Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 84

Platelets are generated from megakaryocytes in bone marrow. Megakaryocytes
extend proplatelet-like protrusions into microvessels. These intravascular extensions
are sheared and torn from their transendothelial stems by flowing blood [ 156 ].
Proplatelets are thus released in blood, at least partly due to applied hemodynamic
forces.
Platelets are involved in blood coagulation in injured vessels and thrombosis
in diseased conduits, 56 as well as inflammation, immunity, and tumor progression.
Platelet activation is also influenced by hemodynamic forces.

3.6.1 Platelet Cytoskeleton

Platelet cytoskeleton is involved in shape change, secretion, and clot contraction.
Platelet shape rapidly changes on contact with foreign surfaces. Platelet actomyosin
microfilaments (thrombostenin) organize in contractile stress fibers.
The cortex contains actin filaments and a circumferential bundle of microtubules.
An intracellular tubular system is open to the cell surface via invaginations of the
plasma membrane. Dense tubules contain Ca 2 + stores and enzymes involved in
arachidonic acid metabolism that are required for platelet secretion and aggregation.

3.6.2 Translational Machinery

Platelets lack nuclear DNA but retain megakaryocyte-derived mRNAs and the
translational machinery for protein biosynthesis, in particular transcription factors
(e.g., peroxisome proliferator-activated receptors [nuclear receptors NR1c1-
NR1c3], retinoid X receptors [NR2b1-NR2b2], glucocorticoid receptor [NR3c1],
and nuclear factor-

B), spliceosome components to process mRNA, and the signal-
dependent protein translation machinery. Post-transcriptional and translational
mechanisms regulate platelet function.
Anucleate platelets released into the blood stream from bone marrow megakary-
ocytes contain not only mRNAs for protein synthesis, but also microRNAs to reg-
ulate mRNA translation [ 157 ]. Platelets contain Dicer and Argonaute-2-containing
ribonucleoproteic complexes to process microRNA precursors and control reporter
transcripts.

κ

3.6.3 Platelet Granules

α

The 2 major types of secretory granules include numerous granules and large,

dense granules (Table 3.8 ). Platelet dense granule membranes contain both granu-

56 Platelet aggregation and fibrin formation both require thrombin, Ca 2 + , and other clotting factors.