A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 

Glossary - O




"o" neuron



OLL and OLQ are neurons of the outer labial lateral sensilla that have ciliated endings with striated rootlets. They are thought to be mechanosensory neurons.

Obligate parasite  

A parasitic species that cannot live independently from its host. Converse is a facultative parasite.

See Facultative parasite
See Commensal
See Free living
See Mutualism


Situated in a slanting position; not transverse or longitudinal.

Oblique muscle  

C. elegans have two bilateral pairs of sex-specific oblique muscles that are located in the adult male tail: an anterior pair (aobL/R) and a posterior pair (pobL/R). Stimulation of the PCS neuron during mating results in shortening of the oblique muscles and is thought to facilitate changes the positioning of the tail during the mating process.

Obliquely striated  

A key feature of bodywall muscles in C. elegans (and many other nematodes) in which the muscle filaments (and the whole muscle cells) lie parallel to the body axis, but terminate at staggered sets of dense bodies (Z bands) running at an oblique angle to the length of the muscle. Each thin filament is anchored at a 6o angle to the Z band, whereas in striated muscles in higher animals, thin filaments are usually anchored at 90o to the Z line. The dark staining refractile Z bands can be seen as repeating rows of “striations” by light microscopy running at oblique angles to the whole muscle cells. These striations generally are organized in coordinate fashion among all muscle cells within a quadrant, so that they transcend cell boundaries.

Ocellus/ Ocelli  

A primitive light-sensing organ found in many marine nematode species. Although C. elegans is reported to have a phototactic response (Burr, 1985), it does not have an ocellus. Where present, the ocellus generally consists of a lens, a stack of lamellae, and a pigment cup (eyespot) (Croll, 1975; McLaren, 1976; Bird and Bird, 1991).

See Eyespot


Spear (S)
Onchiostyle (in Trichondorids)


A very large retractable tooth (stylet) found in some other nematode species (Dorylaims), but not in C. elegans, used to pierce plant or animal prey before feeding. It is operated by stylet muscles to extend or retract, and is generally hollow (De Ley, 2006).

See Stomatostyle


A volatile chemical moiety that can cross tissues to stimulate a buried sensory ending and induce a behavioral change, either attractive or repulsive.

See Attractant

Oe neuron RIPL

These neurons provide, through their gap junctions, the only direct connections between the pharyngeal nervous system and the nerve ring.


Sense of smell. C. elegans can distinguish at least seven classes of volatile odors. Two pairs of amphid sensory neurons, AWC and AWA are required for chemotaxis to these volatile odors (Bargmann, 2006).

See Chemosensation
See Taste

Olfactory receptor


The C. elegans genome is predicted to encode over 600 functional chemosensory receptor genes. Unlike vertebrate olfactory neurons which each only express one or very few olfactory receptors, C. elegans chemosensory neurons express multiple receptor genes (Bargmann, 2006). Studies with olfactory mutants linked a single 7-transmembrane olfactory receptor gene, odr-10, to the volatile attractant diacetyl and found that it is expressed in AWA (Sengupta et al., 1996).

Omega figure  

An intermediate stage in the formation of vesicles during endocytosis and exocytosis.

See Coated pit
See Vesicle fusion

Omega turn Omega bend  

A behavior in which the animal avoids further contact with an aversive stimulus by making a sharp 180o turn while continuing to utilize forward body motion, rather than a reversal (Croll, 1975).

See Deep ventral bend
See Pirouette
See Reversal

Onchia/ Onchium Buccal teeth (S)

Large cuticular “teeth” described from larger nematodes (Chitwood and Chitwood, 1950, after Cobb, 1919) that perhaps correspond to the buccal flaps seen in C. elegans (Wright and Thomson, 1981). In some species they may swivel or retract to help in feeding (De Ley, 2006).

