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302 HELMINTH SURFACES
volume, thereby shrinking or swelling in direct
relation to the osmolarity of their environment.
A few species, including H. diminuta, regulate
their volume (i.e. maintain their weight at
a constant level) over a fairly wide range of
osmotic pressures. However, even these worms
rapidly swell or shrink in media that are below
or above that range, respectively. The mechanisms
by which H. diminuta regulates its volume
are not known, but the ability to excrete
organic acid end-products of glucose metabolism
(succinate and acetate) appears to play
a critical role. Osmoregulation is glucosedependent,
and both glucose absorption and
metabolism increase linearly with osmotic
pressure between 210 and 335 mosmol/liter,
whereas rates of uptake of leucine, Na and Cl
are unaffected by osmotic pressure. As noted
above, glucose transport proteins, including
one that couples to Na transport, are abundant
in the apical membrane of the tegument.
In mammals, Na /glucose cotransporters play
a key role in water transport across membranes,
and they probably serve a similar role in
the cestode tegument. A duct-like structure in
the terminal proglottid may also contribute to
osmoregulation in cestodes (see below).
Immune evasion
Little is known about the role of the tegument
in immune evasion by cestodes compared to
trematodes. The glycocalyx is believed to provide
some protection from host immune effectors
and digestive enzymes. In cestodes in
culture, disruption of this layer by drugs or solvents
exposes the apical membrane of the tegument
to damage from immune components.
Another immune evasion strategy is referred to
as molecular mimicry. This process is thought
to occur when parasites evolve antigenic convergence
with their host, and thereby camouflage
proteins exposed on their surfaces. There
are numerous examples of parasites that
express host-related proteins and glycoproteins,
including receptors, on the apical membrane
of the tegument. Though definitive data
for involvement of specific proteins are lacking,
one possible example is the SGLT1-related
protein associated with the apical membrane
of the tegument. Antibodies raised against
residues 564–575 of rabbit SGLT1 recognize a
common epitope in the cestode homolog of
this protein. This protein is strategically located
for disguising the parasite from recognition by
host immune effectors.
Putative osmoregulatory ducts: an
internal surface in cestodes?
Adult cestodes possess a network of ducts that
form within the terminal proglottid and appear
to empty directly into the host intestine. In larval
cysticerci, these ducts do not open to the
outside, but rather empty into the central cyst,
which enlarges with fluid over time. Recent
immunohistochemical studies have localized
high levels of a Na -dependent glucose
cotransporter (a homolog of human SGLT1) to
the inner lining of these ducts. Though no biochemical
or physiological evidence has been
reported for a role for these structures in
osmoregulation, it has been hypothesized that
Na /glucose cotransport may regulate water
transport across the membanes that form these
ducts. In humans, SGLT1 couples the transport
of 210 water molecules per two Na ions and
one glucose molecule, and it is tempting to
speculate that a similar process may occur
within the putative osmoregulatory ducts in
cestodes.
TREMATODES
In addition to a tegument, adult trematodes
possess an internal surface, the gastrovascular
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS