The outer surface of the animal is covered by a tough, but flexible, extracellular cuticle (CutFIG 1). This cuticle protects the animal from the environment, maintains body shape, and permits motility by acting as an external skeleton. The cuticle is secreted by the epithelial cells covering the body (hypodermis, seam cells) and by interfacial cells lining the four major openings to the exterior (anus, excretory pore, vulva and pharynx). The cuticle surface is covered with a surface coat (glycocalyx) that is thought to be secreted by gland cells (the excretory cell, pharyngeal gland cells, or the amphid and phasmid support cells) (Nelson et al., 1983; Jones and Baillie, 1995; Page and Johnstone, 2007). At each larval stage, an entirely new cuticle is generated and the old cuticle is shed, allowing for growth. Significantly, cuticles of each stage differ in their surface protein expression, layer number, relative thickness, and composition, presumably to accommodate for the changing developmental needs and environmental conditions experienced during the life of the animal.

The cuticle surface bears shallow, circumferential-oriented furrows (CutFIG 1, CutFIG 2A&B) (Chitwood and Chitwood, 1950; Cox et al., 1981a). The ridges on either side of the furrows are called annuli. In L1, dauer, and adult cuticles annulations are interrupted laterally by longitudinally oriented ridges called alae that are generated by cells of the lateral seam beneath (CutFIG 1, CutFIG 2B) (see Epithelial system - Seam cells). The cuticle surface is also marked by holes and swellings where some neuronal cilia are exposed to the exterior (e.g., amphids and inner labial sensory organs) (CutFIG 2A) or lie just beneath the surface, respectively (e.g., the outer labial sensory organs, CutFIG 2A, or deirids, CutFIG 2B; see also NeuroTABLE 1).

The cuticle consists of layers or zones that differ in structure and composition. Layers can be distinguished by electron microscopy (EM) cross section as well as en face by several modalities (e.g., thin section and deep etching; CutFIG 3, CutFIG 4A&B). The adult cuticle is approximately 0.5 μm in thickness (Cox et al., 1981a) and is organized into five major layers or zones: the surface coat, the epicuticle layer, and the cortical, medial and basal zones (Cox et al., 1981a; 1981b, Bird and Bird, 1991).  With aging, the thickness of the adult cuticle increases due to expansion of the basal zone (Herndon et al., 2002).

3.2 Cuticle Composition

The different layers of the cuticle appear to consist of distinct molecular assemblies that impart different qualities. The wide range of phenotypes associated with mutations in cuticle components genes suggests a high degree of functional specialization among cuticle proteins and the layers to which they contribute. Mutant phenotypes range from abnormal surface epitope expression (Srf, surface antigenicity abnormal phenotype) and pathogen resistance (e.g. Bus, bacterially unswollen) to altered body shape (Rol, roller; Dpy, dumpy; Sqt, squat; Lon, long) or abnormal cuticle morphology (Bli, blister) (Brenner, 1974; Higgins and Hirsh, 1977; Kusch and Edgar 1986; Politz et al., 1990; Link et al., 1992; Kramer and Johnson, 1993; Nicholas and Hodgkin, 2004). The most abundant structural components of cuticle are collagens and the non-collagenous cuticulins (Sebastiano et al., 1991; Lewis et al., 1994; Kramer, 1997). These molecules assemble into the relatively insoluble, higher-order complexes that form the cuticle matrix. Collagens COL-19 (CutFIG 5A) and BLI-1 (CutFIG 5B) localize to the adult cortical layer and (Liu et al., 1995; Thein et al., 2003) medial layer struts, respectively (J. Crew and J.M. Kramer, pers. comm.); DPY-7 (CutFIG 5C) localizes to the furrows in all stages (McMahon et al., 2003). Although few have been characterized, the cuticle probably also contains many soluble proteins such as enzymes involved in post-secretion modification and cross-linking of matrix proteins or structural proteins associated with the surface coat (e.g. mucins). Lipids and glycolipids are found in the epicuticle layer, an atypical membrane. Carbohydrates are associated with glycosylated proteins of the matrix and of the surface coat (Blaxter and Robertson, 1998).

Larval stage cuticles differ from adult in the type of layers present or their relative thickness (CutFIG 6 and DCutFIG 1). The dauer cuticle is further distinguished from those of other stages in being less permeable and proportionally thicker (10.2% of the animal's cross sectional area; c.f. 4.4% for other stages) because of a reduction in body diameter and an increase in epicuticle layer thickness (Cassada and Russell, 1975; Cox et al., 1981b). See also Dauer Cuticle Chapter.

The major openings of the animal are also cuticle-lined: the buccal cavity and pharynx (see Alimentary system - Pharynx), the vulva (see Reproductive system - Egg-laying Apparatus), the rectum (see Alimentary system - Rectum and Anus) and the excretory duct and pore (see Excretory system). These cuticles are secreted by underlying cells that are generally epithelial, although in the case of the buccal cavity and pharynx, other cell types may contribute (e.g. pharyngeal muscle cells). In contrast to body cuticle, cuticles that line the openings do not appear to be composed of layers. However, some contain specialized cuticular elements. The most striking examples are seen in the pharyngeal cuticle, which contains at least four different types of elements: bridging cuticle, flaps, grinder, sieve, and channels (CutFIG 7A, CutFIG 7B-E; Albertson and Thomson, 1976; Wright and Thomson, 1981). The excretory duct and pore cuticles also contain regions of distinctive patterning (CutFIG 8) These may serve to strengthen the pore and to keep it open.

5 Cuticle Production and Molting

The first cuticle, the L1 cuticle, is laid down at the time of embryonic elongation. Contraction of circumferential actin filament bundles, which lie beneath the surface of the hypodermal apical membrane, induce elongation of the embryo and simultaneously produces ridges and furrows in the hypodermis and an overlying lipid layer called the embryonic sheath (Priess and Hirsh, 1986). This surface is thought to serve as a template for cuticle annulations (CutFIG 9A) (Costa et al., 1997).  A similar contractile mechanism may be employed to pattern later stage cuticles and, possibly, the formation of alae.

Following hatching are four post-embryonic molts whereby an entirely new cuticle is synthesized and the old cuticle is shed. The new cuticle is laid down beneath the existing cuticle, with the outer layers synthesized first and the inner layers last. Seam cells, and to a lesser extent hypodermal cells, acquire large Golgi and have vesicles containing densely staining material, consistent with the notion that high levels of protein synthesis and secretion occur at this time (Singh and Sulston, 1978). The new cuticle is initially highly convoluted and the underlying hypodermis contains folds known as plicae (CutFIG 9B).

Consistent with the cyclic nature of the molting process, synthesis of cuticle components is low between molts and high prior to molts (Cox et al., 1981c). Transcriptional analysis of collagen gene activity reveals multiphasic waves of early (e.g. dpy-7), middle (sqt-1, dpy-13), and late (col-12) gene expression. It is hypothesized that collagens that are produced cotemporally may be incorporated into the same heteromeric complex or cuticle substructure (Cox et al., 1981c; Cox and Hirsh, 1985; Park and Kramer, 1994; Johnstone and Barry, 1996; McMahon et al., 2003). Some cuticle proteins are synthesized at every molt (e.g. COL-12) whereas others are stage-specific e.g., adult-specific COL-19 and BLI-1 (CutFIG 5A&B).

Molting consists of two phases: lethargus, the period of inactivity preceding cuticle shedding, and ecdysis or cuticle shedding (Singh and Sulston, 1978). In the first half of lethargus pharyngeal pumping and locomotion decrease and seam cells lose their granular appearance. Immobility is thought to result from the separation of the basal zone in the old cuticle from the underlying hypodermis. Loosening of the old cuticle begins around the head, then moves to the buccal cavity and the tail. In the second half of lethargus, the worm begins to spin and flip around its long axis. The pharynx contracts and its cuticle lining breaks; the posterior half passes into the intestine and the anterior half is expelled and shed with the body cuticle. At this time, refractile granules are apparent in the pharyngeal gland cell processes and are thought to play a role in ecdysis.  As in insects, molting in C. elegans appears to be regulated by nuclear hormone receptors (Kostrouchova et al., 1998, 2001).

Locomotion requires the transmission of contractile force from body wall muscle to the cuticle.This force is transmitted through a series of mechanical linkages connecting body wall muscle, basal lamina, hypodermis and cuticle (CutFIG 10, CutFIG 11). The inner layer of the cuticle is attached to the apical hypodermal membrane through electron-dense attachment complexes, called hemidesmosomes, from which intermediate filaments extend into the cytoplasm. Similar complexes are also present on the basal hypodermal membrane and when apical and basal complexes are in register, a fibrous organelle (FO) is formed. Anchoring fibrils extend from FOs into the extracellular basal lamina, which is linked, in turn, to muscles through the M line and dense bodies of the sarcomere (Francis and Waterston, 1991; Hresko et al., 1994, 1999; Bercher et al., 2001; Hong et al., 2001; Hahn and Labouesse, 2001) (see also Somatic Muscle). Similar junctional complexes are also associated with non-body wall muscles (e.g. of the vulva, pharynx and rectum) and with non-muscular cells that also make tight transhypodermal contact with the cuticle such as the excretory pore, touch neuron processes, amphids and phasmids (Bercher et al., 2001).

7 Cuticle Mutants

Proper cuticle formation and function is regulated by a number of different genes, many of which are mentioned above and have been described in detail previously (for review see Page and Johnstone, 2007 in WormBook). When cuticle formation is altered, this can lead the changes in the appearance of the animal, producing phynotypes such as long or dumpy (CutFIG 12). Addtionally, animals can exhibit defects in the surface as seen in the structure of their annuli, furrows and alae (CutFIG 12 & CutFIG 13). These altered cuticle structures can also result in molting and movement defects (CutFIG 13).

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