CutFIG 6: Cuticle layer organization in different larval stages.
(Red labels) Layers that are either specific to a particular stage or have distinct characteristics in that stage, relative to the adult. (Adapted from Cox et al., 1981b.)
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.
4 Cuticle Generated By Other Tissue
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.
CutFIG 7A: Cuticular structures of the pharynx. DIC image of an adult animal head (lateral view) showing the compartments of the pharynx (black labels) and the various structures found in lumen cuticle lining (white labels).
CutFIG 7B-E: Ultrastructure of pharynx cuticle elements. B. Buccal cavity. (Image source: [E. Hartwieg and R. Horvitz] N2NOSE/EH5775.) C. Procorpus. D. Metacorpus (ventral is to the left). E. Terminal bulb. (Image source [MRC] JSA.)
CutFIG 8: Ultrastructure of excretory canal cuticle lining. A. TEM, horizontal section. (Image source [Hall] N533 N1 C629B.)
B. TEM, longitudinal section. (Image source [Hall] N513B T1 1527.)
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.
CutFIG 9A: Schematic of cuticle formation and molting. L1 cuticle formation during embryonic development (longitudinal view). (Adapted from Costa et al., 1997.)
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).
CutFIG 9B: TEM of cuticle formation and molting. TEM, longitudinal section, of animal during a molt. (Image source: mab-5 [e1239]; him-5[e1490] [MRC] 3826-11.)
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).
6 Cuticle Attachment Complexes
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).
CutFIG 10: Ultrastructure of cuticle attachment complexes. Schematic (transverse view) and TEM (longitudinal view) of fibrous organelles (FO) formed by hemidesmosomes (Hd). (Image source: fat-3 [Hall] T411.)
CutFIG 11: Proteins associated with cuticle attachment complexes. Epi-fluorescent images of transgenic L3 animals (head, lateral view) expressing a reporter gene for A. mup-4 (strain source B. Bucher) and B. intermediate filament gene mua-6 (strain source V. Hapiak and J. Plenefisch).
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