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CESTODES 299
material bounded by the basal lamella and the
glycocalyx.
The cestode tegument is not homogeneous;
regions of specialized structures are found
over most of the adult animal. These include
adaptations that form the scolex, the region
involved in attachment to the host intestinal
tract. Putative excretory and secretory openings
interrupt the continuity of the distal cytoplasm
and plasma membrane. Sensory nerve
processes terminating in external cilia also
interrupt it, as does the genital atrium. Little is
known about the biochemistry of these specialized
structures. Our discussion of the functional
properties of the cestode tegument will
therefore focus on tegument typical of the
proglottids.
Biochemistry and molecular biology of
structural components
The cestode tegument contains many structural
proteins and enzymes, some of which
are clearly adsorbed from host fluids. In vitro
studies measuring uptake and incorporation of
[ 14 C]leucine by H. diminuta revealed at least 17
microtriche proteins synthesized by the parasite.
Together, the glycocalyx and microtriches
contain at least 30 distinct proteins and glycoproteins,
with molecular masses of 12–237 kDa.
These proteins impart a negative charge to the
surface of the parasite. They are rapidly shed,
exhibiting half-lives of only about 6 h, and are
replaced by new proteins synthesized in the
cytons. Actin, tubulin, collagen and keratin are
present at high levels.
The cestode tegument contains a variety of
glycolipids and phospholipids. In H. diminuta,
lipids comprise 40% of the mass of the
outer tegumental membrane, including the
microtriches, with the major components
being cholesterol, cardiolipids and phosphatidylethanolamine.
The predominant fatty
acids contain 16–18 carbons and are 50–60%
unsaturated. Like other helminths, cestodes
are unable to synthesize cholesterol or other
long-chain fatty acids de novo from acetyl-
CoA. Lipid droplets are formed in the cytons
from fatty acids and sterols that accumulate
following absorption from the host across the
tegument.
Little is known of the physiological or biochemical
characteristics that distinguish the
tegument of the developing cestode from the
adult. Endocytosis may play a more important
role in the absorption of macromolecules
in immature stages, though supporting data
come only from studies on Taenia crassiceps.
Marked structural changes in the tegument
typically follow the molting process, some of
which appear to be regulated by neuropeptides
released beneath the basal laminella. In
Diphyllobothrium dendritium, neuropeptides
are released in response to the elevated body
temperature of the definitive avian host. The
possibility that the tegument actively responds
to external signals is suggested by the presence
of G proteins in the tegumental brush-border
membrane of H. diminuta.
Proteins within the glycocalyx and the
membrane of the microtriches are protected
from digestion by host proteolytic enzymes.
The underlying mechanism(s) is unknown.
One possibility is that the glycocalyx helps
maintain an unstirred water layer between the
parasite and the contents of the host intestine.
Another possibility is that the organic acid
end-products of carbohydrate metabolism
excreted by cestodes form a microenvironment
in the immediate vicinity of the tegument
that is too acidic for host intestinal
proteases to function. Alternatively, the glycocalyx
may contain factors, as yet undefined,
that specifically inhibit host hydrolytic
enzymes. For example, H. diminuta inactivates
trypsin and chymotrypsin by releasing a
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS