HEMOSTASIS IN 
THE ANIMAL KINGDOM 
K. Laki* 
Four mechanisms are known by which nature seals 
wounds: the three clotting systems (platelet system, 
transamidase system, cascade system) and vasoconstric- 
tion. All four systems are present in man. One or the 
other of the four systems operates in animals found at 
various levels of the evolutionary scale. The platelet 
system is exemplified in the horseshoe crab, the trans- 
amidase system in lobster. It will be illustrated with 
the interaction of fibrinogen and thrombin of various 
vertebrates and with the role of transamidase in ather- 
osclerosis how comparative studies help to clarify var- 
ious aspects of the clotting processes. 
INTRODUCTION 
When life started on earth in the primordial 
ocean, the primitive organisms obtained their 
nutrients from the surrounding fluid and dis- 
charged waste material into it. Eventually mul- 
ticellular organisms developed large enough so 
that for these, exchange of molecules by diffu- 
sion became insufficient. To make sure that all 
portions of the animal remained close enough to 
the life supporting fluid, body cavities were de- 
veloped to contain the fluid. For further devel- 
opment, it became important to prevent the 
accidental loss of this body fluid. 
The evolution of body cavities, especially the 
closed ones in v^^hich the hemolymph circulates, 
permitted the animal to venture also onto dry 
land, an event which emphasized the impor- 
tance of wound-sealing. Apparently, various 
mechanisms evolved to close a wound. The es- 
sential feature of these mechanisms is that in- 
jury triggers a series of events which lead to 
the sealing of the wound. 
The accidental opening of the body cavity 
may be reduced or closed "simply" by muscular 
contraction. Lower invertebrates such as Areni- 
cola (earthworm) utilize the contraction of the 
body wall musculature for wound-sealing. Vaso- 
* National Institute of Arthritis and Metabolic Diseases, National 
Institutes of Health, Public Health Service, U.S. Department of 
Health, Education and Welfare, Bethesda, Maryland 20014. 
constriction is more important in vertebrates 
where a closed vascular system has to be pro- 
tected from fatal loss of blood. 
In most instances, however, fatal loss of the 
body fluid is prevented by the formation of a 
"plug." The ingredients needed for the "plug 
formation" are either the circulating cells, or 
materials dissolved in the body fluid (plasma) . 
The hemostatic cells (hemocytes) may just ag- 
gregate at the site of injury, or break up and 
release material which forms a fibrous network 
to plug the wound. When stasis of the body fluid 
is carried out by the elements of its plasma, a 
protein component of the plasma (coagulogen) 
becomes altered and comes out of solution in the 
form of long filaments, the entangled mesh of 
which plugs the wound. So far two basic mecha- 
nisms are known by which the clotting proteins 
become modified to form filaments. In one case, 
an activated enzyme connects the molecules of 
the clottable protein. In the other also, an acti- 
vated enzyme splits off small parts of the clot- 
ting protein which comes out of solution and 
aggregates into long filaments. 
The information available indicates that in 
their main features, the clotting mechanisms 
are similar in all the animal phyla. For exam- 
ple, the blood of the honeybee is reported to con- 
tain factors V, VIII, IX, X, XI and XII.i 
The ancestry of these clotting mechanisms 
may be traced back to the metazoa. What com- 
plicates the picture of considering the various 
clotting mechanisms as being homologous is 
that in some species, especially in invertebrates, 
(there are some 70,000 insects), one or the 
other clotting factors predominate or are en- 
tirely absent. In the Limulus, plasmatic clotting 
is absent. It relies exclusively on the clotting 
protein released from circulatory cells. Lobster, 
on the other hand, converts its clotting protein 
473 
