| Mammalian platelets are pale blue granular fragments (much smaller than RBC) that are shed from multinucleate megakaryocytes in the bone marrow; avian and reptilian platelets are true cells with nuclei. They maintain the integrity of the endothelium and act as part of the clotting process to repair damaged endothelium, where they ensure mechanical strength of the clot. Increased platelet counts (thrombocytosis) occur as a reaction to
consumption following injury, when large juvenile platelets may also appear; after splenectomy, as splenic stores are liberated to the circulation; after vincristine treatment, which increases platelet shedding from megakaryocytes; and in megakaryocytic leukemia. Decreased platelet counts (thrombocytopenia) are caused by autoimmune reactions, thrombotic/thrombocytopenic purpura, bone marrow suppression and aplasia, bone marrow neoplasia, and
equine infectious anemia. Signs are petechiation and ecchymosis more than frank hemorrhage, and little may be seen until the platelet count is <20 × 109/L. Platelet functional abnormalities present similarly, but platelet numbers and morphology are normal.
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