Nematode cuticles are organized as layers of collagenous extracellular matrix arranged in outer (cortical) zones and inner (basal) zones. The cortical zone of the dauer cuticle is thickened relative to non-dauer cuticles. The dauer cuticle’s basal zone is striated due to convergence of longitudinally-oriented layers with circumferentially-oriented layers. Loose fibrillar structures separate the hypodermis and striated zone (DCutFIG 1) (Cox et al., 1981; Page and Johnstone, 2007).
DCutFIG 1: Structural features of dauer bodywall cuticle. A, B. Arrangement of cuticle zones in adults and dauers. (Adapted from Cox et al., 1981.) C. Cross-section of adult cuticle showing absence of striated zone. (Image source: WormAtlas CutFIG 4A.) D. Cross-section of dauer bodywall cuticle showing striated zone. (Image source: [D. Riddle] N2 starved dauer 50-2-1 #1183.)
Cuticular thickenings, called alae, run laterally along the length of the body like train tracks (DCutFIG 2&3). In cross section, the dauer alae appear as four prongs which are symmetrically oriented around a central track. Lateral alae are also present in the L1 and adult cuticles. The dauer alae are structurally distinct from L1 and adult alae and may project to relatively greater height. Dauer alae can usually be identified along the lateral edge of the midbody using light microscopy with DIC optics. For this reason, the presence of dauer alae is a commonly-used morphological marker for dauers.
DCutFIG 2: Lateral alae in dauer larva. A. DIC light microscopy image of dauer lateral alae. Upper panel, white box indicates magnified region below. Circle-tipped arrow lies in the central track of the alae, between the dorsal and ventral ridges. (Image source: Sylvia Lee, Cornell University.) B. Outlines of cuticle in cross-section showing lateral alae in dauers, adults and L1s and absence of alae in L2, L3 and L4 cuticles. Dauer, L1 and adult alae profiles were adapted from Cox et al., 1981. Outlines of stages lacking alae were hand-drawn.
DCutFIG 3: Progression of dauer alae in different head regions. Numbers in upper panel indicate approximate positions of slices shown in lower panels. Presence and elaboration of lateral alae corresponds to presence of seam cells (orange rods in upper panel). (Image source: upper panel, WormAtlas SeamFIG2; lower panels, [D. Riddle] TEM, N2 starved dauers, 50-6-2 229, 293, 24M.)
2.3 Buccal Plug
In dauers, the anterior bodywall cuticle completely covers the buccal cavity to inhibit feeding. Light photomicrographs and longitudinal electron micrographs show that the plug projects across the front of the buccal orifice, at the level of hyp1 and the arcade cell epithelia (compare to InterFIG 1). Cross-sectional electron micrographs reveal three flaps of cuticle which join tightly as a three-pointed star. The buccal plug may protect the dauer from desiccation as well as maintaining the non-feeding status (DCutFIG 4&5).
DCutFIG 4: Cuticle plug covering the buccal opening in the dauer larva. A&B. SEM of mouth region in L2 larva (A) and dauer larva (B) showing opening to buccal cavity. (Image source: Riddle, 1988.) C. Cross-section SEM of L2 larva mouth showing opening to buccal cavity. (Image source: [D. Riddle] N2_L2_28-14-006.) D. Cross-section SEM of dauer larva showing cuticle flaps creating the plug which occludes the buccal cavity. (Image source: [D. Riddle] CB2620_dauer_69_5-1-021.)
DCutFIG 5: Lateral view of cuticle plug. A. Transverse SEM of wildtype dauer showing cuticle plug. (Image source: [D. Riddle] N2 dauer 50-7-1_30.) B. DIC micrograph of a starvation-induced wildtype dauer larva showing cuticle plug over mouth. (Image source: Wendy Iser, NIA.) C. Higher-magnification view of dauer buccal plug. (Image source: [D. Riddle] N2 dauer 50-7-1_43b.)
3 Pharynx Cuticle
3.1 Buccal Cavity
In cross-section, the buccal cavity cuticle appears structurally similar in dauers and non-dauers. The cuticle lining the buccal cavity appears to be thicker than the bodywall cuticle or the pharyngeal cuticle. Reinforcing struts are can be visualized in transverse views of the buccal cuticle. These struts may be thicker in dauer cuticles than in non-dauers (DCutFIG 6). The interior of the buccal cavity is lined by three sheets of cuticle. The joints where the buccal cuticle sheets meet is reinforced in all stages, but this reinforcement appears to be more extensive in dauers than in L2 larvae (DCutFIG 7).
DCutFIG 6: Transverse view of buccal cavity cuticle. Transverse SEM of dauer nose showing cuticle lining the bodywall, buccal cavity and anterior pharynx. Small arrow indicates reinforcing strut. (Image source: [D. Riddle] N2 dauer 50-7-2-43.) Inset, transverse view of buccal cuticle in an adult showing reinforcing struts (arrow). (Image source: [Hall] N533_11152.)