See Flaps
See Tooth

Oocyte Ovum/ Ova  

The female haploid gamete prior to fertilization by a spermatozoon. Two haploid gametes (oocyte and sperm) combine to produce the single-cell diploid embryo. The female gamete is generally equivalent to what would be termed a secondary oocyte in higher animals, being formed after the meiotic division of a diploid germ cell. As the oocyte matures, it remains in syncytial contact with the rachis to gain nutrients and organelles from nearby gametes, many of which will undergo germline programmed cell death after sharing these components with the surviving oocyte

The term is also frequently used to refer to a “fertilized oocyte”, which is then the diploid cell resulting from the fertilization event and is actually an early embryo.

Oocyte recognition    

Process by which a sperm recognizes the proximity of an immature oocyte and attempts to fuse with it to initiate fertilization (L’Hernault, 1997).


The process by which immature germ cell mature into oocytes within the somatic gonad (for more detail see Greenstein, 2005).

Oogonium/ Oogonia (archaic) 

A female germ cell, a precursor that must divide several times in order produce one or more mature oocytes. In C. elegans, these are female germ cells lying in the unsheathed portion of the distal arm of the gonad.

See Germ cell

Oolemma/ Oolema  

This term has two different possible meanings, of which the former one is more common: 

1) The plasma membrane of the developed oocyte just prior to fertilization, including its “surface coat” of glycoproteins (Lee and Lestan, 1971; McLaren, 1973; Anya, 1976). Some of the glycoprotein coating may be contributed by secretions from the somatic gonad.

2) The portion of the eggshell that is derived from secretions of the uterus (Bird and Bird, 1991). This usually includes the outer vitelline layer.

See Uterine layer
See Vitelline layer
See Zona pellucida


A commonly used strain of E. coli used to feed C. elegans on culture plates. Due to a metabolic defect in the bacterium, this strain is limited in its growth potential. Hence the OP50 bacterial strain does not continue to grow too much (overgrow), producing a reproducible “lawn” of bacteria on the surface of the agar that becomes exhausted as nematodes consume the original lawn (Stiernagle, 2006).

Oral cavity    

See Buccal cavity


The complete collection of open reading frame sequences (ORFs) within the genome of a given animal, which may be expressed at any time in the animal’s developmental history.

See Genome
See Proteome
See Transcriptome

Organizing center/ Organizer



A cell whose intercellular signaling activities induce the position and/or orientation of morphogenesis in a nearby tissue, acting in cell nonautonomous fashion. Examples known in C. elegans development could include the role of the anchor cell in guiding the AC/VU decision and the role of the DTC in organizing the polarity of the gonad.




The opening of a cavity or chamber.




Any kinesis mechanism in which the forward speed is changed. In C. elegans, the animal’s slower forward motion when moving on a lawn of bacteria could be considered an example of orthokinesis (Dusenbery, 1980).

See Kinesis

Osmiophilic granules  

Something that is readily stained with osmic acid, a fixative commonly used in electron microscopy and which appears electron opaque. Yolk granules are an example of such kinds of bodies.



Active regulation of osmotic pressure to maintain homeostasis of the body's water content. Regular pulsations have been noted in the excretory duct by light microscopy in C. elegans and other species under certain conditions that suggest the excretory system is required for osmoregulation (Nelson and Riddle, 1984; Bird and Bird, 1991). These pulsations are only seen in dauer larvae in C. elegans, but have been noted to vary in rate according to the tonicity of the environment surrounding the whole animal, suggesting that outflow from the canal cell regulates the water balance of the whole animal. Some researchers believe that the amphid sheath cell may also play a role in osmoregulation in some nematode species (Wright, 1980; Ashton and Schad, 1996).

Osmotic avoidance    

Avoidance of high osmolarity. An observed behavior in normal C. elegans individuals in which they prefer not to enter regions of high osmotic pressure (such as high salt) (Culotti and Russell, 1978; Hart, 2006). Normal animals can be confined on a culture plate by a relatively narrow ring of high salt in the agar that they will not cross, but dauer larvae are more willing to pass over the barrier to sample new territory.



The sexual mating of two unrelated strains in order to introduce a desired genetic feature from one parental strain into the second strain. An outcross is typically followed by repeated backcrosses into the second strain in order to “fix” the gene in the new genetic background as a homozygote, while reducing and then eliminating contributions by all other non-selected genes from original parent strain. Outcrossing in C. elegans is relatively infrequent in wild populations, but is required in male/female species in each generation, resulting in drastically different levels of genetic diversity within wild populations (Graustein et al., 2002; Barrière and Félix, 2005).

