- Muscular system development and organization
The coordinated specification and functional assemblage of cells and tissues into the contractile organ system in the animal. C. elegans muscles are of two types: single sarcomere with focal attachment points at the ends (alimentary system and sex muscles) and obliquely striated muscles with many sarcomeres and no one substantial focal attachment point (body-wall muscles). Components of C. elegans muscles are similar to other animals and include heavy and light-chain myosin, actin, tropomyosin, troponin-like proteins, and paramyosin. Unlike other muscle systems, C. elegans muscles send neuron-like processes to neuropils that contain motor neuron axons rather than motor neurons sending axons to innervate the muscle. Contractile tissue is found throughout C. elegans and is required for locomotion (body wall muscle), eating (pharyngeal muscle), egg laying (vulval and uterine muscles, and gonad sheath), male mating (male tail muscles), and defecation (enteric muscles).
- Sarcomere assembly
The sarcomere is the basic unit of a muscle cell and is comprised of thick and thin filaments made up of myosin and actin, respectively. Muscle sarcomere assembly involves many proteins and occurs in many steps, one of which is the attachment of sarcomeres to the sarcolemma (the membrane of the muscle cell). The steps involved in initiating the correct placement of sarcomere-sarcolemma attachments and other sarcomere substructures are poorly understood and are being addressed through studies of C. elegans mutants. These attachment sites are very similar to vertebrate adhesion complexes.
- Pharyngeal development
The progression of events that leads to the formation of a functional pharynx, the feeding organ just posterior to the mouth or buccal cavity and anterior to the intestine. In C. elegans the pharynx is divided into anterior and posterior regions. The anterior region includes the corpus (procorpus and metacorpus - first bulb) and the posterior region includes the isthmus and terminal bulb (second bulb). Cells first commit to a pharyngeal fate during gastrulation. Establishment of this cell fate is directed by PHA-4, a FoxA transcription factor and four Tbox transcription factors, TBX-2, -35, -37, and -38. The linear gut tube is formed during later embryogenesis. Cell fate commitment during pharyngeal development occurs through a combination of positive feedback loops, positive autoregulation and repression of alternative fates. Other steps in the development of this organ include tissue morphogenesis, muscle differentiation and establishment and maintenance of apical/basal polarity.
- Synaptogenesis
The formation of the chemical synaptic junction that mediates communication between neurons and other neurons or muscle cells. These junctions can be identified in electron micrographs as darkened specialized areas on the presynaptic side of the junction that contains clusters of synaptic vesicles.
- Cuticle biogenesis
The C. elegans cuticle is a protective exoskeleton of specialized extracellular matrix (ECM) consisting primarily of collagen, lipids, and glycoproteins and is required for viability. (Chisholm and Hardin 2005; Page and Johnstone 2007). The cuticle determines the shape of the body and, through connection from the epidermis to muscle, provides anchoring points for muscle contraction. The cuticle also serves as a model for ECM formation and function with molecules and pathways involved in cuticle biogenesis conserved in vertebrates (Page and Johnstone 2007). The outer epithelial layer, the epidermis, of the embryo undergoes a series of cell fusions to make large multinucleate, or syncytial, epidermal cells, which secrete the materials needed to make up the cuticle. This protective layer is produced five times during C. elegans development, with each molt ending with an entirely new cuticle.
- Defecation
In C. elegans the expulsion of intestinal contents occurs every 45-50 seconds. This cycle is characterized by a pattern of muscle contractions under both muscle and neuronal control. The steps of the defecation cycle are a posterior body contraction (pBoc), an anterior body contraction (aBoc), and the final expulsion step (Exp) where the enteric muscles contract, opening the anus and allowing the intestinal contents to be released. Each step is independently controlled as mutations exist that affect one step but do not alter the timing or occurrence of the other. Further, Ca++ oscillations in the intestine, rather than neuronal stimulation, have been shown to control the initiating pBoc step. The contractions of the enteric muscles are controlled by GABA motor neurons AVL and DVB through an excitatory GABA-gated cation channel. The periodicity of the cycle is influenced by the presence of food, is temperature compensated, and can be reset by mechanosensory input.
- Egg laying
C. elegans hermaphrodites exhibit a periodicity in the rate and temporal pattern of egg-laying. Egg laying is modulated by diverse environmental cues. Egg laying behavior has served as an important phenotypic assay for the genetic dissection of neuronal signal transduction mechanisms. Studies in C. elegans have elucidated the roles of specific neurons in the egg-laying motor circuit, which release multiple neurotransmitters affecting distinct parameters of egg-laying muscle activity, and the possible mechanisms for sensory control of egg-laying behavior.
- Locomotion
The movement of the animal in relation to its environment requires coordinating an awareness of environmental cues with the firing of neuronal circuitry affecting the simultaneous contraction and relaxation of opposing muscle groups. C. elegans exhibits many types of movement, the two major types are crawling and swimming. Each of these movements have been further characterized by dominant body shapes, trajectories, angles, speeds, etc., peculiar to the movement. Fundamental to survival of the worm is the ability to sense and move towards or away from different stimuli. Forward and backwards movements can be induced in the lab through the stimulation of the mechanosensory neural network.
- Feeding
Identification of and response to a potential food source is a life-critical process. C. elegans has proved to be a model organism for studying the molecular and cellular mechanisms involved in seeking a food source and discriminating its value. These studies have shown that C. elegans is capable of forming a memory of particular foods and is capable of modifying its eating behavior upon subsequent exposure to the familiar food. In addition, research has shown that this modification in behavior is mediated by extrinsic, such as C. elegans pheromone and bacterial molecules, and intrinsic chemical cues, such as serotonin levels. In C. elegans feeding can be observed by watching pharyngeal pumping, which is composed of a posterior-directed contraction of the grinder followed by an anterior-directed relaxation.
- Response to pathogens
C. elegans is susceptible to disease or death brought on by a number of different microbial or fungal pathogens. While some of these pathogens, e.g., Drechmeria conispora and Microbacterium nematophilum are more specific to nematodes, other pathogens, e.g., Pseudomonas aeruginosa, Salmonella enterica, etc., are also pathogenic to humans. Genetic studies of C. elegans response to these pathogens have shown the nematode to employ three main mechanisms to defend against pathogen attack. First, as a behavioral response, C. elegans has been shown to use olfactory cues to distinguish different bacteria and respond with avoidance to those that are deemed harmful. Second, C. elegans has evolved physical barriers to infection that include a cuticle of collagen and chitin that protects the worm from its environment. This cuticle is also replaced at each larval molt, decreasing the worm's exposure to harmful bacteria that may be hitching a ride. In addition, C. elegans has evolved a pharyngeal grinder capable of pulverizing bacteria, keeping live bacteria from entering the gut. Third, C. elegans nematodes have inducible innate immune responses that are analogous to stress response pathways present in other organisms, for example, the PMK-1/P38 MAPK signaling pathway induced in response to Salmonella enterica.