The secretory-excretory system of the nematode Caenorhabditis elegans
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The secretory-excretory system of C. elegans, reconstructed from serial-section electron micrographs of larvae, is composed of four cells, whose nuclei lie on the ventral side of the pharynx and adjacent intestine. (1) The first of these, the pore cell, encloses the terminal one-third of the excretory duct which leads to an excretory pore at the ventral midline. This cell, a modified hypodermal cell, secretes the cuticle that lines the excretory duct within the pore cell. Laser ablation of this cell seriously disrupts osmoregulation of the nematode. (2) The second cell, the duct cell, surrounds the duct with a lamellar membrane from the origin of the excretory duct at the excretory sinus to the pore cell boundary. Laser ablation of the duct cell nucleus results in disruption of osmoregulation and prevents the secretion of the duct cuticle within the duct cell. (3) The third cell, the large H-shaped excretory cell, extends bilateral canals anteriorly and posteriorly nearly the entire length of the worm. The excretory sinus within the cell body joins the lumena of the canals with the origin of the duct. Like the pore and duct cell, the excretory cell is essential for osmoregulation. (4) The fourth cell, the binucleate, A-shaped gland cell extends bilateral processes anteriorly from cell bodies located just behind the pharynx. These processes are fused at the anterior tip of the cell, where the cell enters the circumpharyngeal nerve ring. The processes are also joined at the anterior edge of the excretory cell body, where the excretory cell and gland are joined to the duct cell at the origin of the duct. Secretory granules may be concentrated in the gland near this secretory-excretory junction. The dauer larva (a developmentally arrested third-stage larva) uniquely lacks secretory granules, and the gland cytoplasm is displaced by a labyrinth of large, transparent spaces. Exit from the dauer stage results in the return of active secretory morphology in fourth stage larvae. The gland cell is not involved in osmoregulation, in growth control, in fertility, in longevity, or dauer larva formation.
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