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144 Veterinary Histology of Domestic Mammals and Birds
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7.13 Blood cells of a horse. Pappenheim stain (oil immersion, x1000).
Platelets (thrombocytes) 20–30 μm), which has a round, often indented, nucleus
For blood to act as a transport medium, it is necessary for and several nucleoli. Multiple endomitoses (replication
a precise balance to be maintained between blood coagula- of chromosomes without nuclear division) occur within
tion and fibrinolysis. These regulatory mechanisms have the megakaryoblast, forming a cell with a lobed, polyploid
an important role in guarding against disruption of hae- nucleus (promegakaryocyte).
modynamics. Lesions of the blood vessel walls result in These cells differentiate into megakaryocytes (diam-
vasoconstriction, initiation of clot formation and activation eter 50–70 μm). The nucleus is highly lobulated, and the
of the coagulation cascade. Of primary importance in this cytoplasm contains numerous ribosomes and azurophilic
process, in addition to plasma factors such as fibrinogen granules. At maturity, megakaryocytes have abundant
(Factor I), prothrombin (Factor II), tissue thromboplastin cytoplasm and a diameter of up to 100 μm, making them
(Factor III) and calcium (Factor IV) are cellular elements of one of the largest individual cells in the body (Figure 7.14).
the blood referred to as platelets (thrombocytes). Megakaryocytes exhibit pseudopodial extensions and
deep invaginations of the plasma membrane. The latter
Development of platelets (thrombopoiesis) extend into the cell and make contact with a dense net-
Thrombopoiesis is the formation of platelets in the red work of membranes of the smooth ER and with vesicular
bone marrow (Figure 7.14). Differentiation of haemopoi- inclusions. A three-dimensional intracellular compart-
etic stem cells gives rise to the megakaryoblast (diameter ment is formed, enclosing areas of cytoplasm containing
7.14 Thrombocytopoiesis (schematic).
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