See Backcross
See Crossing
See Selfing

Outer labial sensilla





They can be subdivided into two groups, the lateral (OLL) and the quadrant (OLQ) labial sensilla.

See Inner labial sensilla
See Labial
See Labial sensilla image gallery

Outer vitelline layer  

The outermost portion of the eggshell. Some or all of the outer vitelline layer may derive from secretions from the uterine epithelium.

See Vitelline layer

Ova/ Ovum  

The haploid female gamete, or oocyte, prior to fertilization. More rarely, sometimes may be used to refer to a fertilized egg (zygote).


That portion of the reproductive tract that contains the mitotic germline and developing oocytes. It includes the distal arm of the gonad, the loop, and the proximal arm up to the spermatheca. The distal arm of the reproductive tract has also been termed the distal ovary, and comprises the germinal zone in the adult gonad.

See Oviduct

Ovarian sheath Gonadal sheath (S)
Oviduct sheath (S)
Somatic sheath (S)

A single layer (composed of 5 pairs of cells) that covers the germ line component of each arm of the gonad. These somatic cells are intimately associated with the ovary and help promote germline proliferation and gametogenesis.

See Gonad sheath


Crowding (S)

A situation when there are too many nematodes to be supported by amount of food and space. This will usually lead to dauer formation.

See Dauer formation


The proximal arm of the reproductive tract, or a portion of the proximal arm. This term has two conflicting usages. Some authors (Nicholas, 1975) use the term to encompass both the gonadal sheath, the spermatheca, and the uterus, those portions of the gonad which contain maturing oocytes and fertilized eggs, and deriving from the somatic gonad lineages. Other authors (for instance Kemphues and Strome, 1997) use the term to refer specifically to the more distal portions where immature oocytes are generated, thus referring only to the somatic sheath portion. This term probably never refers to the unsheathed portion of the distal arm.


An elongated muscular form of the uterus known in some parasitic nematodes such as Ascaris suum, designed to propel fertilized eggs out of the animal (Seurat, 1920; McLaren, 1976). This corresponds roughly to the proximal uterus and vulva of C. elegans, but the um and uv muscles seem underdeveloped compared to some other species.


Egg laying species (S)

Producing eggs which are laid outside the body prior to hatching. Species  bearing live young without forming an eggshell around the intermediate embryonic stage are called “viviparous”.

See Ovoviviparous

Oviposition/ Oviposit

Egg laying


The behavioral sequence involved in egg-laying, including any muscle movements that propel the egg along the length of the uterus, and those muscle movements which expel the egg from the vulva (Trent et al., 1983).

See Egg laying




Gonad that produces both male and female gametes.


Producing eggs (having well-defined eggshells) which then hatch internally, known as a “bag of worms” phenotype. This condition is sometimes seen in very old C. elegans, which are otherwise oviparous, but whose egg laying abilities decline late in reproductive life. Many egl mutations interfere with normal egg laying behavior and can lead an ovoviviparous state, where internal “hatchees” (L1 stage young) break out of the uterus and begin to eat the tissues of mother.

See Oviparous
See Bag of worms


After oocyte maturation, signals from the oocyte stimulate contractions by the sheath and dilation of the spermatheca. The spermatheca is pulled over the oocyte leading to penetration and fertilization of the oocyte by a sperm cell.

See Egg laying

Oxygen sensing    

A chemokinesis behavior in which the animal responds to exogenous oxygen concentration by altering its rate of motion and turning to try to remain within a certain preferred range of oxygen. Animals typically prefer to remain within the range of 5-16% dissolved oxygen, but this preference can adapt (Cheung et al., 2005; Gray et al., 2004). Oxygen levels are detected by several body cavity neurons (AQR, PQR, URX).

Edited for the web by Laura A. Herndon. Last revision: September 19, 2011. This section should be cited as: Herndon, L.A. and Hall, D.H. 2010. Glossary O. In WormAtlas.  doi:10.3908/wormatlas.6.15