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NEMATODES 317
Functional biology
Biological functions of the structure
The cuticle–hypodermis complex provides a
physical barrier that separates the cells and
extracellular fluids (pseudocelomic fluid, PCF)
of the worm from the aqueous environment in
free-living stages, or from host body fluids in
parasitic stages. Nematodes are pseudocelomates
and possess a fluid-filled body cavity. The
nematode pseudocelom differs from a true
celom by the absence of both a mesentery
(membranous lining of the body cavity) and a
muscle layer surrounding the intestine. Fluid in
the pseudocelom is in constant contact with
almost all of the cells in nematodes, and only
one cell layer separates PCF from the external
environment. Externally, that cell layer consists
of the hypodermis–cuticle complex; internally,
it is formed by the intestinal cells, and in rostral
regions by the tubular system.
The cuticle plays a critical role in the mechanics
of nematode movement. Nematodes lack a
rigid skeleton or circular muscles in the body
wall to provide antagonistic systems against
which longitudinal muscles can act to produce
movement. They utilize instead a hydrostatic
skeleton that is dependent on a high internal
(or turgor) pressure within the pseudocelom,
and limitations on expansion of the body imposed
by the cuticle. Internal pressures recorded
from the pseudocelom of A. suum average
70 mmHg above ambient, and values as high
as 225 mmHg are recorded during locomotion.
These values are much higher than transmural
hydrostatic pressures recorded in other invertebrates.
The biochemical mechanisms that
underlie the high internal pressure in nematodes
are unknown. Most species rapidly desiccate
upon exposure to air, and transport studies
using radiolabeled water indicate that the
cuticle in some species, including A. suum, is
highly permeable. Paradoxically, solute levels
in pseudocelomic fluid are generally isosmotic
or even hyposmotic to the fluid in the digestive
tract of the host. No simple biophysical model
can explain how nematodes maintain a high
internal pressure in the absence of a measurable
external driving force.
In addition to providing a scaffold against
which the hydrostatic skeleton can be levered,
the elasticity of the cuticle is essential for locomotion.
Although the cuticle collagen fibers are
inelastic per se, their unusual arrangement in
the composite basal layer confers elasticity to
the tissue. The fibers are organized into cylindrical
helices which spiral around the body in
opposite directions, such that fibers in the middle
layer run at a 75° angle to the longitudinal
axis of the parasite. The longitudinal muscles
are attached to the cuticle at its inner surface.
As these muscles contract, the helix is drawn
apart. This changes the shape of the body,
which becomes longer and thinner as localized
volume reductions occur within regions of the
pseudocelom beneath bundles of contracting
muscle fibers. The importance of the organization
of these helical layers to movement is illustrated
by C. elegans mutants with helical coils
that run in parallel, instead of at an angle to the
top and bottom layers. One unusual phenotype
exhibited by these worms is the inability to
move in a straight line. Instead, they ‘roll’ to the
left or right, and move only in small circles.
Most of the enzymes that catalyze crosslinking
reactions, which in turn control the structural
organization of proteins in the cuticle,
are unknown. However, molecular mechanisms
that underlie muscle attachment to the
cuticle are partially delineated for C. elegans.
The gene mec-8 encodes a regulator of RNA
splicing that controls processing of the gene
sym-1. Sym-1 encodes a protein that contains
a signal sequence plus 15 contiguous leucinerich
repeats. The product of sym-1 helps attach
body wall muscle to the cuticle. Disruption of
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS