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We have described the ultrastructure of the C. elegans secretory-excretory system. This four-cell organ develops embryonically with the exception of the pore cell (designated G2p) which surrounds the excretory duct early in the second larval stage, replacing an embryonic pore cell, G1 (Sulston and Horvitz, 1977). The entire system is descended from the early embryonic stem cell, AB, which gives rise to primary ectoderm (von Ehrenstein and Schierenberg, 1980; Sulston, White, Thomson, Schierenberg, and von Ehrenstein, personal communication).
have been attributed to nematode excretory systems are (1) excretion of
metabolic waste, (2) osmoregulation, (3) secretion of molting exsheathment
fluid, and (4) secretion and export of hormones to target tissues. The excretory
cell of C. elegans has a morphology consistent with any or all of these
functions. The excretory canals are exposed to the pseudocoelom, and they form
extensive gap junctions with the hypodermis. The energy required for active
transport across canal membranes could be made available by the abundant
mitochondria within the canal cytoplasm, and the surface area at which exchange
increased by canalicular branching of the lumena. The exchange process
conceivably could work in either direction, to excrete wastes into the canals or
to resorb, perhaps selectively, substances from the lumena. Any substances
collected in the excretory canals are presumably transported from the organism's
body via the excretory sinus, through the excretory duct to the excretory pore.
In addition, it is possible that substances may be excreted from the hypodermis
directly through the cuticle, which is water permeable in all
In other nematode species, osmoregulation is perhaps the best documented
excretory function. Weinstein (1952), Croll et al., (1972), and Atkinson
and Onwuliri (1981) observed that changes in the rate of excretory duct pulsation
correspond with changes in the osmolarity of the environment. In general, the
more sudden the change toward a hypotonic medium, the faster were the pulsations
of the duct. Waddell (1968) observed excretory duct pulsation in L3 (infective
stage) larvae of Stephanurus dentatus (pig kidney worm), but fluid from
L4s or adults was excreted without duct pulsation. Excretory duct pulsation
also has been observed in Panagrellus and Heterodera (Narang,
Under routine laboratory conditions, we were able to detect pulsation of the excretory duct only in dauer larvae. It is possible that only relatively fast rates of excretion involve duct pulsation. In C. elegans, a rapid change in water balance occurs at the L2-dauer molt, when radial shrinkage of the body results in an increased density of the dauer larva relative to the L2. Electron micrographs reveal that the volume of the hypodermis is preferentially reduced. The excretory system may be involved in the establishment or maintenance of this state. Thus, dauer larvae may employ excretory pulsing to maintain an appropriate water balance, whereas other stages of lower density may not need to do so. Additionally, the specialized dauer larva cuticle may be less permeable to excreted products, imposing a greater demand on the dauer excretory system.
We observed no
structural features suggesting neuromuscular control of excretory duct pulsation
in dauer larvae. Instead, the normally collapsed duct may be periodically forced
open by internal pressure from the excretory cell. Once accumulating fluid has
opened the duct along its entire length to the excretory pore, internal pressure
would be quickly released, and the duct could collapse once again. The extensive
folding of the duct cell plasma membrane around the duct may provide for the
rapid expansion and contraction of the duct during excretory pulsing. Similar
lamellar folds surrounding the excretory duct also have been observed in
Panagrellus redivivus (Narang, 1972).
possible function of the duct cell membrane is exchange of materials between the
duct contents and the duct cell cytoplasm. The duct cell may selectively
reabsorb salts or other substances entering the duct. The lamellar membrane
amplifies the cell surface area at the duct, and abundant mitochondria within
the duct cell could provide the required energy for active transport. We would
not propose such a function for the pore cell, because its plasma membrane lacks
the lamellar structure.
of the gland cell obviously suggests that a large amount of material is
synthesized within this cell, packaged into granules, secreted into the duct,
and transported to the exterior of the worm along with fluid from the excretory
sinus. The presence of secretory granules in all growing stages examined
suggests that secretion is either constant or occurs at repeated intervals. At
least some, if not all, of the secretory granules are membrane-bound. If they
release their contents into the duct by fusing with the plasma membrane of the
gland cell at the secretory-excretory junction, the apparent complexity of the
secretory membrane at this point may simply reflect the accumulation of membrane
in the restricted area which is entirely bound by a tight
junction. We do not know whether the type of tight junction found in the C.
elegans secretory-excretory system is functionally identical to the zonula
occludens found in vertebrate epithelium. However, it meets the major
morphological criteria established for tight junctions in vertebrates.
Permeability of these structures to heavy metal tracers has not been
morphology of the secretory membrane in animals of various developmental and
physiological states suggests that this structure may be an organelle. Although
some of the dense material associated with the secretory membrane may be
deposited during fusion with the electron-dense granules, none of our
micrographs actually show granules fusing with the secretory membrane, nor have
we seen portions of the membrane budding from the cell. The secretory membrane
retains its dense, convoluted, tubular morphology even in dauer larvae, which
lack the secretory granules altogether. Examination of previously published
light micrographs (Romanowski et al., 1971) indicates that a similar
structure may exist in other nematodes as well.
Studies on Phocanema decipiens indicate that a molting hormone acts on the excretory gland to bring about release of a "molting fluid" through the excretory pore into the space between the old cuticle and the newly synthesized, underlying cuticle (Davey, 1966; Davey and Kan, 1968). A leucine aminopeptidase was identified as a component of the molting fluid, and it was proposed that such enzymes may be involved in release or partial degradation of the discarded outer cuticle. The morphology of the gland in C. elegans also is consistent with a role in secretion of molting enzymes. The results of laser ablation of the gland cell, duct cell, or excretory cell nucleus in L2 larvae, however, led Singh and Sulston (1978) to conclude that the C. elegans excretory system is not essential for molting. These workers observed the loosening of the older cuticle to begin at the tip of the head rather than at the excretory pore. Furthermore, the amount of secretory granules in the excretory gland does not cycle with the molts, but increases steadily during development (with the exception of the dauer stage), and the greatest number of granules are found in the adult. The gland cells of all feeding stages of C. elegans (including the adult) are PAF-positive, whereas the glands of starved animals and dauer larvae are not. The ultrastructural comparisons suggest that PAF staining is correlated with gland activity.
The gland may not be essential for development or reproduction in the laboratory. Nevertheless, it may function in many ways to enhance survival in the natural soil environment. One substance released by C. elegans during all growth stages is a pheromone that in crowded cultures enhances entry into, and inhibits exit from, the dauer larva stage (Golden and Riddle, 1982). This pheromone or other informational molecules could be secreted via the excretory duct.
The gland cell
ultrastructure does not preclude the possibility that substances may be secreted
into the pseudocoelom, either by direct diffusion across the plasma membrane, or
by fusion of secretory granules with the plasma membrane at sites other than the
secretory-excretory junction. In addition, secreted products could be
transported throughout the body of the animal via the excretory
The striking differences in gland cell morphology between dauer larvae and other developmental stages suggest that release or degradation of secretory granules accompanies dauer formation, and that secretory activity is not present in the dauer stage- It also seems likely that gland function is not required to initiate exit from the dauer stage. The elimination of secretory granules and vacuolation of the cell is correlated with the dauer state itself and not with nutritional condition. No cause and effect relationship between dauer formation and gland function, however, has yet been established. We plan to perform laser microsurgery experimerits, in which both gland cell nuclei will be destroyed during the first larval stage, in order to test the possible effects of cell ablation on dauer larva formation.
Adapted by Yusuf KARABEY for WORMATLAS, 2003