DCutFIG 7: Cross sections of buccal cavity cuticle. A&C. Cross-sectional SEM of buccal cavity in L2 larva. Slice in panel C is slightly posterior to panel A. (Image source: [D. Riddle] A, N2_L2_28-14-018; C, N2_L2_28-14-120]. B&D. Cross-sectional SEM of buccal cavity in dauer larva. Panel D is slightly posterior to panel B. (Image source: [D. Riddle] N2 starved dauer 50-2-1, 93 and 124.)
3.2 Pharynx lumen
Cuticle also lines the length of the pharynx lumen (see also Dauer Pharynx). The dauer pharynx is relaxed and does not pump. In general, the pharynx cuticle appears similar in dauers and non-dauers. The dauer pharynx cuticle may have greater reinforcement, as suggested by the presence of electron dense strands running circumferentially within the cuticle sheet (DCutFIG 8).
DCutFIG 8: Cuticle lining of dauer pharynx lumen. Arrows indicate cuticle lining the pharynx lumen. A&B. Procorpus. C&D. Terminal bulb. (Image source: [D. Riddle] A&C N2_L2_28-14 175 and 1419; B&D N2 starved dauer 50-6-2 293 and 50-2-1 973.)
4 Cuticle of the Rectum and Excretory Pore
The other body orifices present in the dauer are the excretory pore and rectum. The excretory pore (EP) is lined by cuticle and serves as a portal for release of materials from the body. Pulsations of the excretory duct have been observed in dauers, suggesting that the excretory system is active (Nelson et al., 1983). The dauer EP cuticle is thickened relative to that in non-dauers, but it lacks the striated zone present in the dauer bodywall cuticle (DCutFIG 9A&B). The dauer rectum appears to remain open in SEM images of dauers (DCutFIG 9C&D). Some infective stage nematodes (equivalent to the C. elegans dauer) develop an “anal plug” sealing the rectal opening. The anal plug composition is unknown, but is thought to result from rectal gland secretions (Vincent et al., 1979). While SEM images suggests the anus is not occluded in C. elegans dauers, the internal composition of this structure has not been studied.
DCutFIG 9: Cuticle lining of excretory pore (EP) and rectum. A. SEM of excretory pore in adult hermaphrodite. EP, excretory pore. (Image source: [Hall] N513A_1463.) B. SEM of dauer EP, indicating thickened EP and bodywall cuticle. The EP cuticle lacks the striated zone of the bodywall cuticle. (Image source: [D. Riddle] daf-4 dauer 51-7-1_68). C&D. SEM imaging indicates that the dauer rectum (arrow) remains open in C. elegans, and is not covered by a plug as in some other nematode species. This dauer animal was prepared for SEM by Ken Nguyen and Leslie Gunther in the Hall lab, and photographed on the scanning electron microscope (SEM) by Juliana Castrillon, a student at the BioBus in New York (www.biobus.org).
5 The Cuticle's Role as a Regulator of Influx and Efflux
Cuticle structure has the potential to affect body growth dynamics. Developing C. elegans larvae grow linearly during four sequential larval stages. The C. elegans bodywall cuticle is non-rigid and expands to accommodate body growth. The cuticle also allows for some diffusion of materials into and out of the body through a permeability barrier established by the hypodermis (Stawicki et al., 2011). This route for diffusion is impaired by the thick dauer bodywall cuticle. Thus, the structure of the dauer bodywall cuticle facilitates retention of internal molecules and protection from external solutes (DCutFIG 10).
In contrast to the bodywall cuticle, the cuticle lining the buccal cavity is rigid and inflexible (see Dauer Pharynx). Expansion can only occur during the molt, when a newly-secreted enlarged cuticle increases buccal cavity diameter. The rigid buccal cavity restricts the rate of food intake, through a straw-like effect (Knight et al., 2002). The dauer buccal cavity may be modestly shrunken, which could further limit entry of nutrients into the animal and maintain a non-feeding state. However, buccal closure by the bodywall cuticle would seem to obviate the need for protective mechanisms within the buccal cavity proper.
DCutFIG 10: Cuticle specializations of the dauer larva limits fluid and ion exchange between the body and the environment. A. Alimentary system promotes intake and efflux of fluids and nutrients through the mouth and anus, respectively, in non-dauer stages, such as the L2. Transport through the alimentary system is halted in the dauer due to physical closure of the buccal opening and the anus. B. Ion and fluid exchange through the excretory system is probably maintained in the dauer, as evidenced by the open excretory pore. The thickening of the dauer cuticle does not seem to restrict this opening, and might actually help it to resist forces that could cause collapse of the opening. C. Thickening of the body wall cuticle prevents exchange of fluid and ions across the hypodermal route in the dauer. The sum of these changes may be significant for promoting dauer survival in desiccated environments. Dark arrow indicates unrestricted flow; medium arrow indicates limited flow; crossed open arrow indicates blockage of flow. Double-headed arrow suggests two way flow is likely; single headed arrow indicates that one way flow is likely.
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