Platelet Size in Throinbocytopenias and Thrombocytosis

Planimetric studies were carried out on canine blood platelets fixed with formaldehyde. In the normal dog, the mean platelet area was 5.25 sq/i (SEM ± 0.34 sq/i). During the first 3 days following acute experimental thrombocytopenia, platelet area increased, averaging 9.5 sq/i (p < 0.01). The reactive thrombocytosis following splenectomy or nephrectomy was not accompanied by a rise in large platelets. Platelet size decreased to 3.9 sq/i in dogs made thrombocytopenic by intravenous injections of mitomycin C. The morphologic changes induced by the various experimental procedures suggest that: (1) newly formed platelets are larger than platelets of a normal population; (2) some large platelets are possibly released by macromegakaryocytes; (3) other large thrombocytes probably represent fragments of granular megakaryocytes; (4) senescence of platelets in the circulation is associated with decreasing size; and (5) large young platelets are preferentially retained in vivo on glass beads.