- 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.
- 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.
- 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.
- 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.
- Neurotransmission
Neurons communicate across synaptic junctions with target cells, such as neurons, muscles, or specialized secretory cells through chemical messengers that are released from the neuron and bind to and activate receptors on the target cell. Pre-synaptic release of neurotransmitters can be evoked, such as through mechanical or chemical stimulation, as well as can occur spontaneously at a low rate. Depending on the neurotransmitter released and or the receptors of the post-synaptic cell, the activation of receptors can trigger excitatory or inhibitory actions in the target cell. These neuronal communications can also result in short term post-synapatic cellular changes to the membrane potential or can cause the activation of signaling cascades, resulting in longer term changes in the cell.
- Embryogenesis
The process by which an embryo forms and develops, marking a time of rapid cell proliferation, specification, and differentiation. Embryogenesis in C. elegans takes about fourteen hours at 22 degrees C, starting with fertilization of the oocyte with self-sperm from the hermaphrodite or sperm from a male. During the first two hours, the zygote forms and early cleavages establish the embryonic axes. Somatic and germ-line founder cell fates are also determined. During the next five hours, most cell proliferation completes, the embryo undergoes gastrulation, and cell differentiation and organogenesis begins. Cell differentiation, organogenesis and morphogenesis are completed during the final stage of embryogenesis. The nervous system becomes active and muscles are stimulated during this last stage, resulting in the embryo twitching within its egg shell and eventually hatching as an L1 larva.
- 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.