trans

298 HELMINTH SURFACES
Important exceptions are noted. For example,
although Caenorhabditis elegans is a freeliving
species, insights it offers into the genetic
bases for structural and functional properties
of the cuticle, gut and tubular systems in
nematodes justifies its inclusion as a model
organism. Comments are primarily restricted
to adult stages, which are usually the most
pathogenic (with some notable exceptions,
e.g. Echinococcus spp. and Onchocerca volvulus)
and represent the principal targets for
chemotherapy.
CESTODES
Although cestodes have been less thoroughly
studied than trematodes or nematodes, we
begin with these parasites because of their
anatomical simplicity. The absence of both an
alimentary tract and a distinct excretory system
simplifies the interpretation of functional
properties of the external surfaces.
External surface
Structure
Cestodes interact with their environment only
across the tegument. In addition to the functional
properties associated with the external
surface in other helminths, the cestode tegument
is also structurally adapted to perform the
functions normally associated with an intestine.
Indeed, the cestode body plan has been
conceptualized as an ‘inside-out intestine’.
Consequently, research on the cestode surface
has been somewhat biased toward features
relevant to its role in nutrition.
The adult tegument forms a coherent
boundary between the fluid compartment of
the fibrous interstitium and the environment.
gc
mi
bl
cc
FIGURE 13.1 Schematic of the tegument and underlying
tissues in Hymenolepis diminuta. Abbreviations:
gc, glycocalyx; mi, microtriche; bl, basal lamella;
cc, cytoplasmic channel; cm, circular muscle; lm, longitudinal
muscle; n, nucleus; cy, cyton (drawing courtesy
of J.W. Bowman).
Several structurally distinct components are
evident (Figure 13.1). The outermost layer is
the glycocalyx. This acellular, translucent
mucopolysaccharide–glycoprotein coat, also
referred to as the laminated layer, adheres to
the tegument plasma membrane, which is the
limiting boundary of the tegumental syncytium.
The surface is evaginated into microtriches,
structures that are reminiscent of the apical
microvilli on vertebrate intestinal epithelial
cells. Microtriches amplify the functional surface
area of the parasite 2- to 12-fold, depending
on the species and stage.
The inward-facing membrane of the tegumental
syncytium is bounded by a fibrillar
basal lamella. The cytoplasm and organelles
within the syncytium are synthesized by cell
bodies (subtegumental cells, cytons) that lie
beneath the basal lamella. Somatic muscle
cells are anchored to the fibrillar components
of the basal lamella, so that contraction
is coupled to movement of the body. Our
analysis of the cestode surface focuses on the
cm
lm
n
cy
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS