Sharks can't get cancer?
Although it is not true
that sharks do not develop cancer, they do have a remarkable cancer shield. Tumors
are uncontrolled cellular growth and to support their very high metabolism,
they secrete a hormone called 'angiogenin.' This causes nearby blood vessels to
grow new branches that surround the tumor, bringing in nutrients and carrying
away waste products. A part of the tumor then breaks away and establishes another
cancerous colony elsewhere in the body. Research has revealed a promising
anti-tumor agent obtainable from shark cartilage. Shark cartilage contains a
compound opposed to the effects of angiogenin, called 'angiogenin inhibitor.'
This prevents the formation of new blood vessels so that the proto-tumor
'chokes' in its own waste products.
Sharks seem to possess
only one class of broad-spectrum serum antibody - similar to that found in
human infants. An antibody is a blood protein produced in response to a
specific antigen. In humans, this broad-spectrum class of antibody is replaced
by more specific antibodies as the child matures and is exposed to a greater
variety of pathogens. Sharks however keep their non-specific immune response throughout their
lives. This generalized immune system is one of the reasons sharks are able to
detoxify many potentially harmful compounds quickly without need of previous
exposure. It has even been suggested that sharks may be helpful in our fight
against HIV and AIDS. This was originally disregarded as no one could see why a
shark would develop a response to something it is very unlikely to encounter.
Finally, Sharks have
all the internal organs one would expect of a gill-breathing vertebrate as well
as something 'extra' called an 'epigonal organ'. This lies underneath each of
the kidneys and is quite unique to elasmobranchs (cartilaginous fish.) The
epigonal organ seems to be composed of lymphoidal tissue and seems to be
concerned with hemopoetic functions. Recent data also indicates that the organ
is the site of T-cell differentiation in elasmobranchs, playing an important
role in the immune system of these fishes. Without long bones to serve as a
site for leucocyte development, sharks have jury-rigged a scrap of available
tissue to bear that function.
McA
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