. Platelet Membrane Glycoproteins
Important platelet membrane glycoproteins and their functions are as follows:
• Gp Ib-IX-V: This is a constitutively active receptor that mediates vWF-
dependent adhesion of platelets to subendothelial collagen.
• Note: Von Willebrand factor (vWF) is a glycoprotein crucial to primary
hemostasis through platelet and subendothelial collagen adhesion, and
the intrinsic coagulation cascade, through factor VIII stabilization.

29. Platelet Membrane Glycoproteins
Gp IIb/IIIa: On activation,
serves to bind fibrinogen
and thus mediates
aggregation. Also receptor
for vWF, fibronectin, and
thrombospondin.
Gp Ia-IIa: Constitutively
active receptor for
collagen and mediates
platelet adhesion
independent of vWF.

31. Platelet
Antigens

32. Platelet Antigens
• Platelets possess HLA antigens and platelet-specific antigens.
• HLA class I antigens induce alloimmunization (immune response
to nonself antigens ) and cause refractoriness to platelet
transfusions when platelets are obtained from random donors.

33. Platelet Antigens
• The platelet-specific antigen systems are now known as human
platelet antigen (HPA) systems.
• Platelet-specific antigens play an important role in neonatal
alloimmune thrombocytopaenic purpura (NATP) and in post
transfusion purpura.

35. Thrombocytes
functions
Platelet functions:
•Maintenance of
Vascular Integrity.
•Formation of the Primary
Hemostatic Plug.
•Accelerate thrombin
formation.

36. Thrombocytes
functions
Maintenance of Vascular
Integrity.
• Platelets, or thrombocytes, are true
blood cells that maintain blood vessel
integrity by initiating vessel wall repairs.
• Platelets rapidly adhere to the surfaces
of damaged blood vessels, form
aggregates with neighboring platelets
to plug the vessels, and secrete
proteins and small molecules that
trigger thrombosis, or clot formation.

37. Thrombocytes
functions
Primary Hemostatic Plug.
•Platelets are the major cells
that control hemostasis, a
series of cellular and plasma-
based mechanisms that seal
wounds, repair vessel walls,
and maintain vascular
patency (unimpeded blood
flow).

38. Role of Platelets in
normal Hemostasis

39. Role of Platelets in Haemostasis
• Activation of platelets refers to adhesion, aggregation, and release
reaction of platelets which occurs after platelet stimulation (i.e.
after vascular damage).

40. Role of Platelets in Haemostasis
• Platelets do not normally adhere to each other or to the
vessel wall but can form a plug that aids in cessation of
bleeding when vascular disruption occurs.
• Injury to the intimal layer in the vascular wall exposes
subendothelial collagen to which platelets adhere. This
process requires von Willebrand factor (vWF), a
protein in the subendothelium that is lacking in patients
with von Wille brand disease.
• vWF binds to glycoprotein (GP) I/IX/V on the platelet
membrane.

41. Role of Platelets in Haemostasis
• Following adhesion, platelets initiate a release reaction that
recruits other platelets from the circulating blood to seal the
disrupted vessel. Up to this point, this process is known as
primary hemostasis.
• Platelet aggregation is reversible and is not associated with
secretion. Additionally, heparin does not interfere with this
reaction and thus hemostasis can occur in the heparinized
patient. Adenosine diphosphate (ADP) and serotonin
are the principal mediators in platelet aggregation.

42. Role of Platelets in Haemostasis
• Adhesion: This means binding of
platelets to nonendothelial surfaces,
particularly subendothelium which is
uncovered following vascular injury.
• von Willebrand factor (vWF)
mediates the adhesion of platelets to
subendothelium via GpIb on the surface
of platelets.

43. Role of Platelets in Haemostasis
• Release reaction (secretion):
Immediately after adhesion and shape
change, process of release reaction or
secretion begins. In this process,
contents of platelet organelles are
released to the exterior.
•

45. Role of Platelets in Haemostasis
• Aggregation: This may be defined
as binding of platelets to each
other. ADP released from platelets
or from damaged cells binds to
specific receptors on platelet
surface.

