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NEMATODES 329
elaborated by microvilli which greatly increase
its surface area for nutrient absorption. A
related phenomenon may occur in adult female
O. volvulus, in which the hypodermis hypertrophies
as the worm matures, coincident
with atrophy of the intestine.
Biochemistry and molecular biology of
structural components
Information on the biochemistry and molecular
biology of structural components in the
nematode gastrointestinal tract focuses mainly
on the regulation of endodermal differentiation
and on collagen synthesis. Proteins involved in
these processes are reasonably well conserved
among C. elegans and the few parasitic species
examined, including A. suum and H. contortus,
which should facilitate research in this area.
The structural organization of collagen aggregates
in the basement membrane of A. suum
clearly distinguishes them from cuticular collagens,
though biochemical explanations for
the differences remain unresolved. The importance
of tubulins in the nematode intestine is
suggested by data showing that depolymerization
of intestinal tubulin is the initial (and
perhaps lethal) action associated with the benzimidazole
anthelmintics.
Functional biology
The importance of the intestine for nutrient
absorption, excretion, and ionic and osmoregulation
varies among species and stages. In
general, the intestine is more important in
gastrointestinal than in tissue parasites. This
generalization may be challenged as better
culture systems for nematodes are developed
or when transport studies can be extended to
in vivo conditions. Since adult parasitic nematodes
often do not survive extended in vitro
incubations, it is likely that essential components
are absent from the culture systems
used, and these deficiencies may affect intestinal
function. Little is known about biochemical
changes in the intestine associated with
development in parasites. However, considerable
insight has been gained from C. elegans.
The identification of genes that are only
expressed in the gut (see examples below) permits
a detailed analysis of the development of
this specialized tissue.
Digestion
In intestinal parasites, digestion begins as food
particles enter the pharynx, and even earlier in
species that feed by extracorporeal digestion.
Nippostrongylus brasiliensis, for instance,
releases histiolytic enzymes from the pharyngeal
and subventral glands that partially digest
host mucosal cells, which are subsequently
ingested by the pumping action of the pharynx.
The release of digestive enzymes onto food
particles is greatly reduced in saprophagous
species, which feed on material already partially
digested by the host. In most animal parasites,
pharyngeal glands secrete digestive
enzymes onto ingested food particles as they
enter the pharynx. In A. suum, esterase, amylase,
maltase, protease, peptidase and lipase
are all secreted into the pharynx during feeding.
The pharynx of species that feed on host
mucosal cells may also contain hyaluronidase
and additional enzymes specialized for digestion
of host tissues.
A role for the nematode intestine in digestion
can be inferred from the presence of numerous
glands that are single cells that empty their contents
directly into the lumen. Enzymes localized
within the lumen of the nematode intestine
include an array of hydrolases typical of digestive
tissues; proteases are the best characterized.
Carboxy and thiol proteases are found in
the gut of many parasitic nematodes. A cysteine
protease from H. contortus degrades fibrinogen
and may help prevent clotting of ingested
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