Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

II. Mechanisms of Hemostasis
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FIGURE 10-1 Simplified sequence of platelet activation
and aggregation. Abbreviations: AA, arachidonic
acid; ADP, adenosine diphosphate; PAF, platelet activating
factor; TF, tissue factor; TXA 2 , thromboxane A 2 .
and the recruitment of additional intracellular signals. The
various reactions are controlled by an integrated network
of biochemical pathways ( Levy-Toledano, 1999 ). Results
of in vitro studies, involving the exposure of platelets from
various mammalian species to a single agonist, indicate
that there are species differences in the relative importance
of various membrane receptors and signaling pathways
( Gentry, 1992, 2000b ). However, it is difficult to extrapolate
the results of in vitro studies to the physiological setting.
Under in vivo conditions, circulating platelets are exposed
to multiple agonists and, once platelets are activated, the
phenomenon of “ crosstalk ” between the various extracellular
and intracellular signaling pathways occurs ( Fig. 10-1 ).
Although most of the current knowledge regarding platelet
biochemistry has come from human and murine studies, it is
likely that there are more similarities than differences in biochemical
mechanisms among species. In addition to a full
complement of biochemical pathways, an adequate number
of circulating platelets with structural integrity are required
for platelets to fulfill their central role in hemostasis.
1. Platelet Production
Platelet formation is the culminating event of megakaryopoiesis.
In bone marrow, long, cylindrical processes of
megakaryocyte cytoplasm are pinched off, to form proplatelets,
and released into the circulation ( Nagata et al .,
2003 ). These fusiform, or elongated, platelets undergo
morphogenesis into mature platelets through additional
fragmentation processes involving the fusion of cytoplasmic
vesicles with the plasma membrane ( Benke, 1993 ).
Fusiform platelets constitute about 15% of the platelet
population in the rat and guinea pig, and elongated platelets,
up to 20μ long, can occur in horses ( Tablin, 2000 ).
The mature mammalian platelet is small, 5 to 7μ long and
frequently less that 3μ wide ( Tablin, 2000 ). Early clinical
studies indicated that among the physiological roles
of interleukins, including IL-1, IL-3, IL-6, and IL-11,
was their ability to stimulate platelet production. More
recent studies have confirmed that thrombopoietin (TPO)
is the primary biochemical regulator of steady state megakaryopoiesis
and platelet production ( Kuter and Begley,
2002 ). TPO, also known as c-Mp1 ligand, is a lineage-specific
cytokine that is constitutively synthesized in the liver.
It is removed from the circulation by binding to the c-Mp1
receptor on platelets and bone marrow megakaryocytes
( Kaushansky, 2002 ). The TPO molecule consists of 331
amino acids organized in two domains, a receptor-binding
domain that shows some homology to erythropoietin and
a highly glycosylated carboxy-terminal portion that stabilizes
the TPO protein. Although TPO increases the size,
ploidy, and number of megakaryocytes, and also stimulates
the expression of platelet-specific markers, it does
not appear to increase the rate of platelet shedding from
late-stage megakaryocytes ( Choi et al ., 1996 ). Circulating
TPO levels appear to be directly influenced by platelet
mass because TPO levels rise and remain elevated during
persistent thrombocytopenia. It is likely that TPO-independent
mechanisms are also involved in platelet production
because TPO / mice are healthy and show no signs of
spontaneous hemorrhage, despite being thrombocytopenic
( Alexander et al ., 1996 ).
The normal circulating life span of mammalian platelets
is in the range of 3 to 10 days. The mechanisms involved
in the clearance of senescent platelets from the circulation
are not yet known, but evidence is emerging that platelet
aging is associated with the universal cellular phenomenon
of loss of membrane asymmetry. For example, the proportion
of canine platelets with PS exposed on the plasma
membrane increases with age and is accompanied by