Structure and Function of Platelets: Topic 1

Question 1

where do platelets come from?

Answer 1

they are derived from bone marrow cells called megakaryocytes

mature megakaryocytes are colossal cells residing in the bone marrow space

Question 2

what is thrombopoiesis?

Answer 2

megakaryocyte cells give off fragments of their cytoplasm that eventually reach the circulatory system

at this point, these cytoplasmic pieces are called platelets

hence platelets have no nuclei

Question 3

do platelets have nuclei?

Answer 3

no

pieces of megakaryocyte’s cytoplasm and cell membrane bud off to form platelets (hence platelets have no nuclei).

Question 4

what’s a healthy platelet count?

Answer 4

150,000 to 400,000 platelets per microliter of blood

Interestingly, this represents only two thirds of circulating platelets because the spleen stores the rest

Question 5

where are platelets stored?

Answer 5

spleen

Question 6

what is thrombocytopenia? when can it occur?

Answer 6

platelet deficiency

can occur in patients with enlarged spleens

occurs as a result of medications or diseases

Question 7

what is thrombocytosis?

Answer 7

excess of platelets

migration of platelets from the spleen to the peripheral blood occurs in some disease processes causing an excess of platelets

Question 8

what is the consequence of thrombocytopenia?

Answer 8

platelets are essential for the clotting process.

fewer platelets leads to
increased bleeding (the holes in the vessels cannot be plugged) or even spontaneous internal hemorrhage

Question 9

describe the cytoplasm of a platelet

Answer 9

cytoplasm is divided into two main regions:

1. an outer, pale hyalomere
2. a central granulomere, which contains bluish granules

Question 10

what is the granulomere?

Answer 10

central area of the platelet is densely filled with bluish granules

Question 11

what are the types of granules located in the granulomere?

Answer 11

1. alpha (also known as specific) granules

2. dense (or delta) granules

Question 12

what’s inside alpha granules of the granulomere in platelets?

Answer 12

• platelet-derived growth factor
• platelet factor 4
• fibrinogen
• von Willebrand factor (vWF)
• adhesion molecule P-selectin

alFa granules have platelet Factor 4, Fibrinogen, and von willebrand Factor (vWF). Also, alpha granule contain adhesion molecule P-selectin.

Question 13

what’s inside dense granules of the granulomere in platelets?

Answer 13

• serotonin
• adenosine diphosphate (ADP)
• adenosine triphosphate (ATP)
• calcium

Dense granules contain Diminutive molecules like calcium, serotonin, ADP and ATP

Question 14

what is the hyalomere of a platelet?

Answer 14

the pale, outer part of the platelet containing cytoskeletal fibers, which include actin and myosin

fibers allow the platelet to maintain its shape and contract when necessary, such as in the final step of coagulation, when the platelet plug contracts to stop blood loss

Question 15

what do actin and myosin do for platelets?

Answer 15

fibers allow the platelet to maintain its shape and contract when necessary, such as in the final step of coagulation, when the platelet plug contracts to stop blood loss

Question 16

what does the platelet membrane do?

Answer 16

displays a variety of receptors that bind a multitude of molecules triggering different physiological actions

Question 17

what receptors are found on the platelet membrane?

Answer 17

1. glycoproteins Ib (GPIb)
2. glycoprotein IIb/Illa (GPIIb/IIIa)
3. P2Y12

Question 18

what does the glycoproteins Ib (GPIb) receptors in the platelet membrane do?

Answer 18

This binds to vWF, which itself is attached to the subendothelium of the injured vessel

So the GPIb-vWF interaction allows the platelet to stick to the injured vessel wall –> this process is called adhesion

Question 19

what does the glycoprotein IIb/Illa (GPIIb/IIIa) receptor in the platelet membrane do?

Answer 19

this receptor binds to fibrinogen, allowing platelets to attach to each other, forming a clump

this process is called aggregation

Question 20

what does the P2Y12 receptor in the platelet membrane do?

Answer 20

this receptor binds ADP, resulting in an increase in cAMP within the cell, and an increase in the expression of GPIIb/IIIa receptors

Question 21

what is the main role of platelets?

Answer 21

to stop hemorrhage by plugging the holes of damaged blood vessels in a process known as primary hemostasis

secretion of granules aids in this process and further accelerates the formation of a clot

Question 22

what is another name for clot?

Answer 22

thrombus

Question 23

what keeps platelets in the inactive form when they’re not needed?

Answer 23

Under normal circumstances, endothelial cells in blood vessels constantly secrete chemicals (prostacyclin, also known as prostaglandin I2 and nitric oxide) that prevent platelets from becoming active and secreting their contents

more imporantly, endothelial cells inhibit platelet activation and degranulation by hiding proteins, such as collagen, found beneath them. This is crucial because collagen is a very potent activator of platelets

Question 24

what chemicals keep platelets inactive?

Answer 24

1. prostacyclin, also known as prostaglandin I2

2. nitric oxide

Question 25

what activates platelets?

Answer 25

collagen

Question 26

what happens when endothelial cells are injured?

Answer 26

when endothelial cells are injured, they slough off the vessel, exposing the subendothelial collagen and stopping the production of “anti-platelet factors”

this process attracts platelets to the injury site, causes them to swell, become “gluey” and message other platelets to undergo the same transformation by releasing their granules

Question 27

what does serotonin do?

Answer 27

secreted by platelets

causes blood vessels to constrict and even close small arteries

this reduces the leakage of blood and stagnates the coagulation factors in the place where they need to exert their effect

Question 28

what does von willebrand factor do?

Answer 28

vWF is also secreted by endothelial cells and is always present in the blood

serves as the glue that allows platelets to attach to the collagen

Question 29

what does calcium do?

Answer 29

it’s a critical requisite for the coagulation cascade

Question 30

what is the function of platelet receptors?

Answer 30

adhesion and aggregation of platelets!

Question 31

what is Bernard-Soulier syndrome?

Answer 31

platelet adhesion is altered

results from a hereditary deficiency of GpIb

since platelets cannot adhere to the subendothelium, formation of the platelet plug does not occur

Question 32

what happens during platelet adhesion?

Answer 32

GpIb directly binds to vWF, and vWF in turn strongly attaches to the exposed collagen

Question 33

what happens during platelet aggregation?

Answer 33

ADP binds to the receptor P2Y12, causing intracellular GpIIb/IIIa to move and insert on the membrane surface

GpIIb/IIIa binds to fibrinogen, one of the coagulation factors

fibrinogen, also released from alpha granules, glues platelets in the process of aggregation

Question 34

what is Glanzmann thrombasthenia?

Answer 34

platelet aggregation is altered

platelets can’t aggregate because of an inherited deficiency or absence of GpIIb/IIIa

results in an increased bleeding time

Question 35

how is the soft platelet plug reinforced?

Answer 35

soft platelet plug needs to be reinforced somehow because otherwise the mechanical stress imposed by the flow of blood could remove it

fibrinogen helps with this!

the coagulation cascade will ultimately convert the fibrinogen molecules into fibrin, creating in turn a very strong platelet plug

Question 36

what are the three major steps involved in primary hemostasis?

Answer 36

1. secretion
2. adhesion
3. aggregation