trans

304 HELMINTH SURFACES
projecting side chains of gangliosides and
oligosaccharides extends outward from the
apical membrane of the tegument. The function
of this structure, referred to as the glycocalyx,
is unknown. Cytoplasm bounded by the
tegumental membranes in schistosomes contains
mitochondria and two major inclusions:
discoid bodies and multilaminate vesicles.
Discoid bodies are precursors of the dense
ground substance of the cytoplasm as well as
of the surface bilayer. They may also break
down to form crystalline spines which are
most prominent on the dorsal surface of male
schistosomes (Figure 13.2). Multilaminate vesicles
consist of concentric whorls of membrane
which migrate to the surface and form the
outer bilayer. Mitochondria in the syncytial
tegument are sparse, small and contain few
cristae.
In the teguments of schistosomes and fasciolids,
nuclei, most mitochondria and ribosomes
are contained in subtegumental cells
(cytons) located beneath the layers of muscle
that underlie the tegumental syncytium. The
cytons are connected to the syncytium through
cytoplasmic channels that contain numerous
microtubules. Inclusion bodies synthesized in
the subtegumental cells arise from Golgi bodies
and are transported to the syncytium through
these channels. This process is interrupted by
protein synthesis inhibitors or microtubule
poisons. The tegumental surface membrane is
continually replaced by multilaminate vesicles
that migrate to the apical region of the syncytium
and fuse with the plasma membrane.
Biochemistry and molecular biology of
structural components
The schistosome tegument contains several
phospholipids, the most abundant being
phosphatidylcholine, with relatively high levels
of phosphatidylethanolamine, sphingomyelin,
lysophosphatidylcholine, phosphatidylinositol
and cerebroside glycolipids. Palmitic acid and
oleic acid are the most abundant fatty acids.
These molecules are derived from the host, as
schistosomes, like cestodes, are unable to synthesize
cholesterol or long-chain fatty acids
de novo. Schistosomes cannot degrade longchain
fatty acids by -oxidation. However, they
can interconvert fatty acids and cleave the polar
head from phospholipids. The apical membrane
of the schistosome tegument is continuously
replaced from multilaminate and discoid
bodies. Estimates for the turnover rate of the
apical membrane range from a few hours to
several days. This wide range of estimates is
probably due to the fact that some types of
molecules (e.g. phosphatidylcholine) used as
probes in these studies turn over much faster
than others.
The schistosome surface membrane contains
a range of enzymatic activities, including
alkaline phosphatase, Ca 2 -ATPase and glycosyl
transferase. Ouabain-binding sites on the
surface of the tegument indicate the presence
of Na /K -ATPase. A large number of associated
proteins in schistosomes has been detected
immunologically. Some are embedded in the
tegumental membrane, whereas others are
anchored by glycosylphosphatidylinositol (GPI).
At least one is dependent on palmitoylation
for membrane attachment. Unfortunately, little
is known about the function of these surfaceassociated
proteins. Paramyosin is an example
of a protein that was identified and characterized
after it was shown to be a partially
protective antigen in the tegument. Biochemical
studies on antigenic proteins have also
revealed that the surface spines are composed
of paracrystalline arrays of actin, and that
glutathione S-transferase is present in the
tegument.
Carbohydrates exposed on the surface of
schistosome glycocalyx include mannose,
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS