This document summarizes key information about platelets: Platelets are small, granulated, and non-nucleated cell fragments that are 2-4 micrometers in diameter. They are produced from megakaryocytes at a rate of 400,000-150,000 per cubic millimeter of blood, and have an average lifespan of 8 days. Platelets play a critical role in hemostasis, the process of stopping bleeding, through adhesion, activation, aggregation, and formation of a platelet plug. Their cytoplasm contains various proteins, organelles, and granules that enable activation and responses to injury.

1. Platelets
small
granulated
Non-nucleated
round or oval
2 – 4 µ in diameter
mm³/400000 – 150000
life span average 8 days
detached bits from megakaryocyte

2. fibroblasts
Production of platelets is
regulated by thrombopoietin
secreted by
Normally
endothelial cells
certain leucocytes
of platelets are present % 75 – 60
in the circulating blood
25 – 40 % of platelets are present
in the spleen

3. Structure of platelets
Granules
Mitochondria
Golgi zone
Glycogen
Microtubules
Dense
body
Membrane
Dense
tubular
System
Microfilament
Dense body in
granule
Open canalicular
system

4. Structure of platelets
I- Plasma membrane
plasma membrane contains***
glycoprotein receptors
For
• collagen
• fibrinogen
• vessel von-Willebrand factor
Phospholipids
I
ُ nclude
• platelet factor 3

5. *** The plasma membrane invaginates
into the interior of the platelets to
form the open canalicular system
serves as a pathway
uptake of extra
cellular calcium
release of intracellular
substances
*** There is also a coat of glycoprotein on
its surface which helps its adhesion to
injured endothelium; but not to normal
endothelium

6. II – Platelet cytoplasm
1- Contractile proteins
actin
thrombosthenine
myosin
enable activated platelets to change
their shape

7. 2- A skeleton of microtubules that keeps
the disc shape of platelets
Residuals of- 3
endoplasmic reticulum
Synthesis
of enzymes
Golgi apparatus
Calcium
store
4- Mitochondria and enzyme system for synthesis
of ATP and ADP

8. .Lyzosomes containing hydrolytic enzymes- 5
Glycogen granules for production of energy- 6
.anaerobically
7- Enzyme system that synthesize prostaglandins
from phospholipids of the Platelet membrane.

9. types of granules 2- 8
A) Dense granules
contain non-protein
substances
ATP, ADP, Ca++)
)and serotonin
b) Alpha granules
contain the secreted
proteins
• clotting factors
• fibrin stabilizing
factor XIII
• platelet derived
growth factor

10. HEMOSTASIS
By Prof Sameh Shamaa
Prof Of medical Oncology and
Internal medicine
Mansoura Faculty Of Medicine

11. HEMOSTASIS
Def:- stoppage of bleeding from the blood
vessels
Mechanisms
(I) v.c of blood vessels
(II) platelet plug formation
(III) Blood coagulation (fibrinogen fibrin)
(IV) Clot retraction
(V) fibrinolysis to dissolve the clot

12. PRIMARY HEMOSTASIS
includes the processes that result in the
.formation of the platelet plug
-:Necessary factors
The blood vessels : the vessel walls esp. the .subendothelial layer
The platelets: plasma glycoproteins 2fibrinogen
Willebrand factor ,which also
presents inside the platelets
-

13. :Mechanisms
.v.c of the bl. vessel-1
Platelets adhesion to subendothelial layer, ( Willebrand- 2
(factor is necessary for this stage
:adhesion of platelets- 3- platelets secretion
their activation and secretion of ADP,adrenaline,
noradrenaline –>
aggregation & activation of other
.platelets
.Aggregation of platelets- 4
.Formation of capillary plug- 5

14. Coagulation of Blood
Def :- represent the conversion of fibrinogen
(soluble protein( to fibrin (insoluble(
meshwork which occludes the point or
vessel rupture.

15. First Step :Activation of factor X
BY One of 2 systems:
I-urgent system
(Extrinsic system.(
II-delayed system
(Intrinsic system.(

16. systems of coagulation
I-urgent system.
Extrinsic system.
12-20'' (seconds(
In vivo only.
Due to tissue damage.
II-delayed system
Intrinsic system.
4-8' (minutes(
In vivo & in vitro
due to contact with foreign surface
↓
↓
Tissue factor
activation of contact system
↓
↓
X < ------------------------------------IX a < ---------------- IX
↓
Xa
↓
2- prothrombin
thrombin
3-fibrinogen
Fibrin

17. EXTRINSIC SYSTEM
FACTORS NICESSORY ARE:
Factor X
Tissue factor and Factor VII
Tissue F.
VIIa
Xa
Blood vessel
VII
X

18. INTRINSIC SYSTEM
Necessary factors: XII (Hageman factor(
- Contact system XI
Kallikrene
kininogene
- F. IX
- F. VIII
- F. X
- Ca. ++
- phospholipids of the platelet’s membrane

19. Contact System:
Foreign surface
|--------------------------------------------------|
Kalierne XII
kininogene
Fragmentation
XIIa
XI
XIa
Rest of intrinsic pathway
IX

20. Rest of intrinsic pathway
IX
Platelets
Ca ++
IXa
X
VIIIa
VIII
Xa
II
IIa

21. Second Step: of Coagulation
Thrombin Formation: (IIa)
Factors needed:
- prothrombin (II)
- Xa
- V (acceleririe)
- phospholipids
- Ca + +
Platelets
++Ca
II
IIa
V,Xa

22. 3 Step :Fibrin Formation
rd
Fibrin Formation:------------------------
IIa
XIII
Ia
(Fibrinogen) -------------------(Soluble fibrin)
Insoluble protein
XIIIa