General info - Exc. Canal Cell - Gland cell - Duct cell & Pore Cell - Cell identification - Cell list - Back to Contents
Four distinctive cell types make up the excretory system: one pore cell, one duct cell, one canal cell, and a fused pair of gland cells (See ExcFIG1). Although in embryonic life G1 cell acts as the excretory pore cell (=excretory socket cell), postembryonically this function is carried out by the posterior daughter of G2 cell. The excretory canal cell functions in part as a kidney, excreting saline fluid via the duct and pore in order to maintain the animal's salt balance (osmoregulation) and probably to remove metabolites (Nelson and Riddle, 1984; Buechner et al., 1999). The excretory gland cell is connected to the same duct and pore and secretes materials from large membrane-bound vesicles. The nature of this secretion is unknown, but has been postulated to play a role in molting (among others, see B.G. Chitwood and M.B. Chitwood. 1950). These secretions of the excretory system empty through a cuticle-lined excretory duct and pore on the ventral side of the head, just below the isthmus of the pharynx. The cells and their shapes are highly variable in nematodes (B.G. Chitwood and M.B. Chitwood. 1950). The connections are rather simple in C. elegans except for the convoluted shape of the duct. This system has no obvious innervation, although Nelson et al (1983) noted a close apposition between the anterior limit of the gland cell and the nerve ring.
The cell body of this H-shaped, extremely large cell lies just
behind the isthmus of the pharynx, on the ventral side, and extends four long
"canals", two running anteriorly in the lateral hypodermis, and two
running caudally in the lateral hypodermis (See ExcFIG2).
The excretory cell lies within the hypodermal tissue, and is closely linked
to the hypodermis by an extensive system of large gap junctions between their
lateral membranes (See ExcFIG3: GJ:gap junction).
Each canal also forms a basal surface that remains in contact with the pseudocoelom
over the full length of the canal. The excretory cell shares a common basal
lamina with the neighboring hypodermis. The nucleus is extremely large, has
a large nucleolus and is located on the ventral side of the terminal bulb of
pharynx. The excretory cell forms gap junctions with the duct cell and the gland
cells.
Each canal has a central lumen, and these lumens connect together within the
cell body to form a single excretory sinus that lies just anterior to the excretory
cell nucleus (seeExcFIG4; Exc: excretory cell,
GL: gland cell, DC: Duct cell, open arrows point to lumens of the excretory
canals). The collected outflow of the individual canals feed from this sinus
into the excretory duct via a specialized intercellular junction called the
secretory membrane, that must be permeable to the outflow of fluid. There may
be filamentous material extending from this junction into the duct lumen. The
secretory membrane spans a fairly large area, and is fenestrated to create a
set of short narrow channels, lined by very dense material, which connect the
canal sinus to the duct cell lumen (See ExcFIG7).
A system of beaded canaliculi feed into the apical lumen along the length of
each canal. These canaliculi radiate from all sides of the lumen over short
lengths to fill most of the canal cytoplasm (See ExcFIG
3). The canaliculi probably act as collecting tubules to filter salts
and fluid from the canal cytoplasm into the lumen. The plasma membrane surrounding
the lumen is locally specialized, having an electron dense thickening on the
cytoplasmic surface which may reinforce the shape of the lumen to prevent it
from deforming during fluid outflow (Buechner
et al., 1999). The shapes of the canaliculi are more plastic, and in different
conditions the canaliculi may appear as smooth, narrow tubes, a set of connected
beads on a string, with 50 nm diameter beads connected by narrow (10 nm)
necks, or the canaliculi can break up into a set of larger (90 nm) vesicles
which are disconnected from their neighbors and from the lumen. Many exc mutations
are known to cause gross cyst formation along the canal lumen (Buechner
et al., 1999; Suzuki
et al., 2001; Fujita et al., 2003). Some of these encode structural
proteins necessary to reinforce the lumenal membrane on the cytoplasmic face
or the extracellular face, such as sma-1 (spectrin) and let-653 (mucin) respectively
(McKeown
et al., 1998;
Jones and Baillie, 1995).
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Two identical (exc gl L and R) cells fuse to form the A-shaped
excretory gland (Nelson
et al, 1983). These large cells have separate cell bodies lying in the pseudocoelomic
space and connected to the ventral hypodermis just posterior to the pharyngeal-intestinal
valve (See a 3-D reconstruction of excretory gland cells by R. Newbury & Moerman lab. Cell labels are shown in ExcFIG7b (you can also visualize the movie by clicking on this figure). 3-D movie was created from confocal images of a strain expressing the
GFP marker linked to the promoter for B0403.4 [dpy-5(e907) X;sIs 13607 [rCes B0403.4::GFP + pCeh361]], using Zeiss LSM 5 Pascal
software v. 3.2). Each extends a large process anteriorly along the dorsal surface of the
ventral nerve cord where they fuse twice to form a ring-like process near their
anterior limit (see ExcFIG5. Asterisks point to
where two gland cells fuse) (See beta-Gal staining of excretory gland here).
Their outflow ends at a specialized permeable junctional complex (the "secretory
membrane") through which the contents of the gland cell must be dumped
into the lumen of the excretory duct (See ExcFIG6).
This junctional region is contiguous to the similar secretory membrane where
the excretory canal lumen connects with the duct lumen, and there is probably
some interchange directly between the canal and gland cells via some of the
same secretory membrane (See ExcFIG7:
red arrows: fluid crossing secretory membrane). The cell bodies
and processes of the gland cells are filled with characteristic large dense
core vesicles (200 nm diameter) whose exact contents and function are unknown.
The gland grows as the animal develops, and the number of vesicles within the
gland seems to increase gradually throughout larval stages and into adulthood,
although there is no obvious change in synchrony with the molting cycle (Nelson
et al., 1983). These vesicles become depleted in dauer larvae. Release of
the vesicle contents has not been observed directly, but there is no evidence
that the vesicle can pass through the secretory membrane; it seems likely that
release involves exocytosis at this junction. The vesicles become less electron
dense in the adult gland.
Each cell body is very large and is characterized by an extensive and highly
dilated RER, many mitochondria and many ribosomes. The gland cells appear to
form no specialized intercellular junctions with other neighbors in the hypodermis,
and may lie somewhat detached within the pseudocoelomic cavity. Nelson
et al (1983; see their Fig 18) suggest that the gland may receive synaptic
input at their most rostral portion where the two gland processes fuse. The
gland processes travel in close proximity to the ventral ganglion and caudal
portions of the nerve ring over many thin sections, but convincing synaptic
contacts are very rare if any.
Two specialized transitional epithelial cells form a narrow cuticle-lined tube (the duct and pore) connecting to the ventral body wall which links the outflow of the excretory system to the opening of the excretory pore on the ventral surface of the body (See ExcFIG9) (Nelson et al, 1983). The duct cell lies next to the terminal bulb of the pharynx, just anterior and lateral (either left or right) to the excretory cell body (See cell identification of the excretory system). The duct follows a tortuous path within the ventral hypodermis, and it always looks empty and fully open by TEM. The duct cell surrounds this tube completely for the initial two-thirds of its length, while the pore cell surrounds it for the remaining one-third portion. It should be noted that in the embryo, G1 cell functions as the excretory pore cell (=excretory socket cell), and in L1 larva, G2 becomes the pore cell. Posterior daughter of G2 (G2.p) then becomes the excretory pore cell of later stages while the anterior daughter (G2.a) gives rise to two neurons. Adherens junctions link the duct cell to the pore cell and the pore cell to the ventral hypodemis at their apical borders (See ExcFIG8; aj: adherens junction. Note: Ventral nerve cord (VC) divides into two equal fascicles to pass around the pore). Both the duct and pore cells feature plasma membrane stacks much like other hypodermal cells, and contain similar cytoplasmic contents. Nelson et al (1983) report that in the dauer larva, the excretory duct pulses visibly under DIC optics, followed by release of fluid through the pore.
Excretory cell nucleus is easily visible as ventral-anteriorly located to the posterior bulb of the pharynx. It is a large nucleus with a fried-egg appearance. The excretory duct is located anteriorly to the excretory cell and opens to outside via the excretory pore at the ventral side of the animal.
1. Excretory
canal
cell (=exc cell)
2. Excretory
gland cell (syncytial):
3.
Excretory duct
cell
4.
Excretory pore cell
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