Platelets and Megakaryocytes Flashcards
What is the minimal required number of platelets to prevent spontaneous bleeding & maintain endothelial integrity?
> 5,000/uL
What are 5 characteristics of platelets?
Mitochondria
no nucleus
open surface connected canalicular system
dense tubular system
granules
What 2 types of RNA is found in platelets?
mRNA
miRNA (regulatory role)
What are 2 sources of proteins for platelets?
from megakaryocytes
endocytosis
What is the most abundant granule type in platelets?
Alpha
What do alpha granules contain?
different proteins
coagulation factors (VWF or Factor V)
growth factors
What is the open surface connected canalicular system?
Internal membrane that is continuous with the plasma membrane
What does the release of alpha granules cause?
generates a complex microenvironment that is important for
Haemostasis
inflammation
wound healing
What do dense granules contain?
Ca2+
serotonin
ADP/ATP/GDP/GTP
-vely charged phosphates
What do platelet lysosomes contain?
Hydrolases
What are platelets generated through?
Fragmentation of cytoplasm of giant bone marrow cells called megakaryocytes
What are MKs derived from?
Haematopoietic stem cells (HSCs)
What drives the development of MKs from HSCs?
THrombopoietin
What is the process MKs develop by?
endomitosis
What is endomitosis?
Nuclear DNA is duplicated but cell does not divide
MK keeps polypoid nucleus (16N)
MK cytoplasm is remodeled into proplatelets
distal constrictions break off new platelets
What is CD41?
Integrin proteins made of 2 chains that is needed for platelet aggregation
What are demarcation membranes?
Precursor of membranes for future platelets (plasma membrane and OCS) that is connected to plasma membrane
What 2 things are proplatelet formation dependent on?
Microtubules
Actin filaments
What are formed at proplatelet tips?
Nascent platelets loaded with granules
What do microtubules do in proplatelets?
Mediate transport of granules from MK cell body to proplatelet tip
What accelerates platelet release?
Shear forces from blood flow (limiting factor)
Where are immature MKs located and where do they go during maturation?
Start at osteoblastic niche of bone marrow & migrate to subendothelium of venous sinusoids
What is another site other than bone marrow that platelet formation can occur?
Lung capilllaries
