54
trout leukocytes suggest that there may be major differences between
different species of fish. Unlike the bluegill, rainbow trout con
tained PHA-responsive cells only in the thymus and LPS-responsive cells
only in the anterior kidney in a manner analogous to the compartmental
localization of T- and B-cells in birds and mammals. However, accurate
comparisons of the rainbow trout and bluegill are tenuous due to experi
mental differences. Unfractionated leukocytes, rather than isolated
lymphocytes, were cultured on]y at 19C in the trout studies. It was
also deemed necessary to switch serum supplements to obtain signifi
cant responses to different mitogens with trout cells. There were also
differences in optimal mitogen doses as well as length of time for
maximum mitogenic stimulation between the two species.
It is thus conceivable that true differences in the lymphoid
systems exist between different species of fish. For example, there
are reports that thymuses of some fish species involute with age while
others do not (37). It is suggested that a third species group may
exist in which the thymus differentiates (or de-differentiates) into a
lymphoid organ similar to the anterior kidney, as apparently is the case
with bluegill.
Differences in environmental temperature tolerances may also effect
the in vitro cellular responses. Rainbow trout live in colder environ
ments, and thus evolutionary pressures may have affected the subpopu
lations of lymphocytes to a point where discernible differences in In
vitro temperature responses may not be recognizable. Further in vitro
studies with other species are necessary before adequate comparisons of
this nature can be made.