trans

NEMATODES 313
the epicuticle is the true limiting ‘membrane’
in nematodes. Several observations refute that
concept. For example, the epicuticle is not
dissociated by treatments that dissolve most
15m
FIGURE 13.4 Light micrograph (500) highlighting
fine structure of the cortical and medial (aqueous)
layers of the cuticle of Ascaris suum. Micrograph was
stained with trichrome. Scale bar 15 m. Abbreviations:
c, cortical layer; ml, medial layer; bl, composite
basal layer; h, hypodermis.
membranes, and lipophilic markers do not
exhibit the lateral mobility characteristic of
membranes when inserted into the surface of
nematodes. The epicuticle in most species
examined is devoid of intramembranous particles
that are normally associated with cell
membranes. Finally, under in vitro conditions,
charged solutes diffuse across the cuticle at
relatively high rates, indicating the absence of
a functional lipid barrier at the surface.
However, the epicuticle contains at least
some lipid components. In Brugia malayi,
for example, a cuticular lipid fraction was isolated
that contains high levels of lysophosphatidylethanolamine
and a novel species
that rapidly degrades to two or more products
upon exposure to atmospheric oxygen. This
lipid extract contains saturated and unsaturated
fatty acids in the range 14–22 carbons, with
the majority being mono- and di-unsaturated
18-carbon species. Furthermore, this parasite
secretes a glutathione peroxidase across its
ep
c
ml
bl
h
m
FIGURE 13.5 Schematic of adult Ascaris suum cuticle and hypodermis. Abbreviations: ep, epicuticle; c, cortical
layer; ml, medial (aqueous) layer; bl, composite basal layer; h, hypodermis; m, muscle (drawing courtesy of
J.W. Bowman).
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS