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Glossary - M




M cell

Sex myoblast (S)

A postembryonic mesoblast that lies in the posterior body region and gives rise to sexually dimorphic muscles in males and hermaphrodites. A descendant of MS.ap, M is born in an anterior position on the left side of the embryo and then migrates to the posterior and right side. In the hermaphrodite, the M lineage gives rise to six cell types: body wall muscles, nonmuscle coelomocytes and four classes of sex muscles including those of the uterus and vulva. In males, M descendants give rise to cells in the tail including some bodywall muscles and to sex muscles of the tail (Herman, 2006).

See Sex myoblast

M line


A planar structure within the muscle sarcomere which lies midway within the thick filament (A) band, attaching to the plasma membrane at its base and running up towards the inward surface of the myofilament lattice. The plane of the M line runs parallel to the axis of the A band, slightly off axis from the longitudinal body axis. It may organize the orientation of the myosin lattice and perhaps the stagger between adjacent sarcomeres (Waterston et al., 1986).

"m" neuron




Sensory head neurons with ciliated endings, but not part of a sensillum and are not open to the outside. Functions with 2 other classes of mechanosensory neurons to mediate nose touch avoidance and initiate backwards movement.

M phase Mitotic phase (S)

A stage in the cell cycle during which mitosis occurs. It is recognized best by light microscopy rather than by TEM methods. (For overview of cell division in C. elegans see Oegma and Hyman, 2006).

M1, M2, M3, M4, M5  

These are the names of individual pharyngeal motor neurons (Albertson and Thomson, 1976).

m1-m8 (pm1-pm8)   Pharyngeal muscle cells, which make 8 consecutive rings of radial musculature encircling the pharyngeal lumen. Most are arranged in a three-fold symmetrical manner. All except pm6-8 are syncytial; pm1 has 6 nuclei while each pm2-5 cell has 2 nuclei, as a result of cell fusions (Albertson and Thomson, 1976; Avery and Thomas, 1997).
m8 muscle cell (pm8)  

A single muscle cell lying at the posterior end of the pharynx which completely encircles the pharyngeal lumen. m8 may act as a sphincter to control the lumenal opening, or possibly the pharyngeal-intestinal valve just behind it, to restrict regurgitation from the intestine (Albertson and Thomson, 1976).

Main body syncytium  

Refers to the hyp7 syncytium, the largest component of the hypodermis. This tissue eventually contains 110 nuclei and encloses the major portion of the animal’s body length in a circumferential toroid that includes the ventral and dorsal hypodermal ridges, as well as most of the lateral hypodermal cords over that region.

Major sperm protein MSP  

The dynamic polymerization of MSP drives C. elegans sperm motility via "treadmilling"(Smith, 2006).


A rare (1/2000) sexual form in normal Caenorhabditis elegans whose body undergoes a dramatic sexual transformation during development, especially of the tail, in order to breed sexually with the hermaphrodite (Emmons, 2005). Unlike the hermaphrodite, the male’s reproductive tract forms only

Tetraploid adults can generate two classes of hermaphroditic progeny: “high frequency male producers” which yield much higher fractions of male progeny, and “low frequency producers” which yield relatively few males (Nigon, 1949; 1951).

See Hermaphrodite
See High incidence of males
See Intersex
See Spontaneous male

Male mating behavior     Males go through specific behaviors to initiate and undergo mating with hermaphrodites (see mating movie). These behaviors include: responding to contact, turning, vulval location, spicule insertion and sperm transfer (Barr and Garcia, 2006).
Male specific  

Cells, tissues, or functions that are found only in the male animal but not in the hermaphrodite.

Male stock


A culture of C. elegans in which male animals are produced at relatively high frequency by repeated backcrossing of males into a relatively small number of hermaphrodites. Since hermaphrodites can self-propogate and produce virtually all hermaphrodite progeny when not mated, it is difficult to maintain a culture with a high proportion of male progeny except by this backcrossing method, or by choosing a him mutant strain to increase the rate of male production.

See Spontaneous male

Male tail   The male tail contains both male-specific and sexually dimorphic structures that allow the male to initiate mating with the hermaphrodite and to transfer sperm into her vulva.
Mantle   A specialized extracellular matrix surrounding the touch receptor neurons. Its production appears to require the touch cell processes to be in close apposition of to the hypodermis (Driscoll and Kaplan, 1997).
Marginal cell Marginal fiber (archaic)  

Transitional epithelial cells of the pharynx that separate each of the three syncytial pharyngeal muscle cells from each other. There are a total of 7 cells: 3 mc1, 3 mc2 and a syncytial mc3 cell with 3 nuclei. These cells are probably non-contractile, but are filled with radially oriented intermediate filament bundles. They lie in rows at the corners of the lumen and supply reinforcing strength to the organ.


Transformation of cell fates to produce male cell fates in place of female cell fates. A masculinizing mutation may transform the ratio of germ cell fates to produce more sperm and fewer oocytes. Transformation of somatic cell fates may produce body parts in a genetic hermaphrodite to form a male tail, cloaca or vas deferens.

See Feminization

Mate-finding Mate searching (S)  

Behaviors that cause sexually adult animals to be attracted and to approach members of the opposite sex. In C. elegans, such behaviors include the “leaving behavior” in males, in which they depart from a food source to find a suitable hermaphrodite (Simon and Sternberg, 2002; Lipton and Emmons, 2003; Lipton et al, 2004). There is apparently no detectable preference among fertile hermaphrodites to aggregate in the presence of adult males (Simon and Sternberg, 2002).

See Leaving behavior
See Pheromone

Maternal effect Maternal expression (S)  

Expression of maternal effect genes is required in the mother for normal development of her offspring.

See mes

Maternal effect lethal Mel (S)

A special class of sterilizing mutations that prevent the development of the progeny of hermaphrodites homozygous for the mutation. In contrast to the zygotic lethals, which prevent the development of the individuals homozygous for the mutation, the gene product from maternal effect lethals is required in the mother for the successful completion of the development of her offspring (Kemphues, 2005). After mating with a wild type male, progeny are generally viable (rescued) and develop normally as heterozygotes.

Mating behavior

To attract males for mating, hermaphrodites provide chemosensory and mechanosensory cues (Barr and Garcia, 2006). Males respond to these cues and undergo a complex behavior pattern to successfully copulate with the hermaphrodite (see mating movie). This behavior includes: response, turning, vulva location, spicule insertion, ejaculation and plugging (Barr and Garcia, 2006). Hermaphrodites also exhibit behaviors that resist his efforts at effective mating (Emmons, 2005).

See Male mating behavior
See Mating resistance

Mating efficiency  

A measurement of the mating ability of a male adult, in particular the animal’s fertility as opposed to its mating behavior per se (Hodgkin, 1983; Hodgkin et al., 1988). Assays for male mating efficiency are used to determine the ability of mutant males to be used in genetic crosses (Emmons, 2005).

Mating plug


See Copulatory plug

Mating resistance

Hermaphrodites have been shown to exhibit two behaviors associated that reduce the likelihood of effective mating with a male: sprinting (hermaphrodites move forward quickly away from a male after being touched by him) and sperm expulsion (after male ejaculation, contractions of the hermaphrodites uterus may expel the seminal fluid through the vulva) (Emmons, 2005).

See Sperm expulsion
See Sprinting

Matrix Ground substance (S)  

Flocculent or lacey dense substance which fills vesicles or regions of space inside the animal; may be intracellular or extracellular.

Also refers to the large internal space of the mitochondrion which contains a mixture of hundreds of enzymes that catalyze the oxidation of pyruvate and other small organic molecules, copies of the mitochondrial DNA genome, special mitochondrial ribosomes and tRNAs (Alberts et al., 2002).

See Extracellular matrix

Matrix layer Intermediate layer (S)  

A portion of the medial layer of the cuticle.

See Basal layer
See Cortical layer


The killing of an adult mother by her progeny. This is a common endpoint in cases of internal hatching in C. elegans, either due to poor success at egg-laying (since progeny are produced internally by self-fertilization), or due to mutations that block normal development of the vulval opening (egl phenotype).  Theoretically, there may also be cases in which the progeny produce a toxic agent that proves fatal to the mother (Mylonakis et al., 2002).

See Bag of worms
See Egl


Male Cell of Mystery MCML

Male-specific interneuron born in L4 stage, daughter of the AMso cell, undergoes transdifferentiation from a glial to neuron fate. Its cell body lies among the glial cell bodies in the male head, and has synaptic connections in the adult male nerve ring; required for a form of associative sexual conditioning in the adult male
Mechanoreceptor neurons



These neurons generate electrical signals in response to mechanical stimuli and transmit those signals to other neurons. One group, the touch receptor neurons (ALML/R, PLML/R, AVM, and PVM), are nonciliated, while the ciliated MRNs include ASH, CEP, ADE and PDE, as well as 42 male-specific sensory neurons that innervate the male tail (Goodman, 2006).




Detection of mechanical stimuli to the body. C. elegans sense a variety of mechanical stimuli including: gentle touch, harsh touch, nose touch and texture (Chalfie, 2006).

See Nose touch

Mechanosensory ending

Mechanocilium (S)
Mechanoreceptor (S)


A rather wide variety of neuronal specializations have been ascribed a sensory role which is imputed to be mechanosensory, but rather few have proven physiological responses. Most of these endings involve morphological specializations embedded in or immediately under the cuticle, sometimes showing physical attachment via hemidesmosomes or adherens junctions. Several involve endings embedded in the hypodermis of a sensillum. Many neurons project long morphologically unspecialized processes in longitudinal nerves which are suspected to allow for stretch sensation (see proprioception), but where no obvious physical attachments are noted even by electron microscopy. The dendrites of the touch neurons, which transduce light touch, are not really “endings” in the conventional sense as their major processes extend longitudinally for many microns, glued to the cuticle and hypodermis via an external mantle.

See Nose touch

Mechanosensory nerve    

Cephalic nerve cord + labial nerve, which travel as a bundle from the lip to nerve ring cell bodies.


The biophysical process by which a mechanosensory ending converts a mechanical stimulus into a neuronal signal. Most of the proteins thought to form the transduction channels in ciliated MRNs are members of the transient receptor potential (TRP) channel superfamily. Members of the DEG/ENaC superfamily have been found to compose channels that operate in nonciliated MRNs (Goodman, 2006; Driscoll and Kaplan, 1997).

Median zone

Medial layer (S)
Mesocuticle (S)

An internal compartment of the cuticle that may show substantial variations between developmental stages, or between species. This layer may be entirely missing in larval cuticles, but is very prominent in adults. It may include several distinct layers, including the fibril layer, the matrix layer and a thin boundary zone, which separates it from the underlying basal zone of the cuticle. This region can display struts, globular bodies and fluid-filled spaces.

See Basal layer
See Boundary zone
See Cortical layer
See Fibril layer
See Matrix layer
See Strut


The collection of animals that are too small to be easily seen by eye (those larger species comprise the “macrofauna”), but are larger than the species that can only be observed with a microscope (the “microfauna”). They can pass through a .5 mm sieve, but are retained on a .045 mm sieve. C. elegans, with an adult body length of one micron, is just barely visible to the eye, and belongs among the meiofauna. Nematodes generally are thought to comprise 70-100% of the meiofauna biomass in most habitats, and their total metabolism may equal 15% of that of the macrofauna (Nicholas, 1975).

Meiosis M phase (S)  

A characteristic series of steps (constituent parts of meiosis) through which dividing germ cell nuclei mature into fertilization-competent haploid oocytes or sperm; these include meiosis prophase I (which itself includes 5 steps), completion of meiotic division I (meiotic metaphase I and anaphase I), and meiotic division II (meiotic metaphase II and anaphase II), and telophase (Alberts et al., 2002). In C. elegans the processes of meiotic maturation, ovulation and fertilization are all temporally coupled (Greenstein, 2005). The meiotic divisions of C. elegans spermatogenesis are coordinated with the morphogenesis of fibrous body-membranous organelles which segregate into spermatids (L'Hernault, 2006).



An enclosing or separating tissue, usually a lipid bilayer that acts as a boundary.

See Basal lamina
See Basement membrane
See Nuclear envelope
See Plasma membrane
See Stacked membrane

Membranous organelle


Special membrane structure (S)

A very distinctive organelle found in the periphery of the cytoplasm of the maturing spermatid and spermatozoan. They contain very electron dense stacked membrane structures that can eventually fuse with the plasma membrane of the sperm, perhaps leading to formation of the pseudopod. The membranous organelle (MO) has also been called the “special membrane structure” by Wolf et al. (1978) and may be formed from pre-existing “special vesicles” and fibrous bodies, which disappear as the MO’s develop. The organelle consists of two lobes separated by an electron dense collar; the larger lobe is sometimes called the “body lobe”, while the smaller lobe is the “head lobe” (Nelson and Ward, 1980; Achanzar and Ward, 1997). The MO develops in close association with the fibrous body (FB), which is composed of the major sperm protein filaments, and surrounds it, separated by a collar region (L'Hernault, 2006). The membranous organelle eventually fuses with the plasma membrane and contributes to the creation of the motile pseudopod of the activated spermatozoon, providing both membrane and glycoproteins (Roberts et al., 1986). Fibrous material originating within the MO passes through the fusion pore and provides components to the outside surface of the sperm’s plasma membr ane (Achanzar and Ward, 1997).

See Collar
See Fibrous body



C. elegans has demonstrated the ability to learn from mechanosensory, chemosensory and thermosensory input. They exhibit habituation as well as associative forms of learning and show both short-term and long-term memory. Additionally the appear to be able to integrate and remember experiences across different sensory modalities (See review by Rankin, 2004).

See Adaptation
See Habituation
See Learning and Memory
See Imprinting

Meridic medium  

An axenic medium containing one component that is not well-characterized, such as a tissue extract.

See Holidic medium


A condition in which a nematode muscle quadrant contains relatively few cells (ranging from 2-5 cells across, as in C. elegans); as opposed to “polymyarian” muscles (in some very large species such as Ascaris) where there are many cells side by side within a quadrant (Schneider, 1866; Chitwood and Chitwood, 1950). In “holomyarian” muscles, there are never more than 2 cells across within each quadrant.

See Holomyarian

Mes Maternal effect sterile

Mutations in these genes causes a sterile effect due to loss of mature germ cells implying that the mes genes are essential for germline proliferation and/or maturation (Capowski et al., 1991; Paulsen et al., 1995). Some mes mutants appear to contain necrotic germ cells (Garvin et al., 1998).

Mesenteron Intestine (archaic)

See Intestine



Blast cells (S)

Precursor, or stem cell, of the muscle cells and coelomocytes. In C. elegans, non gonadal mesoderm arises from a single postembryonic mesoblast cell (M). M is born on the left, next to the pharynx. It migrates posteriorly, remains on the midline for some time, but then shifts to the right-hand side of the intestine (Sulston et al., 1983).

See M cell


Medial layer (S)

See Median zone


In higher animals, this term refers to the cell layer from which the musculature develops. In C. elegans, this tissue consists principally of the daughters of just two cells in the early embryo, the MS (primary mesoderm) and D (secondary mesoderm) blast cells. The term is used to collectively refer to both these blast cells and to all developed musculature in a given animal of any age.

See M cell


Specialized zone of cuticle lining the mesostom or buccal regions, secreted by the pharyngeal epithelium.



A portion of the buccal cavity underlain by the buccal epithelial cells.


Median bulb (S)
First bulb (S)


The anterior bulb of the pharynx.




A type of stretch receptor, known for some nematode species, but not C. elegans (Lorenzen, 1994).

Metarhabdion Glottoid apparatus (S)  

Specialized zone of cuticle lining the metastom, a portion of the buccal passage formed by the pm1 muscles.  In particular, this cuticle includes the flaps that may govern flow in/out of the pharynx proper.

Metastom Glottis (S)  

Archaic term referring to that portion of the pharynx formed by the pharyngeal muscles pm1.

Micro-adherens junctions    

Extremely small transient junctions that have been proposed to occur between hypodermal filapodia in advance of dorsal/ventral cell fusions in the embryo (Podbilewicz, 2000).


An extremely thin elongated cytoplasmic filament, composed of actin protein, which can assemble into any of several distinct forms, including a gel-like meshwork (g-actin), a tight bundle of parallel fibers (larger bundles are often called stress fibers, smaller bundles are often called f-actin), or can be interspersed with other filamentous proteins (e.g. myosin) to form a myofilament lattice, as in muscle cells. These are each important “cytoskeletal proteins” which physically hold the cell’s organelles to one another internally, and which connect via plasma membrane attachments (intercellular junctions) to hold cells together into a coherent tissue. The actomyosin (actin + myosin) lattice is especially important in establishing asymmetric distribution of proteins in one-cell stage embryos (Gonczy and Rose, 2005) and in the force generation in all muscles.

When cytoskeletal proteins are compared in cross section, microfilaments < intermediate filaments < thick filaments < microtubules.

See Actin


Due to its transparency and easy of manipulation, C. elegans is an ideal organism for microscopic observaton of both live and fixed animals or tissues.

See Confocal microscopy
See DIC microscopy
See Electron microscopy
See Four dimensional microscopy
See Multiphoton fluorescence microscopy


A robust macromolecular array which forms long extended cytoskeletal elements with a hollow core. The tubule is made principally of tubulin protein, which forms parallel strings of “protofilaments” along the sides of the tubule. Individual species of tubulin proteins generally form homomeric macromolecular arrays such that each tubule consists of an odd number of protofilaments. In C. elegans, 11, 13, or 15 protofilament microtubules are most common. Microtubules can rapidly change in length, except when stabilized by capping proteins, and can assemble into arrays with one another to form microtubule bundles (as inside the nerve axon) or flexible asters (as in the mitotic spindle). Microtubule-based motors (such as kinesins and dyneins) can slide microtubules past one another to generate considerable forces in moving o rganelles within the cell cytoplasm, or can move cargoes within the

When cytoskeletal proteins are compared in cross section, thin filaments < intermediate filaments < thick filaments < microtubules.

See Tubulin

Microtubule ring

See Ciliary necklace

Microvillus/ Microvilli/ Microvillar


Open tubular extension at the apical surface of some epithelial cells.

1) Found in intestinal cells and some neighboring cells in the alimentary system. Dense collections of microvilli are bounded basally by a terminal web and together constitute a “brush border”.

2) Similar microvilli have been seen in the immature spermatheca prior to adulthood (Southgate and White, unpublished data).

3) Some researchers consider the finger-like extensions of the AFD dendrite to be comparable to microvilli (Ashton and Schad, 1996).

See Brush border
See Finger-like projections

Midbody Central body (S)
Midsection (S)

The body region which is behind the head and neck but in front of the tail, where the tissues are comprised of the bodywall, intestine and gonad tissues.



The middle region of the intestine, lying between the anterior and posterior regions. It consists of a rather simple tubular epithelium that becomes pushed to the dorsal side of the pseudocoelomic cavity by the vulva.




The midline defines a plane running along the dorsal/ventral axis, separating the left and right halves of the body. Most features in the bodywall are symmetric across the midline, except for the major nerve cords. Most internal organs in the adult are not symmetric across the axis. The ventral midline is an important landmark in axon guidance; e.g. “midline crossing” or decussation.

See Ventral midline

Midline crossing



See Decussation


Longitudinal aspect (S)


This term has been used in many different contexts to describe directed movements of individual cells or groups of cells within the body relative to neighboring cells within a tissue. It also refers to movements of cell parts (process extension, growth cone motions), organelle movements (such as nuclei, centrosomes, etc) and relative movements of molecular assemblies within a cell.

The term could also be used to describe directed motions of whole animals or even populations of animals with respect to an environmental signal.

Mitochondrion/ Mitochondria  

Intracellular organelle that completes the metabolism of sugars and produces ATP providing the energy required for much of the cell's processes. Each mitochondrion is bounded by two specialized membranes (an outer and inner membrane) which create two separte compartments - the internal matrix and intermembrane space. They are found in large numbers in tissues that have a high energy requirement such as muscle and sperm cells.

Mixed modality Polymodal (S)

Neurons having more than one principle function within the wiring. For instance, several cells serve both a sensory and a motor function and are termed sensorimotor cells. Many more cells may serve as both sensory cells and as interneurons, or as motor cells and as interneurons, but are less easily categorized since the “interneuron” status is less easily defined.

The same term may also refer to neurons which transduce several different categories of sensation, such as olfaction and osmosensation or chemosensation, or chemosensation and mechanosensation.

Molt Moult (S)
Molting (S)
Ecdysis (S)

The end of each larval stage in C. elegans is marked with a molt where new, stage-specific cuticle is synthesized and the old one is shed (Cassada and Russell, 1975). Molting is almost simultaneous for the cuticular covering of all hypodermal tissues and those of the external openings. The buccal cavity, rectal cuticle and the coats of the excretory duct and sensilla are all replaced at each molt along with the outer cuticle of the bodywall. Before the molt, the animal enters a brief lethargus stage. Molting is

1) Apolysis: The separation of old cuticle from the hypodermis

2) Synthesis: the formation of new cuticle arising from the hypodermis, and

3) Ecdysis: t he shedding of the old cuticle

Cuticle protein synthesis has been found to be high during molting and is very much reduced during intermolt periods. Furthermore, the cuticle ultrastructure and protein composition differ at each molt (White, 1988).

The timing of the four molts is not regulated by any external signal in C. elegans, unlike in many parasitic species, though molting out of the dauer stage is highly sensitive to external conditions. Little is known how the molt cycle is driven internally as C. elegans lacks ecdysone and its receptor. Presumably, another sterol hormone functions in role (Antebi, 2006).

See Apolysis
See Ecdysis
See Exsheathment
See Lethargus

Molting fluid



A secretion from internal stores that may supply enzymes required for molting, thought to aid in the digestion of the old exoskeleton. This fluid may come from the excretory glands (Davey, 1971; Rogers and Head, 1978) and/or pharyngeal glands (Hall and Hedgecock, 1991; Adams et al., 1996). Release of the molting fluid has been postulated to follow reception of an external signal (at least in parasitic nematodes) and release of a hormonal intermediate from neurosecretory neurons.

See Exsheathment
See Molt

Monadic synapse



A synapse having only one postsynaptic process.

See Dyadic synapse
See Triadic synapse

Monocentric chromosomes    

The placement of the microtubules occurs at one specific region (the centromere) and chromosomes move toward the poles during anaphase with the centromere leading.

This is in contrast to holocentric chromosomes where microtubules bind to the chromosomes along their entire length. While C. elegans chromosomes have a holokinetic organization, they do share many features and behaviors with monocentric chromosomes (Albertson et al., 1997).

See Holocentric chromosomes


Possessing a one-armed gonad. Males always have monodelphic gonads, but females/hermaphrodites vary across species whether or not they are monodelphic or didelphic (two-armed) as is the case for C. elegans hermaphrodites (Sommer, 2005).

See Amphidelphic
See Didelphic


A neuron with a single process extending from the soma.

Monoxenic medium    

A simple growth medium for raising nematodes, containing a single well-characterized species (monoculture) of bacteria. A growth culture containing two mixed species would be termed a “dixenic” medium. 

See Axenic medium

An embryo or young animal which has undergone extremely abnormal development due to mutation or physical disruption is often called a (developmental) “monster”.  Such terminal phenotypes (arrest stages) are generally lethal and rarely result in fertile adults (Denich et al., 1984; Schierenberg and Junkersdorf, 1992; Schlicht and Schierenberg, 1991). 

See Arrest stage
See Embryonic arrest
See Terminal phenotype

The development of the shape of tissues, organs and organisms. Morphogenesis of the C. elegans embryo occurs during the last half of embryogenesis, a five hour period which follows the “proliferation phase”. Most cells exit the cell cycle and begin to differentiate in order to produce the detailed features of each tissue prior to hatching. This stage is mostly controlled by the development of the epidermis (Chisholm and Hardin, 2005).

Morula stage


  An early stage of embryonic development in which the embryo consists of compact ball of 12-32 cells. The cells begin to differentiate and then once a fluid filled space forms, called the blastocystic cavity, the morula becomes a blastocyst.
Mosaic development



A general means of tissue development (found prominently in nematodes, including C. elegans) in which individual cell lineages tend to operate independently of their neighbors, such that intercellular regulatory mechanisms are diminished or absent. The whole embryo becomes a patchwork or “mosaic” structure of cell groups that are operating in isolation from most outside influences. If certain stem cells are lost or damaged, no other cells can be upregulated to provide more cells or any regenerative capacity in replacement of the lost tissue. While mosaic development is very prominent in C. elegans embryos, significant examples of regulation have been experimentally determined (Sulston and White, 1980). 

See Regulation / Regulative potential

Mother centriole Parent centriole (S)   In a metaphase or anaphase spindle body, the “mother centriole” lies closest to most microtubule minus ends, and to the centrosome, whereas the “daughter centriole” lies off to the side (O’Toole et al., 2003).
Motile/ Motility  

The relative degree of spontaneous and independent movement of an animal, a cell, an organelle, or cell part (growth cone). Often it refers to the movement of the gamete (sperm), but C. elegans sperm move by a unique crawling mechanism.

Motorneuron Motoneuron (S)
Motor neuron (S)

A neuron which forms synapses (neuromuscular junctions) onto one or more muscle cells.

See Polymodal input
See Sensorimotor neuron

Motor end plate Muscle plate (S)

Mouth Buccal opening (S)

An opening at the anterior end surrounded by six symmetrical lips and sensilla.

MS blastomere

MS founder cell (S)
MSt blastomere (S)


An embryonic founder cell, born on the ventral side from EMS, whose daughter cells, E and MS, give rise to many cell types, most of which are mesodermal, including somatic muscles, glands, coelomocytes, somatic gonad precursors, as well as some neurons.

MSM Medial submedial neuron  

A distinct papillary organ, located slightly posterior in the lips and containing the dendrite of the OLQ neuron.

Multiphoton fluorescence microscopy MPFE

A type of microscopy that combines laser scanning microscopy with multiphoton fluorescence excitation to generate high resolution 3D images of fluorescent signals in live animals.

Multivulva phenotype Muv  

An aberrant condition in which an animal has more than one vulva, or more likely, one functional vulva and one or more extra “pseudovulvae” (see Sternberg, 2005). Many mutants are known to lead to this condition by the generation of excess cells from the Pn.p lineages; these include lin-1, lin-3 and lin-7 (Horvitz and Sulston, 1980; Sulston and Horvitz, 1981). 

See Pseudovulva
See Vulvaless phenotype


A postulated mechanism for the initiation of cell death, in which one cell dies as a result of an external signal from another "killer cell", or dies due to phagocytosis by a neighboring cell while still healthy (Sulston and White, 1980; White et al., 1991). 

See Engulfment
See Killer cell
See Suicide


Somatic cell(s) whose principal function is to provide contractile forces, either to modify body posture or to control the position or motions of particular tissues or organs. In the nematode, which lacks a skeleton, most muscles are anchored to the cuticle. The body’s high turgor pressure provides rigidity to the cuticle, which can then respond to muscle contractions to modify the body shape.

Some muscle cells in C. elegans act alone, but the principal muscles of the bodywall and pharynx operate in groups, either in synchrony, in sequence, or in opposition to provide coordinated, smoothly controlled motions. Individual muscle cells are often small in size, spindle shaped, and consist of single sarcomeres or obliquely striated sarcomeres (as in bodywall muscle).

Several cell types have smooth muscle properties, including the gonad sheath, the uterine muscles, and the anal sphincter muscle. Some muscle types are sex-specific, including the vulval and uterine muscles of the hermaphrodite, and the large variety of specialized muscles in the male tail.

See Bodywall muscle
See Diagonal muscle
See Dilator muscle
See Erector muscle
See Extensor muscle
See Myoepithelium
See Oblique muscle
See Pharyngeal muscle
See Retractor muscle
See Sphincter muscle
See Uterine muscle
See M line
See Z disc
See Actin
See Myosin
See Titin

Muscle arm Innervation process (S)

A process extension from the muscle belly by which the muscle cells reach to the nerve cords to obtain innervation. This extended muscle process is a common feature of practically all somatic muscles, but is not found in the pharynx, where neurons extend processes to the muscle cell directly. The typical bodywall muscle has 3 to 5 muscle arms.

See Muscle belly
See Muscle plate
See Plexus
See Spur

Muscle belly Sarcoplasm (S)

The cytoplasmic swelling surrounding the muscle cell nucleus, which contains many mitochondria and ribosomes, but excludes that portion of the cell devoted to the sarcomeres. Muscle cells are mostly spindle-shaped, with sarcomeres extending to the distal, thinner portions of the spindle, while the nucleus lies within the fatter muscle belly.

See Muscle arm

Muscle fiber  

That portion of the muscle quadrant that contains the myofilament lattice. This term is not often used in nematode literature. It might refer to the myofilament lattice within a since muscle cell, or to the collection of sarcomeres within a single muscle cell.

See Sarcomere

Muscle plate Motor end plate (S)
Plexus (S)

Specialized interdigitating ends of the muscle arms form a localized plexus along certain portions of the longitudinal nerve cords and nerve ring. This plexus, known as the “muscle plate”, is the site of muscle innervation by the motorneurons. This portion of each muscle arm tends to be very thin and electron lucent, lacking any cytoplasmic organelles. The chemical synapses formed by the motor axons are called neuromuscular junctions (NMJs).

Some gap junctions (GJs) may also occur here between nerve and muscle or between nearby muscle arms. The most distinctive muscle plates form just underneath the nerve ring (forming a complete cylinder), and opposite the principal motor neuron axons of the ventral and dorsal nerve cords (as lengthwise bands) (Ware et al., 1975).

See Gap junction
See Neuromuscular junction

Muscle quadrant  

Longitudinal bodywall muscles are grouped into four groups known as the muscle quadrants. Each quadrant comprises a double row of elongated muscle cells that lie very close together, covered by a common basal lamina which separates them from neighboring territories of hypodermis and nerves in the bodywall, and from the pseudocoelomic fluid more centrally.

Muscle stabilization  

Structures and/or developmental events by which the body muscles are anchored to the bodywall in order to provide strong resistance to the forces of muscle contraction, so that muscle contraction causes motion of the whole body rather than damage to the sarcomere.

Mutagenesis/ Mutagenized  

A procedure used to increase the rate of genetic mutations within a population of animals. In C. elegans, common mutagenic agents include ethylmethane sulfonate (EMS), X-rays, and transposon (Tc1) insertion.

Mutagenesis has been used to create large libraries of nematodes harboring deletions in their DNA. Screening these libraries has resulted in the isolation of lines carrying deletions in specific genes. Deletion strains produced by The C. elegans Knockout Consortium are freely available to researchers upon request.

See RNAi


Describes a lifestyle in which two species live together, to the benefit of both organisms. In many cases of mutualism, the relationship is obligate, and neither species can thrive apart from the other. If only one species derives obvious benefits, it is generally considered commensalism. It has been suggested that C. elegans may have a commensal relationship with pillbugs in the wild, living under the shell of the pillbug and perhaps consuming bacteria there (Baird et al., 1994).

See Commensal
See Free-living
See Parasite


Stem cell that divides to produce muscle cells.

Myoepithelial/ Myoepithelium  

Describing any tissue that shares properties of both muscle and of epithelial tissue. Some single sarcomere muscles, such as pharyngeal muscle cells, have this character since they secrete cuticle for the pharyngeal lumen, and cuticle secretion is generally considered an epithelial function. Some smooth muscle tissues, such as the somatic sheath of the gonad, also fit this definition, as they completely enclose the germline cells.

Myofilament Myofiber (S) Myofibril (S)

A generic name referring to both the thick and thin filaments (myosin and actin respectively). They make up the “myofilament lattice”, a well organized network of parallel filaments that are intermingled to form the sarcomere in “obliquely striated” and “single sarcomere” muscles, but can also lie in more d ispersed, disorganized arrays within smooth muscles.

Myofilament lattice  

The organized set of overlapping thick and thin filaments (myosin and actin respectively) that together compose the dominant feature of the muscle sarcomere, anchored on each end to the dense bodies, or Z bands. Each thick filament is surrounded by evenly spaced thin filaments, all lying in parallel; they are believed to slide past one another during muscle contraction. Thin filaments outnumber thick filaments by a ratio of 5:1 in mature bodywall muscles.

Orderly development of the lattice can be monitored under polarized light by light microscopy, or in thin sections by electron microscopy, and is disrupted in many mutants.

See Myofilament
See Obliquely striated
See Sarcomere

Myogenesis Muscle development (S)

The set of developmental steps involved in muscle differentiation and growth, including the gradual construction of the early sarcomere, the mature sarcomere, and the increase in sarcomere numbers per cell.

Myogenic control of locomotion  

A theory that the basic pattern of locomotion in nematodes is patterned by the direct interconnections between muscle cells (via muscle arm gap junctions) and not due to the pattern of synaptic inputs from motor neurons (Crofton, 1971).

See Neurocratic control of locomotion

Myosin Thick filament (S)

Very large elongated macromolecular protein that forms the major cytoskeletal element within muscle cells, the thick filament. Two isoforms of myosin heavy chain, A and B, are localized to different regions of individual thick filaments in bodywall muscle (Miller et al., 1983), binding to a paramyosin (Moerman and Fire, 1997) core protein to form a long straight filament with myosin motors oriented in opposite directions at each end of the thick filament. Each myosin heavy chain contains a motor domain that translocates myosin relative to adjacent actin thin filaments to provide the force of muscle contraction.

Myosin rod  

The central portion of the myosin heavy chain molecule, a coiled-coil domain which is  long and straight, with a globular motor domain on one end and a non-helical tailpiece on the other end (Epstein, 1990; Hoppe and Waterston, 2000).

Myosin tailpiece  

The non-helical portion of the myosin heavy chain protein, which lies at the opposite end of the myosin rod from the motor domain, and is thought to be involved in thick filament assembly (Hoppe and Waterston, 2000).


Membrane protein involved in the assembly and positioning of muscle/hypodermal fibrous organelles (hemi-adherens junctions) (Hresko et al., 1999).

See Fibrous organelle

Edited for the web by Laura A. Herndon. Last revision: January 26, 2010. This section should be cited as: Herndon, L.A. and Hall, D.H. 2010. Glossary M. In WormAtlas.  doi:10.3908/wormatlas.6.13