Morphology, origin and functions of the thrombocytes of Elasmobranchs.

Thrombocytes of Torpedo marmorata Risso and Scyliorhynus stellaris L. (Elasmobranchs) are multiform from rounded to spindle-shaped. They originate in the spleen from prothrombocytes. They appear to be functionally correspondent to mammalian platelets because they form aggregates and adhere to glass and contain the same surface-connected canalicular system (SCCS) as in platelets, as proved by tannic acid treatment. Immunocytochemical staining have demonstrated, moreover, the presence in their cytoplasm of three platelet factors: platelet factor 4, beta-thromboglobulin and factor VIII related antigen. Stimulating agent, like collagen, ADP, noradrenaline, 5-hydroxytriptamine and thrombin induce aggregation of thrombocytes and the empting of their granules and vesicles containing very likely the platelet factors. The above observations leave few doubts on the functional equivalence of elasmobranch thrombocytes to mammalian platelets.