Primitive Coagulation Systems and their Message to Modern Biology

Blood circulating in all living organisms carries with it the ability to maintain a constant flow but to thicken and cease to flow when a lesion occurs in the tissue. This phenomenon, known as blood coagulation, is a primary event in one of the biological defense systems of an organism. No animal species has yet been described in which plasma coagulates without some contribution from cells: including platelets, thrombocytes and amebocytes. Endothelial cells also participate in regulating the rate of coagulation reaction and in lysing the fibrin clot. Hemostatic reactions can be roughly divided into two stages (Fig. 1): one (primary hemostasis) in which the cellular components take part. and the other (secondary hemostasis). in which plasma factors participate. Cell adhesion and agglutination and release of cellular components are biological reactions indispensable for primary hemostasis as well as for triggering secondary hemostasis (1). In mammals. numerous plasma factors and tissue-derived protein factors, cofactors, serine proteases, phospholipids and calcium ions participate in the process of secondary hemostasis (2-5). In 1961, Heilbrunn suggested that plasma clotting in higher animals is the result of a progressive evolutionary extension into the plasma of the cytoplasmic sol-gel mechanism which is present in cells of all the living organisms and underlies such basic reactions as protoplasmic streaming. amoeboid movement, phagocytosis, muscle contraction and cell division (6). However. since knowledge of mechanisms of blood clotting and wound sealing in primitive animals is mostly morphological (7-lo), little is known of the biochemistry. Nevertheless, currently available knowledge permits selection of specific examples which support Heilbrunn's idea (6) in Limulidae (11) and Crustacea (12). For instance. in the horseshoe crab. Linzulus (L.) polyplzemus and Tachjp le~ ls (T.) tridentatus. the clottable protein exists in a specific granule of amebocytes (11.13.14). When these cells contact an activating surface. the protein is released into the surrounding fluid where it forms an insoluble clot. Thus. a hemolymph plasma clot is formed without the participation of plasma factors, unlike events in the mammaiian clotting system. In contrast. in Crustacea such as lobsters. crabs and crayfish. a clottable protein known as vitrogenin-like substance is present in the plasma (14): it is converted to its insoluble form by a cellular transglutaminase released from certain hemocytcs, when they are activated by a foreign surface (12,15). Therefore. hemocytes in circulating blood always have a role in plasma coagulation and there is an apparent evolutionary increase in the number of plasma factors involved in the hemostatic reaction. These extra clotting factors amplify the reaction. thereby increasing its potential activity to meet the demand imposed by the increased circulatory efficiency and higher blood pressure in the more advanced vertebrates. Evidence of a direct evolutionary pathway from invertebrates to vertebrates would mean an intracellular origin for clottable protein and its coagulants and indeed, biochemical studies on limulus clotting reactions support this thesis (16). This short paper will focus on recent advances in our knowledge of limulus clotting factors. especially in comparison with mammalian clotting factors. and of antimicrobial substances participating in biological defense systems.