trans

TREMATODES 305
glucose, galactose, N-acetylglucosamine, N-
acetylgalactosamine and sialic acid. These
sugars are linked to glycolipids and glycoproteins
in the tegument, which also contains
the enzymes required for their biosynthesis.
Surface glycoproteins are anchored to the outer
lipid bilayer by GPI through a process mediated
by an endogenous phophatidylinositol-specific
lipase.
Developmental biology
Schistosome cercariae undergo a profound
structural and biochemical transformation
during and after penetration into the definitive
host. During penetration, cercariae lose their
tails and begin to shed their fibrillar surface
coat. Detachment of the glycocalyx follows
secretion of proteases from the surface; these
proteases may also play a role in tissue penetration.
By one hour after penetration, multilaminate
vesicles begin to migrate from the
syncytium to the outer surface of the tegument.
During this period, numerous microvilli form
from the outer membrane of the tegument,
then are rapidly shed in a process that leads to
the formation of the heptalaminate outer
membrane. This protein shedding process may
play an important role in the development of
concomitant immunity, since it may prime the
host to mount immune responses against
subsequent challenge by cercariae. Formation
of the heptalaminate membrane is complete
within three hours after penetration. At this
point, the tegument of the schistosomulum and
adult stages are morphologically similar, as are
their sterol and phospholipid compositions.
Much higher levels of several surface proteins
are expressed by immature stages than by adult
schistosomes, and many surface proteins are
differentially expressed on the surface of adult
males and females.
Functional biology
Biological functions of the structure
The trematode tegument is structurally adapted
for immune evasion, nutrient absorption, ion
transport and communication with the underlying
neuromuscular system. Numerous pits at
the surface of the tegument markedly increase
the surface area of the parasite, which is consistent
with a transport function (see below). In
schistosomes freshly collected from host tissue,
these pits contain erythrocytes, suggesting that
they are open to the external environment. The
tegument is less pitted in F. hepatica, but
numerous invaginations of the surface effectively
increase its surface area. The double layer
of membrane at the surface of the tegument in
schistosomes is common only among blooddwelling
trematodes; species that inhabit other
environments, including F. hepatica, which
resides in the bile duct, are limited by a standard
lipid bilayer.
The tegument in schistosomes is electrically
coupled to underlying muscle bundles, allowing
changes in some ionic and electrical gradients
at the surface of the parasite to be transmitted
indirectly to the muscle fibers below. Gap junctions
connecting these two tissue layers are
believed to provide the morphological substrate
for this low resistance pathway. This link
between the surface membrane and underlying
muscle may serve an important role in the ability
of the parasite to respond rapidly to external
stimuli, such as mechanical pressure, nutrient
or ionic gradients. In addition, spines, which are
predominantly on the dorsal surface of trematodes,
may serve as holdfasts that maintain the
parasites within blood vessels or, in the case of
F. hepatica, the bile duct.
Immune evasion
Even a cursory summary of the tremendous
body of evidence supporting a role for the
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS