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336 HELMINTH SURFACES
(including H-gal-GP and TSBP) are protective
only in their native forms. When produced in
recombinant systems, none has conferred
protection, even though they retain enzymatic
activity when expressed in yeast, suggesting
that appropriate glycosylation may be required
for the expression of protective immunity.
Tubular system
Tubular systems are present in most nematodes,
and consist primarily of two lateral ducts
that lie next to or within the paired lateral cords
(extensions of the hypodermis that also enclose
the nerve cords). In A. suum, canals that form
part of this system are exposed to PCF along
most of their length. These ducts or tubules,
and the cells that form ampullae into which
they empty before exiting the worm, were originally
assigned an excretory function based on
morphological evidence. The canals accumulate
dyes injected into the pseudocelom, and a
mechanism was proposed whereby the high
internal (or turgor) pressure in the nematode
could provide a driving force for filtering excretory
products released from cells into the lateral
canals. Given the relatively high molecular
weights and charged nature of the dyes used in
these studies, it is difficult to conceive of a
filtering mechanism that would allow their passage
while retaining water and other nutrients
within the pseudocelom, and no morphological
evidence exists for endocytotic or pinocytotic
mechanisms in the canals that could
underlie such a process.
The tubule system appears to play a critical
role in osmoregulation of some nematodes,
including C. elegans. The entire tubular system
has been reconstructed from serial section electron
micrographs, and consists of four cells with
nuclei located on the ventral side of the pharynx.
A pore cell encloses the terminus of an
excretory duct cell, which leads to an excretory
pore at the ventral midline. An H-shaped cell
forms bilateral excretory canals that extend
anteriorly and posteriorly along most of the
length of the organism. These canals form
numerous gap junctions with the hypodermis
and are in direct contact with the PCF. In addition,
an A-shaped excretory gland cell extends
bilateral processes anteriorly to cell bodies
located behind the pharynx. These processes
fuse with the H-shaped cell at the origin of the
excretory duct. Laser ablation studies on C. elegans
have illuminated the role of these cells in
homeostatic regulation. If the pore, duct or
excretory cell is ablated, the nematode fills with
water within 12–24 hours and dies within a few
days. Ablation of the excretory gland cell results
in no obvious developmental or behavioral
defects. These results suggest a role for the tubular
system in osmoregulation, and are consistent
with microscopic examination of the
excretory duct; its rate of pulsation under hyposmotic
conditions is five- to six-fold higher than
under isosmotic conditions.
C. elegans contains at least 12 ATPases,
including several subunits of a vacuolar-type
ATPase that transports H out of cells in other
organisms. The genes are expressed in the
H-shaped excretory cell, the rectum, and in
two cells posterior to the anus. One subunit has
been used to complement a yeast mutant, conferring
H transport function to the organism.
However, phenotypes associated with mutations
in these genes have not been described. It
should be straightforward to use dsRNAi techniques
to determine if this ATPase contributes
to pH regulation in nematodes, as it does in the
vertebrale kidney.
Additional insights into the role of the
tubular system in osmoregulation come from
C. elegans mutants. Mutants defective in the
posterior migration of canal-associated neurons
(CANs) exhibit arrested development and
excess fluid accumulation in the pseudocelom.
BIOCHEMISTRY AND CELL BIOLOGY: HELMINTHS