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Glossary - B





B fiber

B subfiber (S)

A special form of microtubule within the ciliary axoneme (Chalfie and Thomson, 1982).

B cell


Rectal epithelial cell. Postembryonic blast cell within male tail rectal epithelium. The B cell lineage contributes to the male proctodeum.

B motor neurons

Class B motorneurons are a subset of the motorneurons driving the bodywall muscles and drive forward movement. This class of neuron includes VB and DB cells.

"b" neuron


AWB cell (S)


Amphid wing "b" cells, neurons having ciliated sheet-like sensory endings closely associated with amphid sheath.

See AW (Amphid wing) cell

Bacillary band

A modification of the lateral (or medial) hypodermal cells in some nematode species in which non-syncytial hypodermal cells appear to have a glandular specialization, directed outwards to secrete material towards a cuticular pore (bacillary pore), perhaps involved in osmoregulation (Wright, 1963; McLaren, 1976). In some species, cells containing neurosecretory granules project ciliated endings into the bacillary band (gland cell), perhaps permitting granule release towards the pseuocoelom in response to sensory input to those cilia (Wright and Chan, 1973).

Bacillary pore

An elongated opening in the cuticle associated with the lateral alae in some nematode species (Wright, 1963).

See Bacillary band


A procedure in which the progeny of a sexual mating or a new isolate from a genetic screen is mated back into a parental strain. This procedure is commonly used after genetic screens to obtain a desired mutation as a stable homozygote, while eliminating any secondary mutations from its genetic composition. Alternatively, it is used after introducing a desired mutation into a strain via mating, to eliminate any genetic contributions from one parental strain other than the selected mutant gene.

See Crossing
See Inbred / Inbreeding
See Outcross


Backward locomotion (S)
Backing behavior (S)
Backing response (S)
Backing avoidance response (S)
Backing reflex (S)

"Backing" describes several spontaneous and induced behaviors of C. elegans:

1) Backward locomotion; backing behavior: Spontaneous backward crawl. Forward locomotion and backward locomotion are antagonistic behaviors, controlled by distinct neural circuits, although these circuits functionally interact. The neuronal circuit comprising AVA, AVD and the A motor neurons drives backward movement.

2) Backing behavior: Male specific behavior (see mating behavior movie) where after the male's tail comes into contact with the hermaphrodite, the male backs sharply, curving his tail until his ventral side becomes apposed to the hermaphrodite. He then continues backing, sliding along the hermaphrodite's body and turning sharply near its ends (male avoids the head and tail of the hermaphrodite by turning either under or over the hermaphrodite body near but before it reaches the ends) until his tail reaches the vulva (Barr and Garcia, 2006). The PVY interneuron is required for continued backing.

3) Backing response; backing avoidance response; backing reflex: Worms that bump into obstacles (e.g. glass beads) on their paths or that are touched with a fine hair or that encounter high osmotic strength crawl backwards typically for several body lengths. After this, they usually turn around. This response is mediated by the mechanosensory touch neurons.

Bacterial lawn

A thin film of bacteria grown upon the surface of an agar plate as food for nematodes; this is the most common means to raise C. elegans in the laboratory. The lawn is usually kept from overgrowing the plate by the use of an auxotrophic strain of E. coli (uracil-requiring), and adding a limited supply of uracil to the plate. The lawn varies somewhat as the plate ages, growing somewhat thicker until the uracil is exhausted, and usually becoming slightly thicker at the border of the lawn than in the middle. There also may be subtle differences in the lawn depending on the character of the bacterial strain; such differences can affect some behavioral assays and nutritional studies.

See Auxotroph

BAG cell



Neurons, not part of any sensillum, dendrite runs along ILLso. Ciliated endings of BAG neurons terminate in bag-shaped structures that wrap short projections from the ILL socket cells.

Bag of worms

Bag (S)

A terminal phenotype in which fertilized embryos hatch from their eggshells within the body of the hermaphrodite and begin to feed upon the tissues of the parent animal. This process can occur occasionally in older animals, but is very common in certain mutants where egg-laying is inhibited by poor vulva development or inadequate motility of the vulval muscles.

See Internal hatchees
See Oviparous
See Ovoviviparous

Basal body

A modified centriole found distally in many sensory dendrites. The proximal segment of the ciliary axoneme, consisting of an organized ring of microtubules lying at a local enlargement of a distal dendrite. These structures in nematode cilia do not exactly match the organization of classical basal bodies in motile cilia from other animals, and therefore have sometimes been called transition zones.

Alternately, this term is often more loosely used to refer to that portion of the cilium in which the modified centriole is located, which should more properly be termed the transition zone.

See Axoneme
See Centriole
See Transition zone

Basal lamella

The innermost layer of the cuticle, lying between the fiber layers and the cell membrane of the hypodermis.

See Basal zone

Basal lamina

Basement membrane (S)
Lamina (S)
Extracellular matrix (S)

An extracellular structure formed along the basal pole of the cells of a tissue. It is a continuous layer of connective tissue over the surface of the tissue, such as hypodermis, muscle, or pharynx, uterus (ubl), and germline, and serves to separate it from neighboring tissues. This structure consists primarily of collagens, laminin, fibronectin, and other long chain macromolecules (See Kramer, 2005 for more detail).

In higher animals the basal lamina often consists of two or three distinct layers, but in C. elegans it usually appears as a single homogeneous layer which can be thick or thin depending on the tissue. The basal lamina generally looks quite thin in cross section by TEM (after chemical fixation), but can look more broad, fluffy or mesh-like after certain treatments (such as high pressure freezing) or visualization by electron tomography.

Basal layer

Inner layer (S)
Basal lamella (S)

The innermost layer of the cuticle is called the basal layer. It is relatively thin and may display longitudinal striations (alternating light and dark bands; see Chitwood and Chitwood, 1950).

See Cortical layer
See Fiber layers
See Fibril layer
See Matrix layer

Basal slowing response

A slowing in locomotion rate that occurs when well fed C. elegans encounter bacteria. This response is mediated by dopamine signaling (Sawin et al., 2000).

Basal zone

Fiber layer (S)
Fiber endocuticle (S)

A major subdivision of the cuticle, consisting of three “fiber layers” and a basal lamella, all of which lie innermost within the whole cuticle, just outside the hypodermal cell membrane. Each fiber layer may contain coherent parallel fibers running in different orientations, some in helical bands running around the whole body, or circumferentially around the body.

Basement membrane

See Basal lamina

BDU cell



Neurons, processes run along excretory canal and also in nerve ring, unique darkly staining synaptic vesicles.

Belt junction

Belt desmosome (S)
Adherens junction (S)

These prominent, robust intercellular junctions have also been called tight junctions or desmosomes; they form continuous belts at the apical margin of lateral cell borders in most nematode epithelia. They are tightly linked to the internal cytoskeleton and provide strength to the tissue. They are a form of adherens junction, but may contain some gene products that are typical of tight junctions in higher animals.

See Adherens junction

Bent head

A mutant phenotype in which the nematode exhibits a characteristic sharp kink in its head, causing misdirected body motion (Ward, 1973; Pierce-Shimomura et al., 2005).

See Adherens junction

Bergerac strain

One of the two most commonly studied wild type strains of the nematode Caenorhabditis elegans, originally collected from the wild in France (Nigon and Dougherty, 1949). Reference copies are now stored frozen at the C. elegans Stock Center, at the University of Minnesota, to reduce the amount of possible genetic drift from generation to generation. RW7000 subclone is used as the standard STS (sequence-tagged sites) mapping strain.

See Bristol strain


A characteristic feature of plasma membranes, visible in thin section by TEM, in which the lipids, when seen in cross-section, are organized into two discrete dark-staining layers separated by an extremely narrow clear space. The orientation of the lipids is such that non-polar (hydrophobic) portions of the lipids are concentrated towards the middle of the bilayer, while the polar (hydrophilic) “headgroups” lie on the outward faces, exposed to the cytoplasm or the extracellular space. Proteinaceous intramembrane channels lying within the membrane can span both lipid layers to allow passage of materials across the membrane.


A thin layer of bacteria growing on a surface, such as over the body cuticle, within the rectum or in the gut lumen. Sticky biofilms growing on the head or tail of C. elegans can cause swelling of the underlying hypodermis, interfere with feeding or defecation, and lead to delays in growth, larval arrest or death (Höflich et al., 2004). Aging nematodes often show pronounced swelling of the intestinal lumen due to infection.

Biolistic transformation

A method to create transgenic lines in C. elegans by bombardment of live animal with DNA-coated gold microparticles (Praitis et al., 2001).


When referring to a neuron, that which has two processes extending directly from the soma.

Alternatively, a tissue which extends two equivalent processes from a common central region.


A double refraction of light into two rays which occurs when it passes through certain types of material. In C. elegans, electron dense, birefringent structures are specifically noted in the intestine and are called gut granules.

Blast cell

Stem cell (S)

An ancestral cell in a cell lineage that divides once or more to give rise to several daughter cells. The daughter cells may be of non-equivalent cell fates (as in the AB cell or the MS cell) or of equivalent cell fates, as in a mesoblast. Later in cell lineages, the descendants of individual blast cells may become further restricted, as those of epidermoblasts (neuroblasts) and others.


The fluid-filled compartment inside the developing embryo at the onset of gastrulation, surrounded by the early blastomeres, into which some cells begin to migrate at gastrulation to create a multi-layered embryo. This internal space later is converted into the pseudocoelom as the embryo develops.


Any of the early cells that result from divisions of the fertilized oocyte. In some animal species, all blastomeres may have equivalent genetic potential and be interchangeable. However, in C. elegans early segregation of cytoplasmic and nuclear determinants give each blastomere a distinct identity, non-equivalent size and position. Early loss of any blastomere in C. elegans will result in a dead embryo since there is virtually no means to replace its subsequent lineage cells.

See AB blastomere
See EMS blastomere
See Founder cell
See P blastomeres


A hole in the ventral/posterior side of the embryo caused by the inward flow of cells during gastrulation. It is initially formed at the 26 cell stage by the movements of Ea and Ep. Over time, the blastopore enlarges to form the “ventral groove”.


The embryonic stage at which gastrulation begins. At this time the entire embryo still consists of a single mass of cells, except for a small internal space, the blastocoel.


A small protuberance of the cuticle in a localized spot, often near the lips or tail tip. These often occur in certain mutant phenotypes, such as the vab’s.

Alternately, a small acellular protuberance which breaks off from a cell to create a cytoplast as viewed by light microscopy, in some mutant embryos (Hird and White, 1993).


A local swelling or loosening of the cuticle (larger than a bleb), which can occur anywhere along the body. These often occur in certain mutant phenotypes, such as the vab’s, or in normal animals as a precursor to a molt.

Body cavity neurons

A set of sensory cells (AQR, PQR, URX) that respond to ambient levels of oxygen and whose sensilla lie close to the pseudocoelomic cavity (Cheung et al., 2004).

Body lobe

See Membraneous organelle

Body wall muscle

Body muscle (S)

The principal muscle cell type whose contractile activity generates body motion in the nematode. They consist of 95 unfused cells in the adult organized into four muscle quadrants. Their sarcomeres are obliquely striated and lie lengthwise along the bodywall. Body wall muscles send muscle arms to motor neurons to receive innervation.



A common behavior observed in C. elegans when grown on a culture plate. Animals move actively and stop to feed, but tend to collect preferentially on the edge (border) of the bacterial lawn, apparently where the lawn is slightly thicker. Animals (mutants) that fail to collect at the border are known as “nonBordering” animals, or Bor(-). Bordering and social feeding are thought to be under the influence of an external pheromone that is sensed by several amphid neurons (De Bono et al., 2002), and by oxygen levels monitored by body cavity sense cells (Coates and De Bono, 2002; Gray et al., 2004).

See Aerotaxis
See Pheromone
See Social feeding

Boundary zone

Thin region of demarcation within the cuticle that separates the overlying matrix layer from the fiber layers beneath it.

See Basal zone
See Matrix layer

Bristol strain

N2 (S)

One of the two most commonly studied strains of the nematode Caenorhabditis elegans, originally collected from the wild in Bristol, England by L.N. Staniland from mushroom compost (Nicholas et al., 1959, Fatt and Dougherty, 1963). Reference copies are now stored frozen at the C. elegans Stock Center, at the University of Minnesota, to reduce the amount of possible genetic drift from generation to generation.

See Bergerac strain


Thrush (S)

Residual cytoplasts and/or membranous whorls and debris lying in the fan cuticle of the male tail after retraction; most probably derive from retreating hypodermal cells.

Brush border

Bacillary layer (archaic S)

The microvillar specialization at the apical zone of the intestine, including a densely packed set of parallel microvilli extending into the lumen from the plasma membrane, and a rugged layer of cytoskeletal elements which form a dense reinforcing layer, the terminal web, just beneath the microvilli.

See Microvillus
See Terminal web

Buccal capsule

The more anterior portions of the pharyngeal region, often subdivided into subregions of cheilostom, prostom (arcade), mesostom, metastom and telostom. Different authors may include all or only some of these regions when using the term.

Buccal cavity

A cuticle-lined lumenal region which lies just behind the lips at the entrance to the pharyngeal lumen. Buccal cavity is surrounded by the pharyngeal epithelium and the arcade cells. The buccal cavity and the pharynx are separated by a cuticular constriction, the flaps. The nematode’s food is consumed by entry into the digestive tract through the buccal cavity.

Buccal epithelium

Pharyngeal epithelium (S)

See Pharyngeal epithelium

Buccal filter

Several cuticular specializations extend into the lumen of the pharynx and may act to trap food particles within the central portion of the lumen during pharyngeal pumping. Three outward channels, in turn, may permit excess fluids to be ejected from the corners of the lumen, thus concentrating food solids.

See Channels
See Flaps
See Grinder
See Sieve


Pharyngeal bulb (S)

The tubular pharynx in C. elegans shows two enlarged spherical regions (bulbs) along its length which are separated by a narrower tube-shaped isthmus. First bulb = anterior bulb = metacorpus, and second bulb = posterior bulb = terminal bulb. In some other families of nematode, the pharynx may feature one bulb or none.

See Isthmus


Male tail (S)

The adult male tail including the the lateral fan and rays.

This section should be cited as: Herndon, L.A., Altun, Z.F. and Hall, D.H. 2010. Glossary B. In WormAtlas.  doi:10.3908/wormatlas.6.2
Edited for the web by Laura A. Herndon. Last revision: October 3, 2013.