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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section IVAnatomy

Nematodes live almost everywhere. Diverse genera have adapted to free-living habitats in virtually all terrestrial and marine environments, and they parasitize virtually all species of plant and animal (Blaxter and Bird, this volume). Although they are of ancient evolutionary origin, their phylogeny is unclear because there is no fossil record (Fitch and Thomas, this volume). Nevertheless, all nematodes are built on the same basic body plan, which is made up of two concentric tubes separated by a fluid-filled space, the pseudocoelom. The animal's shape is maintained by internal hydrostatic pressure. C. elegans anatomy has been reviewed by White (1988). The outer tube is covered by the collagenous, extracellular cuticle, which is secreted by the underlying hypodermis (Kramer, this volume). At each of the four larval molts, a new cuticle of stage-specific composition is secreted, and the old cuticle is shed. The body musculature is arranged in four longitudinal strips which are attached to the cuticle through a thin layer of hypodermis (Moerman and Fire, this volume). Contraction of the two subventral muscle strips with relaxation of the subdorsal strips, and vice versa, generates sinusoidal movement in the dorsal-ventral plane (Driscoll and Kaplan, this volume). On an agar dish, the animals move forward or backward on either lateral side and are confined to the surface by the surface tension of the water in the medium. The nervous system, gonad, coelomocytes, and excretory/secretory system are the other components of this outer tube (Sulston and White 1988).

The inner tube is composed of the muscular pharynx with its nearly autonomous nervous system and the intestine. Figure 2 shows a schematic cross section through an adult hermaphrodite. The conserved nematode anatomy is generated by conserved developmental patterns. The early blastomeres, called founder cells, are generated by a series of asymmetric, asynchronous cleavages in which the germ-line precursor cell sequentially gives rise to the four founder cells for the somatic lineages and one germ-line cell (Schnabel and Priess; see Fig. 1 in Kemphues and Stone, this volume). The embryonic lineages generate 671 cells, but 113 of these undergo programmed cell death (Hengartner, this volume). By the time a larva hatches from the egg, it possesses 558 cells. Approximately 10% of these are somatic blast cells that divide further to generate additional somatic tissues in the adult.

Figure 2. Diagram of a posterior cross section through the adult hermaphrodite.

Figure 2

Diagram of a posterior cross section through the adult hermaphrodite. (g) Gonad; (h) hypodermal ridge; (i) intestine; (m) muscle; (nc) nerve cord. (Reprinted, with permission, from Edwards and Wood 1983.) (more...)

The cell bodies of most neurons are positioned around the pharynx, along the ventral midline and in the tail. Most of their cell processes form a ring around the basement membrane that surrounds the pharynx, or they join the dorsal or ventral nerve cords (Rand and Nonet, this volume). Most chemosensory and mechanosensory neurons extend afferent processes from the region of the nerve ring to sensory organs near the tip of the head. Other sensory neurons extend their processes along the body or to the tail. The nerve ring receives and integrates sensory information and connects to motor neurons in the head or along the nerve cords.

The bilobed pharynx pumps food into the intestine, grinding it as it passes through the second bulb (Fig. 3). The intestinal cells surround a central lumen which connects to the anus near the tail (see Fig. 1). The excretory/secretory system is involved in osmoregulation and in secretion of glycoproteins thought to make up a replenishable surface coat over the epicuticle. The excretory cell is the largest cell in the animal, with excretory canals running the length of the body that are connected to an excretory/secretory pore on the ventral side of the head.

Figure 3. Electron micrograph of a feeding L4 larva showing a transverse section through the posterior bulb of the pharynx.

Figure 3

Electron micrograph of a feeding L4 larva showing a transverse section through the posterior bulb of the pharynx. The “grinder” disrupts the bacterial cells ( (more...)

The hermaphrodite reproductive system consists of functionally independent anterior and posterior arms. Each arm is reflexed with an ovary that is distal to the vulva, a more proximal oviduct, and a spermatheca connected to a common uterus centered around the vulva (Schedl; Greenwald; both this volume). The adult uterus contains fertilized eggs and embryos in the early stages of development. Vulval contractions, mediated by the hermaphrodite-specific neurons, are required for egg laying.

The male gonad is a single reflexed organ extending anteriorly from its distal tip, then posteriorly to connect via the vas deferens to the cloaca near the anus (see Fig. 1). As with the hermaphrodite ovary, the germ-line nuclei are mitotic near the distal end. Meiotic cells in progressively later stages of spermatogenesis are distributed along the gonad to the seminal vesicle, in which spermatids are stored for release during copulation (Schedlt; L'Hernault, both this volume). Male-specific neurons, muscles, and hypodermal structures are required for mating with hermaphrodites (Emmons and Sternberg, this volume).

Although nematodes have evolved many specializations for their survival, all nematodes are built on a basic developmental and anatomical framework (Bird and Bird 1991; Blaxter and Bird; Fitch and Thomas; both this volume). As the commonalities between C. elegans and parasitic species have become clearer, C. elegans biology and parasitology have interfaced with progressively more detail. Placing C. elegans in a properly detailed phylogenetic framework will help formulate this interface, but such placement remains a challenge because the characters traditionally used for nematode taxonomy have been so limited and the phylum is so old that evolutionary divergence is great, even within anatomically similar genera (Fitch and Thomas, this volume).

It is not understood why all nematodes molt four times. Some species even molt once in the egg and hatch as second-stage larvae (Bird and Bird 1991). Molting is not required for growth as is the case for many insects. C. elegans increases in size by about one-third during each larval stage and again as an adult after the final molt (Byerly et al. 1976). The large intestinal parasite Ascaris is only slightly larger than C. elegans at its final molt, but it increases in size manyfold as an adult. The necessity to change surface composition to survive changing environments is an explanation for molting in parasites (Blaxter and Bird, this volume), and for free-living nematodes that form dispersal stages, but not for many other free-living species. It seems likely that basic developmental cues controlling postembryonic cell lineages, and even developmental plasticity in cell morphology and function, are activated by the molting cycle, and such linkages may have stabilized the molting regimen in evolution. The molting process in C. elegans has been described (Singh and Sulston 1978), but little is known about hormonal control of molting in any nematode. C. elegans does not biosynthesize ecdysteroids, although cholesterol is required in the diet (Chitwood and Feldlaufer 1990).

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK20145
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