trans

328 HELMINTH SURFACES
of tyrosine-based crosslinks between cuticle
collagen-like proteins.
Several other proteins are thought to participate
in immune evasion or suppression. For
example, the surface coat of Toxocara canis
cuticle is shed rapidly in response to binding of
host antibodies. In this process, a glycoprotein
(TES-120) is shed that contains an 86-amino
acid residue domain that is conserved in all
mucins. In vertebrates, mucins are immunosuppressants,
and it is possible that mucin-like
proteins shed from the cuticle serve analogous
roles in nematodes.
These observations have led researchers to
consider antigens not associated with the cuticle
as candidates for vaccine development.
Proteins that line the intestine of animal parasitic
species have received considerable attention
as candidate protective antigens. Because
of their location within the organism, these are
often referred to as ‘hidden’ or ‘concealed antigens’,
and will be considered in discussion of
the internal surfaces of nematodes (below).
Internal surfaces
Structural considerations
Gross and microscopic anatomy
The intestine and the tubule system provide
surfaces that are important sites of molecular
exchange between the nematode and host.
The alimentary tract extends nearly the full
length of the organism, opening at the mouth
and anus. It consists of three regions: the stomodeum
(mouth, buccal capsule and pharynx
or esophagus), the intestine, and the proctodeum
(rectum and anus, including the reproductive
opening in males). The stomodeum
and proctodeum are lined by cuticle. The
buccal cavity of some intestinal nematodes
contains teeth or cutting plates which are modifications
of the cuticle. In some species, the
walls of the buccal capsule are elaborated to
form an extendable stylet used to inject digestive
enzymes that aid in the extracorporeal
digestion of food. These are most prevalent in
species that feed on the mucosa of the alimentary
or respiratory tracts of the host. Nematodes
that have a small buccal cavity and rely
on extracorporeal digestion of host tissues
depend less heavily on the grinding action of
teeth, and cuticle modifications in the buccal
cavity are less extensive.
The intestine consists of a tube of epithelial
cells, one cell layer thick, connecting the pharynx
and anus. The number of cells that comprise
the intestine varies extensively by species,
ranging from as few as 20 in C. elegans to over
a million in A. suum. The intestine has no muscular
layer and so does not show peristalsis. It
is normally collapsed due to the high internal
pressure of the pseudocelom. Against this
pressure, food particles are propelled backwards
only by the pumping action of the pharynx.
The intestine is bounded at its ends by
muscular structures that control valves.
In most nematodes, microvilli line the intestine.
In A. suum, microvilli increase the surface
area for transport by almost 100-fold. They are
coated by a unit membrane and a glycocalyx
similar in appearance to that of vertebrate
intestinal epithelium. The basal membrane of
intestinal cells in nematodes may be smooth
or folded. It forms a continuous layer which
arises as a secretory product of epithelial cells;
mitochondria are concentrated near the basal
lamellae. Columnar cells, particularly in the
anterior regions of the intestine, appear to serve
glandular functions. Some parasitic species
show markedly different structural patterns.
For example, Bradynema, which parasitizes
insects, lacks both a cuticle and a functional
intestine. Whatever functions the gut serves
in other nematodes must be adopted by other
tissues. The hypodermis in Bradynema is
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS