Lecture 9: Platelet Flashcards
What are platelets involved in?
primary hemostasis
halting bleeding following vascular injury
What are platelets made from?
cellular fragments of cytoplasm from megakaryocytes in the bone marrow
What are the 3 zones of platelets?
the peripheral zone
the sol-gel zone
the organelle zone
Peripheral Zone
the stimulus receptor/transmitter region
Peripheral Zone components
Glycocalyx
Plasma membrane
open canalicular system
specialized microfilaments
glycocalyx
important component of the platelet membrane
What are glycoproteins responsible for?
blood group specificity (ABO)
tissue compatibility (human leukocyte antigen [HLA])
platelet-unique immunologic antigenicity
serve as receptors and facilitate transmission of stimuli
What is the purpose of the phospholipids in the Peripheral zone?
serves as a surface for the interaction of the plasma proteins involved in blood coagulation
What is also in platelet surface?
Coagulation factors V and VIII
participate in the formation of fibrin
What is the purpose development of stickiness in the peripheral zone?
essential for the platelet functions of adhesion and aggregation
Sol-Gel Zone
the cytoskeletal/contractile region
contains microtubules and microfilaments
Thrombosthenin
microfibrillar “contractile” protein
form the platelet’s cytoskeleton, which contracts as the platelet’s shape changes
together with microfilaments: actin and myosin
What is the cytoskeleton responsible for?
maintaining its normal discoid shape
Organelle Zone
the metabolic/organellar region
responsible for the metabolic activities of the platelet
platelet granules
most numerous organelles
heterogeneous in size and differentiated by their electron density and chemical contents
Types of platelet granules
Alpha and dense granules
Alpha granules
more numerous
contain a number of different proteins
physiologic role not clearly defined; but PF 4 does neutralize the anticoagulant heparin
Dense bodies
fewer in number
densely opaque granules in (TEM) preparations
storage pool
intragranular concentration of ADP and ATP found in the dense bodies
release reaction
contents of both the alpha granules and dense bodies are released
energy dependent
additional platelets are drawn to the site of the vascular injury- platelet aggregates
dense tubular system (DTS)
another important structure present in the cytoplasm in organelle zone
Similar to sarcotubules in skeletal muscle
site of prostaglandin synthesis and sequestration of calcium
What does the release of calcium from the DTS do?
triggers contraction of thrombosthenin and subsequent internal activation of platelets
The role of platelets in hemostasis includes what?
(1) maintenance of vascular integrity
(2) initial arrest of bleeding by platelet plug formation
(3) stabilization of the hemostatic plug by contributing to the process of fibrin formation
Maintenance of Vascular Integrity
Platelets are involved in the nurturing of endothelial cells lining the vascular system
Platelet adheres to cell, cytoplasm b/n cell and platelet decreases, platelet becomes part of cell
This process has an effect of “nurturing” or “feeding”
How much platelets are used in maintaining vascular integrity?
10% in circulation
Platelet Plug Formation: processes
platelet adhesion
platelet aggregation
platelet release reaction
Platelet Plug formation: Adhesion
a reversible process whereby platelets stick to foreign surfaces
involves the interaction of platelet surface glycoproteins with the connective tissue elements of the subendothelium
Exposure to subendothelial connective tissue, such as collagen fibers begins process
Adhesion of platelets to subendothelial fibers depends on a plasma protein called what?
on Willebrand factor (vWF)
What does adhesion do for platelets?
Platelets adhere to the area of injury at the endothelial lining or to each other when injured
acting to arrest the initial episode of bleeding
Platelet Plug Formation: Aggregation
injured platelet changes shape from discoid to spheric, with pseudopod formation
caused by ADP, which is released from adherent platelets or endothelial cells
Adenosine diphosphate
potent initiator of aggregation
By binding specific membrane receptors
resulting in the transformation of ambient discoid platelets to reactive spiny spheres
Aside from Adenosine diphosphate (ADP) what else is needed for aggregation?
Both calcium and the plasma protein fibrinogen
Platelet Plug Formation: Platelet Release reaction
from dense granules involves the secretion of ADP, serotonin (a vasoactive amine), and calcium
What does the release of ADP do?
further aggregation of more platelets
Stabilization of Hemostatic Plug
The last stage involved in arresting bleeding after vessel damage
achieved through the formation and deposition of fibrin, the end product of coagulation
platelets require energy in the form of ATP for what?
cellular movement
active transport of molecules across the membrane
biosynthetic purposes
maintenance of a hemostatic steady state
Why is The study of ATP and ADP in platelets complicated?
adenine nucleotides are present in two pools
adenine nucleotides metabolically active pool
participates in metabolism
found in the cytoplasm, mitochondria, and membranes
adenine nucleotides inactive storage pool
found in the dense granules and plays a role in the release reaction
