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 - I





I band

I zone (S)

A dark stripe running lengthwise within the sarcomere of bodywall muscles, best viewed by polarized light or antibody staining. I bands run in parallel with A bands to form a series of alternating bright and dark bands (comprised mostly of myosin and actin) (Miller et al., 1983; Waterston, 1988). The I band contains only actin filaments.

See A band
See Actin
See M line

I cell  

Can refer to intestinal cell

“i” neuron ASI (S)  
I1 neuron



Pharyngeal interneuron

I2 neuron



Pharyngeal interneuron

I3 neuron



Pharyngeal interneuron

I4 neuron



Pharyngeal interneuron

I5 neuron



Pharyngeal interneuron

I6 neuron



Pharyngeal interneuron


IL2 neuron (S)




Inner labial neuron

IL1 neuron (S)




Inner labial neuron

Immune response Innate immunity (S)   The mechanism of resisting pathogen infection. Because of the chemical and mechanical nature of the nematode's cuticle, most pathogens infect C. elegans after being ingested. As such, C. elegans has a navigation system that allows it to try and avoid noxious factors. Pathogen avoidance may be mediated by the Toll receptor (Pujol et al., 2001; Ewbank, 2006). Other work has implicated a role for stress response genes in the immune response (Garsin et al., 2003; Ewbank, 2006). See Gravato-Nobre and Hodgkin (2005) for review on innate immunity in C. elegans.

A simple form of learning in nematodes in which an individual animal's exposure to a specific set of conditions, such as thermal (Hedgecock and Russell, 1975) and olfactory cues (Remy and Hobert, 2005), during a critical developmental time period alters the animal's behavior upon encountering similar conditions later in life. Olfactory imprinting was found to be mediated by a single pair of interneurons suggesting a role for chemosensory receptors in this behavior (Remy and Hobert, 2005).

Inbred/ Inbreeding


The repeated crossing of a strain back into itself or with a closely related strain so that the resulting progeny tend to become highly homozygous at most genetic loci.

See Outcross
See Backcross

Inbreeding depression  

A reduction in fecundity associated with repeated generations of inbreeding. This phenomenon is well known for many animal species, including some bisexual nematodes (Nigon, 1949), but is not likely to be as large a factor in the self-fertilizing hermaphrodite though it has been indicated in the determination of the sex ratio of C. elegans populations (Cutter et al., 2003; Stewart and Phillips, 2002).

Indirect development Heterogonic (S)  

Describes a lifestyle in which a parasitic species must undergo one full generation as a free-living species. The free-living animals proceed through sexual reproduction before undergoing developmental arrest at an intermediate larval stage. The arrested larvae must find a host and infect it before resuming development, or die. The infective animals may reproduce through parthenogenesis as females inside the host (Hawdon and Schad, 1991).

Inductive signaling Induction (S)

A mechanism of tissue differentiation where a signal or signals produced extrinsic to the cell or tissue is received at the cell membrane and cause the cell to alter its status. It has been demonstrated in C. elegans in some key developmental steps, such as in early embryogenesis, postembryonic development of the vulva, and in the connection of the vas deferens to the cloaca (Sulston and White, 1980; Kimble and Hirsh, 1979). There is generally no lineal relationship between the inducing cell and those responding to the inducing signal.


The growth of a secondary organism within the cells or body cavity of a host animal. C. elegans has been shown to be subject to infection by bacteria and yeasts (Tan et al., 1999; Labrousse et al., 2000; Aballay and Ausubel, 2002; Mylonakis et al., 2002; Hall, unpublished) though not from viruses, possibly due to the RNAi surveillance system which could provide virus defense (Vastenhouw and Plasterk, 2001). In some cases, infection seems to lead to death of the animal either by toxic interactions or by blocking feeding. Other infections may interfere with sexual reproduction by blocking normal use of the male tail apparatus.

There is no evidence to our knowledge that C. elegans itself can live as a parasite, but may be free-living and/or live commensally with pillbugs, as has been shown for several related rhabditid species (Baird et al., 1994).

See Commensal

Infertile Sterile (S)  

The inability to produce viable offspring. Infertility can stem from problems with gamete production, fertilization or early development of the offspring.

Ingress/ Ingression  

Move inward, as to migrate to a more central position relative to the outside of the body.

Inhibitory synapse Excitatory synapse (A)  

A synaptic connection between two neurons whose stimulation leads to a decrease in electrical activity in the postsynaptic cell (a hyperpolarization).

Innate immunity  

Nematodes are exposed to a variety of bacteria, fungi and viruses that coexist in the soil environment (Gravato-Nobre and Hodgkin, 2005). While some bacteria are palatable as food for C. elegans, others can be toxic. Besides being shielded by a thick cuticle, nematodes also secrete a modified glycocalyx over this cuticle to reduce the stickiness of unwanted neighbors (Höflich et al., 2004). When bacteria, viruses or toxins do become established inside the animal, the coelomocytes are eliminate some of them from the body cavity.  The intestine can be colonized by unfriendly bacteria (Tan et al., 1999; Alegado et al., 2003), but its lumen is also tightly sealed to prevent entry of live bacteria into the worm’s tissues. Bacterial invasion can induce local tissue swelling during the host defense (Gravato-Nobre et al., 2005). The RNAi phenomenon may be a relic innate response to viral invasion.

See Coelomocyte
See Immune response

Inner labial sensilla

IL cells (S)
Internal labials (S)


Sensilla found on the end of each lip in a symmetrical pattern. Each sensilla contain 2 dendrites as well as one sheath and one socket cell.

See Labial

Inner vitelline membrane

Lipid layer (S)
Vitelline envelope (S)


A common misnomer in C. elegans literature.  It refers to the innermost layer of the eggshell, a lipid bilayer which formally should be referred to as the lipid layer.

See Eggshell
See Vitelline membrane

Innervation process    

See Muscle arm

int I, int II etc.  

A naming system for successive rings of intestinal cells along the body axis, with int I being the most anterior set and int IX the most posterior; after Sulston et al., 1983.

Insertion Spicule insertion (S)

The behavioral act of fully extending the male spicules into the vulva. This behavior is coordinated by sex-specific neurons and muscles in the male tail. Sperm transfer occurs immediately following spicule penetration of the vulva (Barr and Garcia, 2006).


See Interdigitation

Interdigitation Intercalation (S)  

The term is often used in similar fashion to “intercalation”, and refers to the pattern by which cells or cell parts become mixed in a regular alternating pattern, often beginning in two parallel rows and forming a new single row.

For instance, muscle arms from separate muscle cells become interspersed at the muscle plate so that chemical signals from a motorneuron’s neuromuscular junction can simultaneously target several cells at once (White et al., 1976). Similarly, hypodermal cells from opposite rows extend past one another to form one row prior to cell fusion in the hypodermis (Priess and Hirsh, 1986; Podbilewicz, 2000). In the latter case, since the whole cells eventually enter one row, it might be best termed intercalation, whereas in the former case, where only the cell processes extend into one row, while their cell bodies remain in separate rows, the process might be better termed interdigitation.

Interfacial cell Transitional epithelium (S) A subclass of hypodermis in which an epithelial cell serves either to bridge two neighboring epithelial tissues or to form an opening in the epithelium.
Intermediate filament IF

A category of strong cytoplasmic filaments that reinforce the shape of various epithelial cells, binding to adherens junctions or hemi-adherens junctions at the plasma membrane. These extremely long thin cytoskeletal proteins are intermediate in size between microfilaments (actin) and thick filaments (myosin) or microtubules. They tend to organize into bundles of parallel filaments to form larger “stress fibers” and long thin “tonofilaments”. In hypodermis, they are an integral part of the fibrous organelle. Various forms of these proteins are expressed in certain nematode cells, including muscles, buccal epithelium, hypodermis, uterine epithelium, the marginal cells of the pharynx, some amphid accessory cells, and probably in several forms of valve cells (Bartnik et al., 1986; Perkins et al., 1986; Karabinos et al., 2001, 2003). While common in neuron processes in higher animals, it is not yet certain if intermediate filaments occur in nematode neurons. Distantly related proteins, the nuclear lamins, occur in the nuclei of all cells.

See Fibrous organelle
See Tonofilament

Intermediate layer Matrix layer (S)  

Layer of the cuticle

See Cortical layer
See Basal layer

Internal cortical layer Internal cortical exocuticle

A portion of the outer cuticle, comprising the major portion of the cortical zone. This layer lies just beneath the epicuticle, and contains amorphous material that sometimes appears electron dense. It lies just above the median zone, or mesocuticle.

See Cortical layer

Internal hatchees



Although C. elegans hermaphrodites normally lay their eggs prior to hatching, older mothers with diminished motor capabilities, or mutants that have egglaying defects, may have progeny that hatch within the reproductive tract. These young larvae (“hatchees”) may reside within the tract for some time, or break through into the pseudocoelomic space, from which they can begin to feed on the tissues of the mother.

See Bag of worms

Interneuron Integrating neuron (S)  

A category of neurons defined by their relationship within a wiring diagram. This is usually the case where the cell receives significant synaptic inputs (particularly from sensory neurons) and sends significant synaptic inputs to other neurons (especially to motorneurons). Although this set of relationships could easily be applied to many neurons within C. elegans circuitry, classification as an interneuron only occurs if it lacks a sensory specialization (which would classify it as a sensory neuron) and lacks any significant motor outputs (which would classify it as a motorneuron).


A mutant animal in which some tissues have adopted a male fate while other tissues have adopted the hermphrodite or female fate to create a living animal which is not wholely of one sexual fate, or which shows secondary sexual features of both sexes. Such outcomes are fairly common in some C. elegans mutants (Nelson et al., 1978; Doniach and Hodgkin, 1984).

Alternately, this term can refer to animals that have one set of reproductive organs of one sex, and non-functional vestiges of organs of the opposite sex, often called secondary sexual features. Some nematode species commonly show intersex features among normal animals in the wild.

See Hermaphrodite
See Pseudomale

Interstitial matrix    

A space-filling substance lying between two other well-defined structures, or filling a small enclosed space.

Intestinal cell  

Refers to the row of 20 large epithelial cells which line the gut lumen, all of which appear to contain identical cytoplasmic specializations. These cells are large and cuboidal with distinct apical, lateral and basal regions. They collectively form a prominent microvillar layer surrounding the lumen and each cell contains many cytoplasmic organelles related to its digestive function. The term is not generally referred to the rectal gland cells at the posterior end of the intestine, which also form some microvilli near the sphincter valve. The intestinal cells form distinct pairwise associations (Int rings) around the lumen, and are strongly linked to their neighboring intestinal cells by zonula adherens junctions.

Intestinal granule  

See Gut granule

Intestinal lumen    

The extracellular space enclosed by the digestive tract, where food is processed by the intestinal cells. The lumen is bordered by numerous villi that cover the apical surface of each intestinal cell. Digestive enzymes are secreted into the lumen and digested products (solutes, salts and liquid) are absorbed into these cells across the villi.

Intestinal muscle

Somatal-intestinal muscle (S)
SI muscle (S)





Two sheet-like cells that connect the intestinal cells to the ventral epidermis.


Gut (S)
Messenteron (Archaic)


A chain of very large cuboidal cells which all descend from a common precursor, the E founder cell and form a wide central lumen lined by many microvilli that form a brush border. Food passes from the posterior pharynx to the intestine where it is digested and then on to the rectum which processes the waste products for excretion.

Intestinal rings form in groups of two and four cells and surround the common lumen creating an epithelium that is only one cell deep at any point, with neighboring cells firmly secured to their neighbors by apical adherens junctions. These cells have very large nuclei and many contain large vacuoles, yolk granules and other inclusions. These inclusions increase in number and electron density as the animal ages.

The anterior intestine, or ventricular region, has a wider, more open lumen. The midintestine consists of a straight simple tube, which becomes flattened to the dorsal side of the pseudocoelom as it passes the vulva. The lumen of the posterior intestine, or prerectal intestine, may be somewhat dilated and is bordered by several rectal gland cells which may aid in digestion. These three regions have no distinct boundaries, but show only very gradual changes between them.

For more information see Wormbook (McGhee, 2007).

See Gut
See Cecum
See Microvilli

Intraflagellar transport



An active mechanism for transport of materials inside the cilium of a sensory neuron, distal to the transition zone. Transport is based upon the activity of microtubule-based motors and can move protein aggregates or other small cargoes vectorially in both directions at relatively high rates of speed. IFT may be important both for cilium formation and for signal transduction by the cilium. In higher animals IFT also occurs in cilia of many epithelial tissues (Mak et al., 2006; Scholey and Anderson, 2006; Inglis et al., 2007).



Finger-like cell extensions which precede cell invasion, as is seen on the anchor cell as it invades the vulval epithelium (Sherwood and Sternberg, 2003).

Ingression (S)


This is a process by which there is movement or folding inward. Common processes in C. elegans where invagination is seen include:

1) The formation of a cleavage furrow during cell division.

2) The initial step of gastrulation during embryonic development where cells move and fold to create different tissue layers.

3) A step during vulval morphogenesis in which cells detach from the cuticle allowing for the vulval sheet to bend inward. This step establishes the beginning of the vulval lumen.


Cell invasion


A biological mechanism through which a cell can cross a basement membrane to penetrate into a neighboring tissue – an important feature in both metastasis (intravasation, etc.) (Sherwood, 2006) and intercellular tube formation (Buechner, 2002). In C. elegans, cell invasion is known to occur during formation of the vulval uterine junction, when the anchor cell crosses the basal laminae of the uterus and of the vulval epithelium to link these tissues (Newman and Sternberg, 1996; Sherwood and Sternberg, 2003). Mutations that weaken the basal lamina can also cause aberrant cell invasions (Huang et al., 2003). Long distance migration and morphogenesis of embryonic and larval cells (sex myoblast, anchor cell) may also involve invasion events. Conversely, in parasitic species, this term can refer to the event by which a nematode enters the tissues of its host species (plant, animal or even another nematode).

Inverse embryo   Mirror image embryo
Ipsilateral Contralateral (A) A cell or cell extension that lies on the same side of the midline as a reference object.
Isothermal tracking  

See Thermotaxis


A narrowing of the pharynx between the first and second bulbs (between metacorpus and postcorpus) around which the nerve ring forms.

Iteroparous/ iteroparity Semelparous/ semelparity (converse)

The animal’s ability to produce young at repeated intervals over its lifespan rather than just once during during a narrow time period (as in “semelparous” animals). Although its lifespan is quite brief, C. elegans should probably be considered to be iteroparous.


A drug commonly used in humans and animals as an anti-parasite medication especially in the treatment of intestinal worms. Research in C. elegans has shows that this drug activates the glutamate-gated chloride channels (Brockie and Maricq, 2006) that has the effect of inhibiting the activity of excitable cells such as muscles and neurons which express these channels. Genetic (Dent et al., 2000; Keane and Avery, 2003) and pharmacological approaches (Pemberton et al., 2001) in C. elegans have helped to identify genes and how ivermectin functions to activate these channels.

Edited for the web by Laura A. Herndon. Last revision: November 13, 2009. This section should be cited as: Herndon, L.A. and Hall, D.H. 2010. Glossary I. In WormAtlas.  doi:10.3908/wormatlas.6.9