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hepatic cells, and the caeca are transformed
into digestive/absorptive organs.
Nutrient digestion and absorption
The importance of the trematode intestine for
nutrient digestion and absorption is based on
several observations. Schistosomes begin to
feed on host blood within 4–6 hours after penetration.
In vitro, immature stages can survive
several days without erythrocytes, but their
growth and development is severely retarded.
Females ingest many more erythrocytes than
males, presumably in response to the demand
for amino acids and proteins used for egg formation.
Lysis and digestion of erythrocytes
begins in the esophagus, and appears to be initiated
by the release of dense secretory bodies
from gland cells in the posterior esophagus.
Hemoglobin is released from the erythrocytes
and passes into the intestine, where it is catabolized.
Adult F. hepatica ingest mainly erythrocytes,
but also other host tissues, including bile
duct epithelium and mucus.
A wide range of enzymes involved in digestion
and processing of nutrients are localized in
gastrodermis epithelial cells, including several
papain-like cysteine proteases, esterases, acid
phosphatases and glucose-6-phosphatase. The
most thoroughly studied hemoglobinases are
the papain-like cysteine proteases SmCL1 and
SmCL2, which are present in gut epithelial cells
of S. mansoni. These enzymes are 50% identical
at the amino acid level, and belong to separate
lineages of papain-like enzymes. SmCL1 is
expressed in the intestine and the tegument,
and is related to human cathepsins F and W.
SmCL2 is also present in the intestine, but may
be expressed more abundantly in reproductive
organs, and is more closely related to human
cathepsins K, L and S. SmCL2 is also closely
related to cathepsins L1 and L2, which are found
in vesicles within epithelial cells of the intestine
of F. hepatica. Other peptidases are present in
schistosome and fasciolid intestines, but their
specific roles in digestion have not been elucidated.
Little is known about changes in the
occurrence or distribution of these enzymes
associated with development of the trematode
gastrodermis.
The trematode gastrodermal epithelium is
probably the principal site of absorption for
large molecular weight nutrients that seem to
be critical for growth and long-term survival
and for egg production. Mechanisms that
underlie the absorption of these molecules are
uncharacterized. Endocytosis occurs to a very
limited extent in F. hepatica, but there is no
evidence for this process in schistosomes.
Ion transport and excretion
Little information is available on the role of the
trematode gastrodermis in transport of inorganic
or organic ions, or in volume regulation.
Whether this tissue contributes to excretion of
metabolic end-products is a matter of speculation.
Given the high level of permeability of the
tegument for water, inorganic ions and organic
solutes, an essential role for the gastrodermis
in excretion is doubtful. However, in schistosomes
and F. hepatica, the by-products of erythrocyte
digestion collect in the gastrodermis
along with cellular organelles and macromolecules
derived from the gastrodermis itself. This
material is periodically regurgitated, a process
that is probably more accurately equated to
defecation in higher phyla. No careful analyses
of the small molecules regurgitated by trematodes
have been published.
Immune evasion
At least one schistosome gut-associated protein
shed into the host’s circulation is implicated in
immune evasion. This large glycosylated protein,
called circulating anodic antigen (CAA),
binds host complement component C1q, but
does not activate it or its precursor, C1. It may
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS