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 - P




P0, P1, P2, P3, P4 blastomeres

P cells (S)
P founder cells (S)
Propogative blastomeres (S) Boveri (1899)

Germline precursors found in early embryo, which undergo asymmetric cell divisions to yield a somatic founder cell (AB, EMS, C, D) and a succeeding P blastomere, gradually segregating germline cytoplasmic determinants into the germline founder cell, P4. These cells each lie at the posterior pole of the embryo. P4 moves inward during gastrulation, following the inward migration of the E founder cell, and comes to rest along the ventral side as a mesodermal cell in the late embryo, before giving rise to germline lineages in the larval stages.

Note that in Wormbase, the P cells are usually marked as P’; thus P1 = P’1 in Wormbase. There is a chance to confuse the “P cells” and the “P0-P12 lineages” (see below).

See P granules


Parental generation

P0.a, P0- P12 lineages

P ectoblasts (S)
P ventral neuroblasts (S)
P neuroectoblasts (S)
Ventral cord precursor cells (S)

Blast cells lying ventrolaterally in the late embryo and L1 larval stage which divide during the late L1 larva in stereotypical reiterated fashion to give rise to many motor neurons which begin function in the ventral cord and preanal ganglion of the early L2 larva, as well as hypodermal cells (which fuse into hyp7) and a series of programmed cell deaths. In addition, the Pn.p daughters of P3 through P8 lineages persist without fusing into the hyp7 syncytium until mid L3 stage when they can be induced to divide further to produce vulval epithelial cells (vulA – vulF) or more hyp7 cells. P0 is also known as W.

P granules Germline granules (S)

Darkly staining cytoplasmic organelles (not membrane bound) that move to the posterior pole of the one cell embryo and are further segregated in the P blastomeres with each succeeding cell division. These granules occur in clusters, and appear to be ribonucleoprotein particles, containing proteins and mRNAs specific to the establishment of the germline (Kemphues and Strome, 1997; Strome, 2005). They occur in all germ cells except mature sperm (Schedl, 1997).

Pachytene region  

A distinctive portion of the distal germline in which all germline cells have entered the pachytene stage. This region lies just distal to the germline’s transition zone, where germ cells coordinately switch into the leptotene/zygotene stage from the premeiosis stage.

Pachytene stage  

A distinct stage in meiotic prophase progression, following zygotene and preceding diplotene. At pachytene stage the chromosomes are condensed to form tightly bound pairs and the nuclei are characterized by a distinctive "bowl of spaghetti" morphology. The chromosomes are enjoined by the synaptonemal complex at this stage.  This feature is best observed by light microscopy in living cells, and is often used to indicate the progression from mitosis into meiosis, especially with regard to the germline.

Papilla/ Papillae  

Sensory endings that create small, visible lumps in the cuticle surrounding the dendritic specialization. Examples include the deirids, post-cloacal sensillae (PCS), hook, and inner labial sensillae of the lips. A longer, bristle-like sensillium would alternately be termed a "seta". Mechanoreceptors might form subtle cuticular features on the lips, but we do not see papillae by SEM for these structures; instead they apparently form nubbins within the cuticle that produce no visible specialization when viewed from the surface.

During morphogenesis, more complex sensilla (male rays, for example) may also adopt the form of papillae prior to their final elongation phase, which in some developmental mutants may remain as papillae. Some papilla may form a closed structure (as in the case of the deirid) or they could be open to the exterior environment (as in the inner labial sensilla). Alternately, the whole structure of any of the six lips may be considered a papilla, containing several neighboring sensilla per papilla (Klass et al., 1976).

See Nubbin
See Seta / Setae

Papillary cell body     A (neuron) cell body associated with a papilla; generally used to refer to the mechanosensory cells of the lips.
Papillary cilia  

The ciliated endings of the sensory neurons of the lips, such as the inner labial or outer labial neurons.

Papillary fiber


A neuronal process emanating from a papilla. Generally used in reference to the mechanosensory nerves running from the lips to the nerve ring.

Papillary gland Sheath cell (S archaic)  
Papillary nerve Labial nerve (S)    
Papillary organ  

The collected sensory and support cells involved in a single papilla.

Par phenotype Partitioning defective (S)

An embryonic defect (par mutation) in which cell polarity of the single cell embryo is disrupted, and cytoplasmic organelles (P granules, chromosomes) and molecular species (mRNAs, protein kinases, and others) fail to segregate (partition) into specific regions of the cell in normal fashion, disrupting cell division and normal cell fate decisions. Correct localization of the PAR proteins facilitates proper cell polarity. (Kemphues, 2000; Gonczy and Rose, 2005).

Paracrystal/ Paracrystalline array  

Large molecules may associate with one another in highly ordered forms to create higher order complexes, known as “paracrystalline arrays”. These arrays may be seen by light microscopy under polarized light as “birefringence”. Such features are a normal aspect some cells such as muscles. Paracrystals can also form when mutated or poorly processed molecules associate into abnormal aggregates, and these may also be scored under polarized light (Waterston et al., 1977).

See Birefringence


A loss of muscle function that usually comes from damage to the nervous system, but can also come from damage to the muscles. It can be a feature of specific genetic mutations that cause paralysis directly, such as in the case of some unc (uncoordinated) mutants, or can result from external causes such as exposure to anoxia and high temperatures. Certain mutations and chemicals (such as levamisole and aldicarb) that promote acetylcholine buildup at the synapses cause a hypercontracted paralysis (Rand, 2006). Infection by pathogens also leads to a paralysis which is often followed by death (Darby et al., 1999).


This protein, which is only found in invertebrate muscle, serves as a template for thick filament assembly in bodywall muscles in C. elegans. Myosin heavy chain proteins bind to paramyosin to form an elongated rod-like multimeric structure, the thick filament.

See Thick filament

Parasite/ Parasitism

Nematode species often live as parasites, feeding on or inside of another “host” species, which may be a plant, animal, or even another annelid. 

Obligate parasites cannot exist and reproduce apart from their host.

Facultative parasites may survive separately from their normal host species, but occasionally infect a host organism. 

Pathogenic parasites cause the death of their host. 

Ectoparasites live on the outer surface their host. 

Endoparasites live inside the body of their host.

Parasites typically depend upon their hosts for more than just a supply of food. They may also receive enzymes and developmental signals that guide and control their further maturation, and help in dispersal (transport to a new host).

See Commensal
See Free living
See Mutualism
See Necromenic
See Phoretic


Parthogenesis (S)
Asexual reproduction (S)

A form of asexual reproduction, used by some nematode species (but not by C. elegans), in which unfertilized oocytes within females progress through direct development into mature embryos upon passage through the spermatheca (as in Meloidogyne javanica (Triantaphyllou, 1962)). This mode of reproduction may be favored by certain parasitic species under conditions that prohibit finding sexual partners, and may alternate with amphimictic matings when away from the parasitized host, under free-living conditions (Bird and Bird, 1991; Poinar and Hansen, 1983). Parthenogenesis can operate either through meiotic or mitotic progression of the developing oocyte (Nicholas, 1975; Anya, 1976). Sperm are rarely present, and do not contribute genetic material in a parthenogenetic species.

See Pseudogamy
See Selfing
See Thelytoky


A small discrete object inside or outside of a cell, lacking a membrane coat. One example is the yolk particles (also known as gut granules) in the pseudocoelom (Hall et al., 1999).

Paternal effect    

Influences provided to the embryo by the male parent.

See Maternal effect

Paternal effect lethal    

A homozygous mutant that can develop successfully to adulthood, but whose sperm are defective and fertilization usually leads to abnormal zygotic development. Use of paternal effect lethal mutants provides a way to genetically define the contributions of the sperm to early embryogenesis.

See Maternal effect lethal


Siring (S)


The animal that has provided the sperm (male germ cell) in a mating is referred to as the paternal animal, or “sire”. Since C. elegans is polyandrous, the progeny of a single hermaphrodite may come from both self-sperm and the sperm of one or more males. Paternity is often scored after male mating (crossing) by the use of a genetic marker.

Path selection

Axon guidance (S)
Pathfinding (S)

  Neurons project axons, often long distances, to reach their targets. Guidance molecules act as cues to attract and repel the axons so that they find the correct path to their destination.
Pathogenic parasite    

A parasitic species that causes illness or disease in its host, eventually resulting its death.


Literally "disease development", this term refers to the events and mechanisms that underlie the development of a disease.



A defined step in the defecation motor program which is controlled by the GABA neurons (Avery and Thomas, 1997; Jorgensen, 2005).

See Defecation motor program

PDA cell Y cell  

Terminal differentiation of Y cell in hermaphrodites. In males Y divides and anterior daughter becomes PDA.

See Y cell


A characteristic of some nematode species, including C. elegans, in which the male tail fan extends posteriorly to meet at the midline behind the most posterior rays producing a blunt-ended tail (Schneider, 1866; Nguyen et al., 1999; Fitch and Sudhaus, 2002; Kiontke and Fitch, 2005).

See Leptoderan
See Caudal alae
See Fan

Pellucid layer    

The developing eggshell just after fertilization when this chitinous layer is being produced just beneath the fertilization membrane.

Periactive zone


A region of the pre-synaptic axon near the active zone which is a site of endocytosis and vesicle membrane recycling, but is relatively poor in mature vesicles. It is thought that this region may have a role in separating synaptic domains from non-synaptic cytoplasm (Zhen et al., 2000; Nakata et al., 2005; Jin, 2005).

Perienteric fluid

Pseudocoelom (S)




Rhythmic contraction of smooth muscle which produces wave-like motion to move contents from one place to another. In C. elegans this term is usually used to describe the contraction of the isthmus of the pharynx during feeding (Avery and Horvitz, 1989) where about 1 out of 4 pumps is followed by a wave of contraction that begins in the middle of the isthmus and moves towards the posterior, carrying food to the terminal bulb.

Possibly it could also be used to describe contractions of the intestine to move food through the digestive tract and also of movement by the gonad sheath and/or spermatheca during fertilization.

Perivitelline space  

The fluid-filled space lying between the eggshell and the vitelline membrane in the early embryo (Bossinger and Schierenberg, 1992; Schierenberg and Junkersdorf, 1992). Prior to creation of the eggshell, it is the space lying between the outer membrane of the one cell embryo and its fertlilization membrane,


A very small (similar in size to mitochondrion) single membrane, cytoplasmic organelle which is responsible for several steps in lipid metabolism. Peroxisomes have been demonstrated to be present in C. elegans tissues by fluorescence microscopy (Thieringer et al., 2003) and electron microscopy (Yokota et al., 2002).


A cell that engulfs an apoptotic cell during cell clearance (See Mangahas and Zhou, 2005).

See Phagosome

Engulfment (S)

"Cell eating". An endocytic process by which a dying cell or cell fragment is surrounded and digested by a neighboring cell. A very rapid process in apoptosis, but may proceed more slowly in necrosis. In early embryos, the birth of some cells which are destined to undergo apoptosis appears to induce rapid apoptosis, as if the dying cell sends out a signal to its neighbor very early in its own degenerative program. It is still not clear whether any phagocytotic events, "murders", occur to kill healthy cells.

See Engulfment
See Endocytosis
See Murder
See Suicide



A cell process or internal chamber that engulfs an apoptotic or necrotic cell, as well as invading bacteria or cell debris, to form a large intracellular vesicle. The phagosomes then fuse with lysosomes producing phagolysosomes in which the ingested material is digested by the lysosomal acid hydrolases. (See Mangahas and Zhou, 2005)

Pharyngeal channels

Tubular radii (S)
Marginal tubes (S-archaic)

Three cuticle-lined channels (tubular radii) lying at the extreme distal edges of the triradiate (rays of the buccal cavity and procorpus). These channels are thought to funnel excess liquid rostrally to exit the mouth during mastication, while food particles are trapped in the sieve.

Pharyngeal epithelium Buccal epithelium (S)

An anterior region of the pharynx, lying just anterior to the corpus, which forms a thin cuticle-lined tube connecting the buccal cavity to the pharynx proper. Nine cells form this tube by two successive rings of nonsyncytial tissue with six-fold symmetry. These cells are not muscular, but are dominated by radial intermediate filaments reinforcing the tube-like lumen.

Pharyngeal gland  

Five gland cells (g1 and g2 cell types - g1AL, g1AR, g1P, g2L, g2R) lie within the pharyngeal tissue and empty their contents via anterior ducts into the lumen of the pharynx. One pair of g1 cells are fused. The chemical contents of these secretions are unknown, but may be involved in digestion or molting.

See Gland cell

Pharyngeal-intestinal valve

Pharyngeal valve (S)
Cardia (S)
Esophago-intestinal valve (S)

A set of six epithelial cells that form a narrow passage connecting the posterior bulb of the pharynx to the anterior intestine (3-D reconstruction movie).The valve is composed of 3 pairs of cells each forming a flattened disc-like ring and is lined by a layer of cuticle. (Note that Albertson and Thomson, 1976 erroneously report only 5 cells here.) The pm8 muscle of the pharynx may act to open the valve when the grinder is active as there are no apparent muscular elements operating within or attaching to the valve. The valve's primary function appears to be to prevent regurgitation of the intestinal contents (Bird and Bird, 1991, after Loos, 1905).

Pharyngeal lumen



Narrow extracellular space through which food and liquid flow in/out of the buccal cavity past the grinder and pharyngeal-intestinal valve and into the intestine. The lumen has several specialized portions, including the pharyngeal channels, and is bounded by cuticle. Cuticular specializations include the flaps, the grinder and the sieve.

Pharyngeal muscle



There are eight muscle segments (pm1-8) in the pharynx each containing 1 to 3 muscles cells, most with three giving the pharynx three-fold symmetry. In most segments, the muscle filaments are oriented radially to help open and close the pharyngeal lumen, though pm7 muscles are oriented obliquely to pull on the grinder region. All cells, except pm8, are innervated by pharyngeal motor neurons.

Pharyngeal nerve    

A set of three longitudinal nerves (I1L, NSML and M2L) running along the anterior/posterior axis within the pharynx, each lying within a narrow groove created between syncytial pairs of muscles. One nerve lies dorsally, in close apposition to the dorsal gland duct, while the other two nerves lie subventrally.

Pharyngeal nerve ring    

A small circle of axonal processes which form within the first bulb of the pharynx and run around the lumen to allow all pharyngeal neurons to form synaptic contacts (neuropil) with other local neurons. No members of the somatic nervous system participate here. A smaller, less well organized neuropil, the terminal bulb commissure, forms at the posterior commissure of the pharyngeal nervous system.

Pharyngeal neurons

Pharyngeal nervous system (S)

The pharyngeal nervous system is composed of 20 pharyngeal neurons which lie completely within the pharynx. These are grouped into several categories and include motor neurons (M1, M2, M3, M4 and M5), interneurons (I1, I2, I3, I4, I5 and I6), and other types of neurons (MC, NSM and MI).

Pharyngeal pumping    

A stereotyped sequence of muscle contractions (motor program) by individual pharyngeal muscles that act to draw food (bacteria) into the pharynx and to masticate that food with the grinder before moving it into the intestine (Avery and Horvitz, 1989). One pump consists of an nearly simultaneous contraction of the corpus, anterior isthmus and terminal bulb followed by relaxation (Avery and Thomas, 1997).

While an animal is in the presence of food, pharyngeal pumping occurs constitutively, but this action slows or stops when the animal moves away from food, during lethargus, in dauer larvae and as the animal ages. The coordination of pharyngeal muscles is under control of the pharyngeal nervous system, which is virtually independent of the nerve ring.

Pharynx Esophagus (S)

The mouthparts at the anterior end of the digestive tract which contain a series of radial muscles, related neurons and epithelial cells, and several gland cells which may aid in digestion. The pharynx is divided into several regions, including the corpus (procorpus and metacorpus - first bulb), isthmus and terminal bulb (second bulb). Specialized cuticular teeth (the grinder) within the pharyngeal lumen may tear open the food prior to digestion in the intestine. The pharynx lies behind the buccal cavity, and is separated from the intestine by the pharyngeal valve.



A bilateral pair (PHA and PHB) of chemosensory endings in the tail tip, containing two sensory neurons each. In the hermaphrodite these endings open through the cuticle via lateral phasmid channels at the base of the tail whip. In the male the phasmid channels become diverted to open slightly more forward, on the ventral surface at the base of the copulatory bursa, posterior to the opening of the cloaca.

See Amphid

Phasmid channel Phasmid lining (S)

See Amphid channel

Phasmid gland Phasmid sheath cell (S-archaic)


Phasmid nerve



A bilateral pair of phasmid nerves extend posteriorly into the tail tip from the lumbar ganglia to caudal sensory structures including the phasmids and tail whip, in close continuity with the caudal limit of the lateral hypodermal cords.

Phasmid sheath

Phasmid gland (S)


The glia-like cell (Phsh) that forms the channel within which the phasmid cilia are exposed to the exterior environment; the cuticular opening of that channel is formed by the phasmid socket cell.

Phasmidia Secernentea (S)  

A major group of nematode species, categorized by:

1) caudal phasmids
2) a tubular secretory/excretory system
3) pore-like amphids located on the lips
4) no caudal or epidermal glands
5) rarely having cephalic setae
6) three esophageal glands
7) live mostly in terrestrial environments

(Bird and Bird, 1991)

Divided into 3 sub-classes: Rhabditia (of which C. elegans is a member), Spiruria (all parasitic) and Diplogasteria. Most parasitic nematodes derive from the Phasmidia.



See Propylene phenoxitol


Any substance (usually an odorant) released from the body that may act as a signal (attractant or repellant) to other members of the same species, either as a sexual attractant or as a social signal (aggregant or repellant).

Sexual pheromones are known from many nematode species (Bird and Bird, 1991). A sexual pheromone (of unknown chemistry) is likely to underlie the distinctive “leaving” behavior observed in C. elegans males (Lipton and Emmons, 2003).

A “dauer pheromone” induces larval C. elegans to alter their developmental status to enter the dauer pathway when animals are overcrowded (Albert et al., 1981). This pheromone, called daumone, has been purified and synthesized 6-(3,5-dihydroxy-6-methyltetrahydropyran-2-yloxy)heptanoic acid (Jeong et al., 2005) and is secreted and sensed by all animals to determine nematode density.

An aggregating pheromone is sensed by the ASH or ADL nociceptive neurons of the amphids, and can induce social feeding behavior in C. elegans (DeBono et al., 2002). Curiously, this unknown chemical signal is aversive, yet stimulates the animals to group together. This social feeding response is antagonized by a parallel signaling event transduced by reception of a secreted signal within the pseudocoelom by other sense cells (Coates and DeBono, 2002).

Secreted signals that cause signaling between different species of animals are called allelochemics (Bird and Bird, 1991). The physical source(s) of pheromone secretion is unknown, but would seem likely to overlap with the postulated sources of the glycocalyx. It is possible that several of the above responses involve the same pheromone, perhaps acting on the same or different receptors at different stage in the animal’s development.

See Aggregation
See Allelochemical
See Bordering
See Dauer
See Leaving behavior
See Semiochemical
See Social feeding

Phoretic Phoresy (S)

A type of commensal relationship between two species in which one animal (the nematode) uses another host species for transportation only, and does not harm the host, or feed on the host when it dies (Hong and Sommer, 2006; Kiontke and Sudhaus, 2006).

See Necromenic




Movement in response to the presence of light. Some nematode species seem to demonstrate generalized changes in behavior when exposed to light, without orienting to the light source (Nicholas, 1975). Some species, including C. elegans are reported to show specific forms of phototaxis, and move towards or away from the light source.

See Kinesis
See Phototaxis




A behavioral change in response to exposure to light that causes the animal to become oriented to the light source. Positive phototaxis carries the animal towards the light, whereas negative phototaxis carries the animal away from the light. C. elegans is negatively phototactic and responds to exposure to light (especially in the UV range) by increasing its body motions to escape from the stimulus (Burr, 1985). Responses in other nematode species reviewed in Nicholas, (1975).

See Taxis


Evolutionary "trees" that represent the relatedness of various species over time.

For a detailed description of rhabditid phylogeny, see Kiontke and Fitch, 2005.


A plant parasite. Many nematode species are plant parasites, generally having specialized mouthparts, such as a stylet, to pierce and suck from plant tissues.

See Parasite


Plant eating

Pi cell


Signaling from the anchor cell (AC) induces 6 nearby VU grand-progeny to adopt the pi cell fate. These cells generate the uv1 and the utse cells which form the ventral uterus.

See Uterus

Pick Worm pick (S)  

Usually made from platinum wire and a glass pipette, it is used to manipulate individual worms on culture plates using a dissecting microscope.


"Cell drinking". Material to be ingested by a cell is taken in by invagination of a portion of the plasma membrane in a process called endocytosis. The two main types of endocytosis are distinguished on the basis of the size of the vesicles formed. Phagocytosis "cellular eating" involves taking in large particles. Pinocytosis involves the ingestion of fluids and small molecules via small vesicles (~100nm).

See Endocytosis
See Phagocytosis

Pioneer axon Pioneers (S)  

An axon that establishes a new path as it grows away from a neuron cell body along the bodywall, often in contact with the hypodermis and overlying basal lamina. In general, a pioneer axon may exhibit a larger complex growth cone than that of a follower axon, so that it can extend filopodia over a wider region to sample the environment for guidance cues.

See Follower axon
See Guidepost cell
See Growth cone

Pioneer neuron  

A neuron that extends a pioneer axon.


A quick bout of reversals and/or omega turns that interrupt forward body motion, and help to reorient the animal on a chemical gradient (Pierce-Shimomura et al., 1999; Miller et al., 2005; Lockery, 2005).

See Omega turn
See Reversal

Plasma membrane

Plasmalemma (S)


A semipermeable lipid bilayer that contains and supports a variety of biological molecules including proteins, sugars and lipids. This membrane creates a boundary between the interior and exterior environment for cells, cellular organelles and tissues, within an organism. It also functions to control what goes into and out of the cell and maintains the cell potential.




Can refer to properties of the whole animal, of a given tissue, of a given cell, to subcellular features in a cell, or perhaps to molecular conformations of one macromolecule or a molecular complex.

Generally refers to the ability of a given element to adapt its shape and/or function to compensate for local changes in its environment, or to stresses brought about by its own previous history of activity.


A condition (generally true in C. elegans) in which the sarcomeres of bodywall muscles all tend to lie in direct apposition to the outer bodywall; as opposed to “coelomyarian” nematode species in which some sarcomeres are formed away from the outer bodywall to face the sides of the muscle cell or the pseudocoelom itself (Schneider, 1860; Chitwood and Chitwood, 1950; Bird and Bird, 1991).

See Circomyarian
See Meromyarian

Pleated junction  

See Septate junction (S)

Pleated septate junction  

See Septate junction (S)


A condition in which one gene (or one gene mutation) has multiple and/or variable effects (phenotypes) in several tissues or cell types, or at several distinct stages in development.

Mosaic pleiotropy arises due to the parallel use of the same gene product in several different types of cells, perhaps at different times in development.

Relational pleiotropy arises due to secondary cascades of intercellular signaling, such that genetic changes in one cell lead to altered intercellular interactions, and thus cause secondary changes in cells that do not require the gene directly.

In C. elegans, the low level of intercellular regulation greatly reduces chances for relational pleiotropy (Ward, 1976).


A structure in the form of a network, especially as it refers to nerves and blood vessels. In C. elegans this term is used to describe the many nerve processes interwoven at a region of synaptic neuropil, or to the interweaving of major nerves as they enter a ganglionic region, such as the nerve ring. It can also refer to interdigitization of muscle arms at the muscle plate.




Shallow folds (pleating) of the hypodermis in the molting nematode that permit the outer surface of this new worm tissue to be more extensive in area than the current outer cuticle. New cuticle is deposited over the plicae, underneath the old cuticle, and after molting, the new larval stage (or adult) will rapidly expand in size to stretch out the new cuticle, removing evidence of the plicae (Bird, 1977). Such plicae can be readily seen in C. elegans before the molts (Hall, unpublished).


The number of homologous sets of chromosome in a cell. Most cells in an organism are diploid and contain two full sets of chromosomes. However, sex cells (sperm and egg) are haploid and contain only one set of chromosomes. C. elegans hermaphrodites contain 4 autosomes and 2 X chromosomes while males only have 1 X chromosome.


1) the gelatinous copulatory plug produced by a male nematode after mating

2) the structure formed by certain cells (sujcs) that block the passage of the spermatheca prior to sexual maturity in the hermaphrodite

3) a refractile thread that blocks the mouth during development of the pharynx, but falls away shortly before hatching

These structures are entirely unrelated.

See Copulatory plug
See Plugging male
See sujc cell

Plug cell Core cell of spermathecal valve (S)  

See sujc cell

Plugging male  

Males of some species of nematode, but not C. elegans, generate a gelatinous plug on the vulva after spicular retraction. This ability is facilitated by the action of the plg-1 gene (Hodgkin and Doniach, 1997), perhaps by helping in the coagulation of the fluid to make the plug (Barr and Garcia, 2006).

Plum stage Two-fold stage (S)
Morphogenesis stage (S)


Plus end  

The growing tip of a microtubule or microfilament which often faces outward towards the periphery of a cell.

See Protofilament


The 8 separate segments that form 8 consecutive rings of radial musculature that encircle the pharynx.

See Pharyngeal muscle


Ventral hypodermal cells that give rise to the vulval toroids.


See Sheath

Polar body  

One of two small cells produced, each during one of the two meiotic cell divisions, which are then discarded in the creation of the haploid germ cell.

Polarity reversal  

Changes in overall orientation in space of a tissue’s developmental pattern, of a cell group’s pattern of cell divisions, or of an individual cell’s distribution of structural elements. Under some circumstances, any given lineage may demonstrate a reversal in the overall pattern of cell divisions (the fate adopted by the anterior daughter may be switched with that of the posterior daughter, etc) either due to steric problems or mutation (Sulston and White, 1980; Horvitz and Sternberg, 1982).

Some tissues display long range polarity in their organization, such as the gonad’s distal ovary vs proximal storage of sperm and in some mutants can show polarity reversal such that a new zone of mitotic activity can be created proximal to the vulva (Schedl, 1997; Pepper et al., 2003).

Some cell types may undergo a transformation of their shape during late stages of morphogenesis that reverse the order of their structural features in space; for instance, the DD neuron can switch the position of its axon and dendritic specialization during late larval development, so that its axonal process and dendritic process exchange places (Walthall et al., 1993).


The tendency of the female to mate with multiple male partners. In C. elegans, the hermaphrodite will mate multiple times with different males (LaMunyon and Ward, 1999).

See Copulatory plug

Polymodal input  

1) a characteristic of some primary interneurons in which they receive synaptic inputs from a variety of different sensory neurons, who in turn are receptive to substantially different kinds of sensory stimulation (chemical, mechanical, osmotic, thermal, etc) 

2) a sensory neuron that maintains multiple dendrites or multiple membrane receptors that allows it to be receptive to a variety of different modes of stimulation

3) a single membrane receptor that is responsive to a variety of substantially different environmental factors

See Nocioceptor


Muscles that contain many muscle cells side by side within a single quadrant as is the case the very large species Ascaris.

See Meromyarian


An abnormal condition in which multiple spermatozoons have managed to enter the same oocyte during fertilization, providing excess copies of chromosomal material. Such events generally do not produce viable offspring.

"Block of polyspermy” is thought to be accomplished by several coincident events at the moment when the first spermatozoon crosses the oocyte’s outer membrane, including a change in membrane potential in the oocyte and the immediate secretion of a “fertilization membrane” to physically obstruct close approach by another sperm.

Pore cell  

A transitional epithelial cell that connects the excretory duct to the hypodermis of the ventral bodywall at the back of the head, producing the excretory pore, an outlet for secretions of the excretory system. Blockage or destruction of the pore is generally fatal to the animal.

Post-cloacal sensillum/ sensilla PCS

The PCS is a male specific structure that functions redundantly with the hook in several steps of mating behavior including: locating the vulva, backing over the vulval region and inducing spicule prodding behavior. The PCS is innervated by three bilateral pairs of sensory endings in the male tail lying on the ventral surface just posterior to the cloacal opening, and pointing in posteriolateral directions (PCA, PCB, and PCC). Each sensillum consists of two neuron endings enclosed by a sheath and socket cell.


Posterior bulb (S)
Second bulb (S)
Terminal bulb (S)

The most posterior portion of the pharynx, containing the pharyngeal glands.

See Second bulb

Postdeirid Posterior lateral sensillum (S)
Posterior deirid (S)

See Deirid


A special form of L3 larva that has emerged from true dauer larva status and is preparing to molt into L4 stage.


Any event or feature which occurs during larval or adult life, but not within the developing egg.

Posterior bulb Second bulb (S)
Postcorpus (S)
Terminal bulb (S)

A major subdivision of the pharynx, lying between the isthmus and the pharyngeal valve. The bulb has an enlarged round shape and contains several large radial pharyngeal muscles, some neurons and three large gland cells (Albertson and Thomson, 1976).

See Second bulb

Posterior commissure Terminal bulb commissure (S)

A minor “nerve ring” in the pharyngeal nervous system, lying within the second pharyngeal bulb, where some members of the pharyngeal nervous system meet and form synapses.

See Pharyngeal nerve ring

Posterior intestine Prerectal intestine (S)
Prerectal region (S)

The more posterior zone of the intestine, lying just anterior to the rectum, and surrounded by the motor elements of the stomatal-intestinal muscle. The lumen is slightly dilated and receives secretions from the rectal gland cells near its termination at the rectal valve.

Posterior-lateral ganglion  

Neuron somata associated with the post-deirid.

Postreproductive Gravid (A)

An aging adult animal that is no longer capable of producing fertile germ cells nor nourishing and laying fertilized embryos, although it may still contain older, nonviable oocytes or sperm.

See Gravid

Postsynaptic cell Postsynaptic element (S)

Either a neuron or a muscle cell that receives a synaptic input from a presynaptic neuron. In rare cases, other tissues, including hypodermis and marginal cells may also appear to be postsynaptic (lying very close to a presynaptic density), but the physiological significance of these contacts is unknown.

Postsynaptic density Postsynaptic specialization (S)

A thin electron dense specialization lying on the cytoplasmic surface of a postsynaptic cell (neuron or muscle) (Rand and Nonet, 1997) or a thickening of the postsynaptic membrane.

These specializations are generally not found in C. elegans synapses except under rare circumstances, perhaps due to poor fixation or their very small size. In higher animals these specializations are often expected as a reliable indicator of a functional synaptic contact, and there are characteristic differences in certain synapses according to their function or cell type; none of these differences have been reliably seen in the nematode.

Preanal ganglion

PAG cells (S)

Pre-anal ganglion (S)

The preanal ganglion lies at the posterior limit of the ventral nerve cord in the tail. It contains 12 neuron cell bodies that send most of their neuronal processes into the ventral nerve cord. A local portion of neuropil involving many pre-anal and lumbar neurons is continuous with the neuropil of the ventral nerve. In the adult male tail, this ganglion gains many additional neurons and a greatly expanded neuropil, functioning in the control of male mating behaviors.

Precipice response  

An aversive response of the animal when encountering a steep edge, as at the edge of an agar chunk. Animals may avoid the edge (Hart, 2006).

Precursor cell  

Ancestral cell in a lineage. 

See Blast cell

Predacious/ Predatory/ Predator Predaceous (S)

The term implies that a species has a select set of target organisms which it searches out for food. By comparison, an omnivore may eat a wider variety of species, perhaps both plant and animal life, with much less selectivity. C. elegans consumes bacteria in the wild, and can be considered a predator of bacteria.

Many other animals and some fungi are predators of nematodes. Some fungi can trap nematodes (including C. elegans) by a small lariat-shaped ring or with sticky knobs, and then consume them (Nicholas, 1975). In the laboratory, mites are often found as predators on cultures of C. elegans.

See Parasite

Predauer larva L2d (S)

A distinctive late L2 larval stage (L2d) whose body shape is beginning to show changes prior to entering the dauer state, although it is still capable of advancing instead to an L3 stage if environmental conditions improve (Golden and Riddle, 1984).

Pregastrulation stage  

The earliest phase of embryogenesis, during which the egg undergoes early cell divisions to grow from one cell to 26 cells during the first hour and a half after fertilization (see 4-D movie).


A free-living larval stage of a species that will later infect a host plant or animal. In parasitic nematodes, the third larval stage is often the last pre-infective stage and L3 larvae may undergo developmental arrest until a host is found. The anatomy and neuroanatomy of preinfective L3s may parallel the anatomy of dauer larva of C. elegans (Ashton and Schad, 1996).

Prerectal intestine  

See Posterior intestine

Presynaptic density Presynaptic specialization (S)
Active zone (S)

An electron dense tuft or bar lying on the cytoplasmic surface of a presynaptic membrane at a synapse, usually bounded by a cluster of synaptic vesicles nearby (Jin, 2005).

Presynaptic terminal   The swelling of the axon in the locale of a synapse including the cluster of synaptic vesicles that surround the electron dense membrane specializations of the presynaptic region (Jin, 2005).
Pretzel stage Four-fold stage (S)

This comprises the final two stages in embryogenesis, “early pretzel” stage and “late pretzel” stage, just prior to hatching. The animal now is folded into four lengths within the eggshell and resembles a pretzel. By early pretzel stage, the head begins to move independently from the body.

Primary ectoderm  

The cells and tissues that form from the AB lineage, principally comprised of anterior hypodermis, neurons and some pharyngeal ectoderm.

See Secondary ectoderm

Primary interneuron  

An interneuron that receives direct synaptic input from sensory neurons. It may in turn direct its own synaptic output to secondary interneurons and/or to command interneurons.

See Command interneuron
See Secondary interneuron

Primary mesoderm  

The cells and tissues that form from the MS lineage, consisting of most bodywall muscles and the muscles of the pharynx.

See Secondary mesoderm

Primary oocyte 1o oocyte (S)

The most mature oocyte within the gonad sheath, lying next to the spermatheca, which awaits fertilization.

Primary spermatocyte  

A blast cell that divides to give rise to four mature sperm and a residual body.


Step of synaptic vesicle cycle that occurs after docking of the vesicle that renders it competent to fuse with the plasma membrane and release its contents (for more detail see Richmond, 2005).

Primordium/ Primordia  

An embryonic or larval tissue, usually nonfunctional, which will later develop further through cell divisions and/or morphogenesis to form a functional adult organ.

Primordial germ cell  

Those early germ cells lying closest to the distal tip cell within the gonad primordium or the distal gonad arm that remain in mitosis to generate the lineages of the sperm and oocytes, the direct descendants of Z2 and Z3. They all appear to be syncytial with one another from mid L2 onwards (Pepper et al., 2003).

Process Axon (S)
Dendrite (S)
Neurite (S)


Process bundle Nerve cord (S)
Fascicle (S)


Process extension Process outgrowth (S)



The most anterior region of the pharynx, lying just before the 1st bulb. This portion of the pharynx is long, thin and muscular. 

See Corpus
Proctodeum/ Proctodaeum  

The most posterior portions of the alimentary system. In C. elegans, the term is almost exclusively used for the male anatomy, rather than the hermaphrodite, and includes the intestinal valve cells, gland cells, all epithelial cells lining the cloacal sinus and all spicule-related epithelial cells. However, the vas deferens cells, intestinal cells and bursa cells are not included.

See Cloaca
See Rectum

Prod Prodding (S)

An intermediate step in male mating behavior during which the male repeatedly extends his spicules towards the hermaphrodite vulva in an attempt to insert them. Once he meets lowered resistance, spicules are fully extended and sperm transfer begins (Barr and Garcia, 2006). Prodding appears to be rhythmic and fast, perhaps as fast as 7-9 Hertz (Emmons in Hart, 2005).

See Insertion
Progeny Offspring (S)

The set of individuals produced by an adult animal are its progeny. Since the nematode is self-fertilizing, progeny can also be subdivided into self-progeny (produced by internal self fertilization) vs cross-progeny (produced by sexual mating between a male and hermaphrodite). Members of succeeding generations (F1, F2, etc.) from one parent animal might also be included among its progeny.

Programmed cell death Apoptosis (S)

A common form of cell death which appears to originate as a decision within the nucleus to initiate a cascade of cellular events inside both the nucleoplasm and the cytoplasm resulting in chromatin clumping, nuclear breakdown, cell shrinkage and eventual death. The dead cell is then engulfed and removed by a neighboring cell. The molecular components required for engulfment have been identified in C. elegans. Unlike necrotic cell death, apoptosis does not generally spread to neighboring cells. The genetic pathway controlling this process has been identified in C. elegans (Conradt and Xue, 2005). During development of the worm, specific cells are generated that then undergo apoptosis.

See Apoptosis
See Necrosis
See Engulfment
See Murder
See Suicide
See Undead cells


Specialized cuticle lining the prostom, formed by the arcade cells.


An increase in overall number of an object, cell type, or animal due to divisions (splitting and growth, mitosis) or new births (reproduction).

Proliferation phase  

The first half of embryogenesis, a six hour period during which the single cell zygote rapidly undergoes many rounds of synchronous cell divisions to produce an embryo containing about 550 cells at the end of gastrulation. At this time, most cells are still virtually undifferentiated. The proliferation phase is followed by a 5 hour “morphogenesis phase” during which cells exit the cell cycle and begin to undergo tissue development.

The same term may also be used in reference to the pattern of cell divisions occurring in the distal germline to produce early gametes, or also to early stages in the development of the male tail’s sexual features in the L3 larva, when many new neurons, muscles and support cells are born, prior to differentiating in late L4 stage.


A lineage pattern in which a stem cell continues to divide repeatedly (and symmetrically) to produce a large number of (equivalent) daughters, totaling more cells than usual, perhaps more than the organism actually will utilize in this class (Sulston and Horvitz, 1981).

See Reiterative

A subdivision of M phase during which the nuclear envelope breaks down into vesicles and the developing mitotic spindle microtubules move from the cytoplasm into the nuclear region and become associated with the chromosomes.

Pronuclear envelope  

A lipid bilayer, the equivalent of the nuclear envelope, that surrounds each pronucleus within gametes and within the early zygote after pronuclear reformation (Gonczy et al., 1999; Gonczy and Rose, 2005).

See Nuclear envelope

Pronuclear fusion  

An event during zygote formation after the migration and rotation of the two (haploid) pronuclei within the one-cell embryo in which they join together to create a diploid nucleus. The act of touching between the pronuclei is also called the “pronuclear meeting” (Cuenca, 2003).

Pronuclear migration  

An event during zygote formation after pronuclear reformation in which the two pronuclei are translocated along the anterior/posterior axis so that they meet near the posterior pole of the one-cell embryo (Krieg et al., 1978; Hird and White, 1993). The maternal pronucleus travels farther, passing through the pseudocleavage constriction to enter the posterior portion of the embryo and meet the male pronucleus; they rotate around one another just prior to fusion. For more detail, see Oegema and Hyman (2006).

Pronuclear reformation

Reconstitution of pronuclei (S)

Soon after the fertilization event, the (haploid) pronuclei of the egg and sperm seem to disappear for about 30 minutes. Egg cell cytoplasm is rearranged by cytoplasmic streaming (Nigon et al., 1960) after which the two pronuclei reappear, now positioned on opposite poles (one anterior and one posterior), an event called “reformation”. These pronuclei will then migrate towards one another, rotate and fuse prior to form the first diploid nucleus of the zygote (Krieg et al., 1978).

Pronuclear rotation  

An event during zygote formation during which the two pronuclei migrate into close apposition in the posterior compartment of the single cell embryo, then move 90o past each other along the left/right axis so that they lie virtually on top of one another prior to fusion. Then they fuse and create a diploid nucleus (Deppe et al., 1978; Albertson, 1984).

Pronuclear stage  

The earliest events in zygote formation, a thirty minute period during which the pronuclei disappear, reform, migrate, rotate and fuse with one another to create a diploid nucleus.


The haploid nucleus of a gamete at the time of fertilization. The pronuclei of the oocyte and sperm, disappear, reform, migrate towards one another, rotate versus each other and fuse to create the first diploid nucleus of the embryo prior to first mitosis (Allberston, 1984Oegema and Hyman, 2006).


A stage in cell division, the transition between G2 phase and M phase in the cell cycle, during which cytoskeletal proteins in the cytoplasm begin to rearrange to form a mitotic spindle, while the chromosomes condense pairwise into sister chromatids within the nucleus.  These features are more easily visualized by light microscopy techniques than by TEM.

Proprioception/ Proprioceptor Kinesthesia (S)

Sense of one's own body that leads to the orientation and coordination of movement. In C. elegans, proper proprioception generates a sinusoidal pattern of movement. This type of locomotion requires contraction of the dorsal and ventral body muscles in an alternating pattern, probably driven by several categories of non-specialized neuron processes (Voglis and Tavernarakis, 2005). It is not known for certain how this sinusoidal wave is propagated, but it may be via the action of "stretch receptors". The “light touch response” of the touch neurons (ALM, PLM, AVM, PVM) could be reasonably compared to stretch sensitive, or proprioceptive, activation. Stretch-sensitive processes have also been imputed among certain neurons in the male tail (Sulston et al., 1984) where the processes may sense the status of spicule eversion or the positions of other mating structures.

In the pharynx, neuronal endings attached by adherens junctions or hemidesmosomes to the cuticle or muscle epithelium have been termed proprioceptive by Albertson and Thomson (1976) and Ward et al. (1975) suggested that some receptors of the lips could be proprioceptive by analogy to the structure of campaniform sensilla in insects, which are believed to detect physical stressing of the integument. It is also possible that some receptors may be responsive to both mechanical sensations produced by objects encountered in the local environment and from stretching of the cuticle, spicules, or buccal cavity during self-produced body motions.

Propylene phenoxitol Phenoxypropanol (S)

A compound commonly used as a reversible anaesthetic for C. elegans (Sulston and Horvitz, 1977).


An anterior portion of the buccal cavity, cylindrical in shape, in register with the arcade cells, just behind the lips. It is lined by arcade cell cuticle.

See Cheilostom
See Telostom

Protandrous/ Protandric reproductive system  

A system in which the germline produces sperm first, and then switches to producing oocytes. This trait describes C. elegans reproduction. Some Rhabditis species can resume making sperm at repeated intervals, interspersed with intervals of oocyte production (Nicholas, 1975).


A large multimolecular complex, residing the in the cell cytoplasm, which is responsible for the breakdown of ubiquinated proteins into small peptide fragments.


The complete set of proteins in a given organism at a given time. The term could also be applied to the collection of proteins found in a particular cell type under specific conditions. For C. elegans protein mining, researchers can use Wormbase.

See Genome
See ORFeome


Term for the short specialization extending from the young sensory neuron cell body to the bodywall. It attaches the early cilium to the cuticle, prior to involution of the cell body and extension of the dendrite, by passive stretching between the cuticle and the receding cell body.


The individual molecular subunits which make up a microtubule. Microtubules can differ in their inherent diameters depending upon how many protofilaments surround the tubule. Under special conditions, protofilaments can be visualized by TEM as they split from the microtubule (at “minus ends”), or can be grouped into sheet-like structures in growing (“plus end”) microtubules (O’Toole et al., 2003).

Protractor muscle Spicule protractors (S)

The two protractor muscles (one dorsal dspL/R and one ventral vspL/R) are attached to the base of each spicule. Contraction of the muscle causes the spicules to extrude from the body. They are directly innervated by SPCL/R neurons.


An object lying relatively close to the center or origin of a structure or reference point (in anatomy this usually is the main trunk of the body). The converse of distal, meaning an object lying farther away.

Proximal arm  

That portion of the hermaphrodite gonad lying closer to the vulva and uterus; usually referring to that portion lying between the vulva and the “loop”. The portion lying farther away from the vulva and uterus, past the loop, is known as the distal arm.

See Distal arm

Proximal epithelium  

A category of epithelial tissue within the uterus, involving those portions proximal to the vulva (e.g. the dorsal epithelium, du, and the ventral epithelium, uv2 and uv3) (Newman et al., 1996).


A distinct period preceding the complete division of the one-cell embryo during which directed flow of cortical cell components cause rearrangements which prefigure an asymmetry to the first cleavage (Hird and White, 1993). These changes include polarized flow of cytoplasm, pronuclear migration, and reorganization of cortical actin networks just under the plasma membrane, perhaps organized by the orientation of the mitotic spindle and a contractile ring during cytokinesis.

Pseudocleavage furrow Pseudocleavage constriction (S)

A belt-like region of indentation causing a noticeable narrowing or constriction of the one-cell embryo along the midline, about halfway between the anterior and posterior poles which closely resembles the cleavage furrow that would occur after a nuclear mitosis as a step in cell division. While this furrow disappears after fusion of the pronuclei, a few minutes later the new diploid nucleus does undergo mitosis and a new cleavage furrow is generated in virtually the same place. The cell then divides the two subsequent cell daughters (Albertson, 1984; Hird and White, 1993).

Pseudocoelom Pseudocoelomic cavity (S)
Body cavity (S)
Pseudocoel (S)
Peri-enteric fluid (S)

A fluid-filled space which separates the bodywall from internal organs. Within this space the digestive tract and reproductive tract lie separately, each enclosed by its own basal lamina. All tissues have their basal side turned towards this cavity and are covered there by basement membranes. Intercellular signals, nutrients and waste products can travel between tissues within this space. Pseudocoelom fluids are prevented from leakage by barrier junctions in the surrounding epithelium.

See Blastocoel
See Coelomic cavity

Pseudocoelomic fluid  

The contents of the cavity; it may contain hypertonic salt solutions to maintain the animal’s turgor pressure, and to provide a medium through which signals and waste products can flow.

Pseudocoelomate Aschelminthes (S)

A classification of primitive animals characterized by the presence of the pseudocoelom. This includes roundworms, rotifers, gastrotrichs, and others. 

Pseudocoelomic membrane  

A spongy structure bounding the pseudocoelomic cavity. In C. elegans it consists of basal laminae of the major tissues, where as in other nematode species it may contain some cell processes, fibrils, and tubular systems (Bird and Bird, 1991).


See Coelomocyte

Pseudogamy/ Pseudogamous  

A form of parthenogenesis in which sperm is necessary to activate egg development, but where the male chromosomes are discarded, with the sperm pronucleus degenerating without fusing to the female pronucleus. This condition can occur in species that are hermaphroditic or bisexual (Nicholas, 1975; Anya, 1976). It is not known to occur in C. elegans, but has been reported for Rhabditis aberrans, another hermaphroditic species (Kruger, 1913).


A mutant intersex phenotype in which an animal exhibits some or all of the male tissues (male gonad, male copulatory structures) while carrying the XX chromosomes. Some fail to copulate successfully with hermaphrodites or females due to lack of normal mating behavior (Klass et al., 1976), but others are capable of producing cross-progeny (Hodgkin and Brenner, 1977), or may be fertile hermaphrodites despite having secondary male features (Herman and Horvitz, 1980).

See Intersex

Pseudopodium/ Pseudopod/ Pseudopodia  

An elongated cell extension, more narrow than a lamellipodium, but broader than a filopodium, which may be extended in advance of a migrating cell or cell process. They are present as leading extensions on spermatozoa, developing neuronal processes, migrating hypodermal processes and somatic sheath cells. Migrating cells displaying trailing pseudopodia include the distal tip cell of the gonad.

See Filopodium/ Filopodia
See Growth cone
See Lamellipodium/ Lamellipod/ Lamellipodia

Pseudopod elongation  

A required component of pseudopod motility, apparently stemming from membrane flow within the pseudopod (Ward et al., 1981; Achanzar and Ward, 1997). The spermatozoon moves by amoeboid action of the pseudopod, extending membrane ruffles and short cell extensions to guide the cell forward, somewhat like the movement of a neuron growth cone.

Pseudopod initiation  

An early step in the creation of the pseudopod in a maturing spermatozoon, apparently due to the fusion of membranous organelles with the plasma membrane to contribute membrane and fibrous material to the growing pseudopod (Ward et al., 1981; Achanzar and Ward, 1997).


An aberrant multicellular structure caused by excessive inductive signals acting upon the VPC blast cells to create an extra (nonfunctional) vulval specialization by inducing extra cell divisions to create extra pi blast cells and their progeny. These can generally been seen by light microscopy as one or more small protrusions along the ventral surface of the adult animal, flanking the location of the true vulva.

See Multivulva phenotype


See Pharyngeal pumping


Staining patterns seen by light microscopy in which the stain highlights discrete spots (puncta) as opposed to diffuse staining. This term is often used with respect to staining for synaptic proteins, in which labeling for either pre-synaptic proteins or post-synaptic proteins tends to be punctate, but also can refer to repeating patterns such as those in seen muscle cells. The “punctate” object should be compact but may be irregular in shape.

Punctum/ Puncta Dot (S)

Individual structure(s) that have small compact shapes, such as synaptic swellings along an axon. The term implies the cohesive compact shape less than the exact size, since the punctate shape of one signal may overlap that of another punctate signal of different size, referring to separate components of a single larger element (Hallam et al., 2002). In a mutant background, the term is sometimes still used to refer to objects that are more compact in wild type, but have become distorted and diffuse due to mutation.

See Synaptic terminal

Edited for the web by Laura A. Herndon. Last revision: February 17, 2010. This section should be cited as: Herndon, L.A. and Hall, D.H. 2010. Glossary P. In WormAtlas.  doi:10.3908/wormatlas.6.16