46. Aggregation

47. Role of Platelets in Haemostasis
• Platelet procoagulant activity: When platelets are activated,
negatively charged phospholipids (phosphatidylserine and
phosphatidylinositol) located in the inner half of the lipid bilayer
become exposed on the outer surface to critical binding site for
several procoagulant proteins. These phospholipids play an active
role in coagulation by providing surface for the interaction of some
coagulation factors.

49. PLTs activation
• TxA2 serves as a positive-feedback mediator during platelet
activation.
• TxA2 acts to activate adjacent platelets, generate more TxA2, and
amplify the action of other, more potent, platelet agonists. When
TxA2 binds to its cell surface receptor TP, platelet activation leads
to platelet-shape change, activation of phospholipase A2, platelet
degranulation of dense granules and alpha granules, and platelet
aggregation.
• TxA2 also induces vasoconstriction of smooth muscle

50. Synthesis of
thromboxane
A2.

51. Platelet procoagulant activity. Platelets provide surface for some
important coagulation reactions
In addition platelets also secrete calcium, FV, fibrinogen, and FXII and
contribute to the coagulation system.

52. To sum up

57. • Thrombocytopenia may be associated with:
• A. Postsplenectomy
• B. Hypersplenism
• C. Acute blood loss
• D. Increased prolifera
• tion of pluripotential stem cells
• Platelets
• A release ADP after activation
• B are also called megakaryocytes
• C can only be activated via their glycoprotein receptors
• D have dense vesicles containing histamine

58. • During primary homeostasis
• A fibrinogen is converted to fibrin
• B the initial vasoconstriction does not require platelet
activation
• C thromboxane A2 causes platelet adhesion
• D activation of lipoxygenase is a vital step

59. • Wh ic h of th e followin g b lood c e lls p la y a n im p orta n t
role in b lood c lottin g ?
• (a ) Throm b oc yte s
• (b ) Ne utrop hils
• (c ) Le uc oc yte s
• (d ) Erythroc yte s

60. • Se ru m d iffe rs from b lood a s it la c ks
• (a ) a ntib od ie s
• (b ) c lotting fa c tors
• (c ) a lb um ins
• (d ) g lob ulins
Wh ic h of th e followin g is c orre c t?
• (a ) Se rum c onta ins b lood a nd fib rinog e n
• (b ) Pla sm a is b lood without lym p hoc yte s
• (c ) Blood c om p rise s p la sm a , RBC, WBC a nd p la te le ts
• (d ) Lym p h is p la sm a with RBC a nd WBC

61. Platelets are formed from what type of cell?
a) Melanocytes
b) Macrophages
c) Astrocytes
d) Megakaryocytes
Clum ping of cells is known as _______
a) clotting
b) m utation
c) agglutination
d) glutathione

62. • Which of the following is correct?
a) Lym ph = Plasm a + WBC’s + RBC’s
b) Plasm a = Blood – Lym phocytes
c) Neuron = Cyton + Dendron + Axon + Synapse
d) Blood = Plasm a + RBC’s + WBC’s + Platelets
• Which of the following plasm a protein is involved in coagulation
of blood?
a) Album in
b) Globulin
c) Fibrinogen
d) Am ylase

63. A release ADP after activation
B are also called megakaryocytes
C can only be activated via their glycoprotein
receptors
D have dense vesicles containing histamine
Platelets

64. A fibrinogen is converted to fibrin
B the initial vasoconstriction does not require platelet
activation
C thromboxane A2 causes platelet adhesion
D activation of lipoxygenase is a vital step
During primary homeostasis

65. A is triggered by thrombomodulin
B always requires the presence of activated
platelets
C is Ca
2+
independent
D depends on Factor X
Activation of thrombin

66. Which of the following is NOT one of the four major
physiologic events of hemostasis?
A. Fibrinolysis
B. Vasodilatation
C. Platelet plug formation
D. Fibrin production

67. Which is required for platelet adherence to injured
endothelium?
A. Thromboxane A2
B. Glycoprotein (GP) IIb/IlIa
C. Adenosine diphosphate (ADP)
D. Von Willebrand factor (vWF)