trans

314 HELMINTH SURFACES
cuticle, which detoxifies fatty acid and phospholipid
hydroperoxides, but not hydrogen
peroxide. These findings have been interpreted
to suggest that the enzyme is secreted
to protect lipids in the epicuticle from oxidative
damage by reactive oxygen species released
by the host. However, there is no direct evidence
that the enzyme is secreted to protect
lipid components of the epicuticle per se rather
than those in some other site within the cuticle–
hypodermis complex, or that this process
even occurs in vivo. The surfaces of other
species of nematodes contain different proportions
of lipid components. In T. spiralis,
for example, lysophosphatidic acid predominates,
comprising 37% of the mass of surface
coat lipids.
The cortex is an amorphous, electron-dense
layer of the cuticle that is composed of at least
two sublayers (outer and inner) which contain
abundant keratin-like proteins and a highly
insoluble protein termed cuticulin. The medial
layer is an aqueous compartment that contains
fine collagenous fibers that are less dense than
those in the cortex. In some species, fluid within
this layer contains hemoglobin. The composite
basal layer contains crosslinked collagen fibers
in two to three distinct sublayers that spiral
around the nematode at an angle 75° to the longitudinal
axis of the worm. Depending on the
species, the cuticle may contain a wide range of
gross structural elaborations, including annulae,
lateral and transverse ridges, and spines.
These structures are usually restricted to the
cortex, though they may extend into the medial
layer. In some filarial species, the cuticle is
traversed by anatomical pores that extend to
the hypodermis. There is no histological evidence
for similar anatomically defined pores in
the cuticles of parasitic intestinal nematodes,
though the existence of functional pores is
suggested by biophysical studies on the
permeability characteristics of the cuticle (see
below). In addition, infective larvae of most
parasitic nematodes retain the cuticle of the
second larval stage. The adherent former cuticle
is referred to as a sheath, and is usually
400–600 nm thick. This structure retains the
epicuticle and cortical layers, and provides the
infective larvae with a second layer of protection
that is highly resistant to physical or chemical
damage, including insults associated with
the host’s immune system.
The hypodermis lies immediately beneath
the composite basal layer. It is multicellular or
syncytial, depending on the species. In most
species, an outer hypodermal membrane is
apparent, and it is likely that its absence in
micrographs of other species is an artifact. The
outward-facing membrane of the hypodermis
probably represents the true limiting membrane
of nematodes. Though typical cytoplasmic
constituents are uniformly distributed in
the hypodermis, nuclei are located only within
lateral ridges that project into the pseudocelom
at the mid-ventral, mid-dorsal and lateral lines.
The ventral and dorsal ridges also contain the
major nerve cords, whereas the lateral ridges in
most species contain small canals (or tubules)
that may serve excretory or secretory functions
(see below). The hypodermis is also traversed
by fibers that anchor the somatic muscles to
the basal cuticle.
Biochemistry and molecular biology of
structural components
The epicuticle is composed primarily of carbohydrate,
including negatively charged residues
and hydrophilic, sulfated proteins that are
partially exposed to the environment. In some
species, the epicuticle can be digested by elastase,
suggesting that an elastin-like protein may
be an important component. However, the precise
chemical composition of the epicuticle is
unknown.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS