trans

312 HELMINTH SURFACES
lungs. Larvae then migrate to the trachea, are
swallowed and return to the stomach before
eventually re-entering the anaerobic environment
of the small intestine, where they
mature and reproduce. At each site within the
host, the parasite is exposed to immune effectors,
many of which are designed specifically to
destroy them.
Other species of nematode encounter
remarkably different environmental challenges.
Filarial nematodes parasitize internal tissue
compartments and never experience a freeliving
stage; they are transmitted by insects that
serve as intermediate hosts, moving from insect
to vertebrate through the process of insect
feeding. Some species evoke elaborate alterations
of host cell morphology and function
to create tailored feeding sites. This pattern
is exemplified by the process of nurse cell
creation by larval stages of Trichinella spiralis
in a mammalian host muscle. Adult female
Onchocerca volvulus elicit the construction of a
nodule from the host response, in which worms
can live and reproduce for up to a decade. Each
of these animals is adapted to the peculiar conditions
provided by its habitat. These include
markedly different pH, ionic and osmotic conditions,
and vary in the abundance, quality and
type of food available.
Though nematodes have evolved a vast array
of surface adaptations suited for specific environments,
the general features of their surface
biology are well conserved. That more is
known about those surfaces in nematodes
than in other helminths is attributable to several
factors. Parasitic nematodes pose more
prominent challenges to both veterinary and
human medicine, and more research investment
has been focused on them for medical
and economic reasons. The large size of Ascaris
suum, which is easily collected from the small
intestines of swine, has greatly facilitated
physiological and biochemical studies that
would be very difficult using smaller species
of nematode. Finally, much work has been
performed on the free-living nematode,
Caenorhabditis elegans, which in 1999 became
the first metazoan for which the primary
genome sequence was fully delineated. The
power of C. elegans genetics illuminated many
basic biological and biochemical phenomena
in nematodes, including many related to external
and internal surfaces. Fortunately, A. suum
and C. elegans provide information that can
generally be extrapolated to other species.
External surfaces
Structure
Gross and microscopic anatomy
Nematodes are bounded externally by a complex,
multilayered cuticle that extends into and
lines the pharynx, rectum, cloaca and other
orifices. Molecules that form the cuticle are synthesized
and secreted by the hypodermis, an
anatomical syncytium that forms a continuous
cellular layer immediately beneath the cuticle,
and is specialized for transport and secretion
(Figures 13.4 and 13.5). Though the cuticle and
hypodermis are morphologically distinct, they
represent a functional unit. Nematode cuticles
are tremendously diverse in structure. Among
species that parasitize vertebrates, the cuticle of
the adult stage typically consists of six distinct
layers: the epicuticle, outer and inner cortex,
medial layer, fiber layer, and composite basal
layer. Larval stages of some species also exhibit
a loosely associated surface coat that is secreted
through the excretory pore or from the esophagus;
its functional properties have not been
delineated.
The outermost layer of the adult cuticle,
or epicuticle, is 6–30 nm thick and often
appears trilaminate in electron micrographs.
Considerable controversy exists about whether
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS