Structure and Function of Platelets: Topic 1 Flashcards
where do platelets come from?
they are derived from bone marrow cells called megakaryocytes
mature megakaryocytes are colossal cells residing in the bone marrow space
what is thrombopoiesis?
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
do platelets have nuclei?
no
pieces of megakaryocyte’s cytoplasm and cell membrane bud off to form platelets (hence platelets have no nuclei).
what’s a healthy platelet count?
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
where are platelets stored?
spleen
what is thrombocytopenia? when can it occur?
platelet deficiency
can occur in patients with enlarged spleens
occurs as a result of medications or diseases
what is thrombocytosis?
excess of platelets
migration of platelets from the spleen to the peripheral blood occurs in some disease processes causing an excess of platelets
what is the consequence of thrombocytopenia?
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
describe the cytoplasm of a platelet
cytoplasm is divided into two main regions:
- an outer, pale hyalomere
- a central granulomere, which contains bluish granules
what is the granulomere?
central area of the platelet is densely filled with bluish granules
what are the types of granules located in the granulomere?
- alpha (also known as specific) granules
2. dense (or delta) granules
what’s inside alpha granules of the granulomere in platelets?
- 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.
what’s inside dense granules of the granulomere in platelets?
- serotonin
- adenosine diphosphate (ADP)
- adenosine triphosphate (ATP)
- calcium
Dense granules contain Diminutive molecules like calcium, serotonin, ADP and ATP
what is the hyalomere of a platelet?
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
what do actin and myosin do for platelets?
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
what does the platelet membrane do?
displays a variety of receptors that bind a multitude of molecules triggering different physiological actions
what receptors are found on the platelet membrane?
- glycoproteins Ib (GPIb)
- glycoprotein IIb/Illa (GPIIb/IIIa)
- P2Y12
what does the glycoproteins Ib (GPIb) receptors in the platelet membrane do?
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
what does the glycoprotein IIb/Illa (GPIIb/IIIa) receptor in the platelet membrane do?
this receptor binds to fibrinogen, allowing platelets to attach to each other, forming a clump
this process is called aggregation
what does the P2Y12 receptor in the platelet membrane do?
this receptor binds ADP, resulting in an increase in cAMP within the cell, and an increase in the expression of GPIIb/IIIa receptors
what is the main role of platelets?
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
what is another name for clot?
thrombus
what keeps platelets in the inactive form when they’re not needed?
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
what chemicals keep platelets inactive?
- prostacyclin, also known as prostaglandin I2
2. nitric oxide
what activates platelets?
collagen
what happens when endothelial cells are injured?
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
what does serotonin do?
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
what does von willebrand factor do?
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
what does calcium do?
it’s a critical requisite for the coagulation cascade
what is the function of platelet receptors?
adhesion and aggregation of platelets!
what is Bernard-Soulier syndrome?
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
what happens during platelet adhesion?
GpIb directly binds to vWF, and vWF in turn strongly attaches to the exposed collagen
what happens during platelet aggregation?
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
what is Glanzmann thrombasthenia?
platelet aggregation is altered
platelets can’t aggregate because of an inherited deficiency or absence of GpIIb/IIIa
results in an increased bleeding time
how is the soft platelet plug reinforced?
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
what are the three major steps involved in primary hemostasis?
- secretion
- adhesion
- aggregation