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The mutable collagenous tissue (MCT) of echinoderms has the ability to undergo rapid and reversible changes in passive mechanical properties that are initiated and modulated by the nervous system. Since the mechanism of MCT mutability is poorly understood, the aim of this work was to provide a detailed morphological analysis of a typical mutable collagenous structure in its different mechanical states. The model studied was the compass depressor ligament (CDL) of a sea urchin (Paracentrotus lividus), which was characterized in different functional states mimicking MCT mutability. Transmission electron microscopy, histochemistry, cryo-scanning electron microscopy, focused ion beam/scanning electron microscopy, and field emission gun-environmental scanning electron microscopy were used to visualize CDLs at the micro- and nano-scales. This investigation has revealed previously unreported differences in both extracellular and cellular constituents, expanding the current knowledge of the relationship between the organization of the CDL and its mechanical state. Scanning electron microscopies in particular provided a three-dimensional overview of CDL architecture at the micro- and nano-scales, and clarified the micro-organization of the ECM components that are involved in mutability. Further evidence that the juxtaligamental cells are the effectors of these changes in mechanical properties was provided by a correlation between their cytology and the tensile state of the CDLs.
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21935473 ???displayArticle.pmcLink???PMC3173489 ???displayArticle.link???PLoS One
Figure 1. Anatomical relations and behavior of CDLs.(A, B) Specimen of P. lividus split into two halves: (A) The Aristotle's lantern (arrow) is observable in the oral half. (B) Enlargement of the Aristotle's lantern showing the anatomical location of the CDLs (arrow). (C–E) Diagrams of sea-urchins with lanterns in different positions; CDLs shown in black. (C) Protracted position; CDLs compliant; (D) Resting position; CDLs in standard state; (E) Retracted position; CDLs stiff.
Figure 2. General view of CDL internal structure.(A) Semi-thin longitudinal section of CDL and (B, C) FEG/ESEM micrograph of resin-embedded CDL revealing the dense collagen array surrounded by a coelomic epithelium. Fig.2A shows clearly the coelomic myoepithelium. (D) FIB/SEM micrograph of milled CDL showing small globular cells between the collagen bundles covered by a ciliated coelomic epithelium. ce, coelomic epithelium; my, myoepithelium; cm, collagen matrix.
Figure 3. CDL extracellular matrix.(A) Cryo-SEM and (B–H) FEG/ESEM micrographs showing a (A) general view of CDL compliant matrix. (B) Detail of collagen fibrils and fibrillin (arrow). (C) Parallel array of collagen fibrils showing a clear D-banding pattern. (D) Enlargement of Fig. 3B showing a fibrillin bundle (arrow). (E) Enlargement of Fig. 3B showing a fibrillin meshwork. (F) Loose fibrillin meshwork (arrow) on surface of collagen fibrils. (G, H) Interfibrillar bridges linking adjacent fibrils (*).
Figure 4. Proteoglycans presence and distribution.Longitudinal section of CDL stained with (A) alcian blue at pH 2.5 and cuprolinic blue (B) (arrows, cuprolinic blue stained precipitates).
Figure 5. Juxtaligamental cells.(A) CSEM micrograph showing a transverse section of a JLC (arrow) within the collagen array. (B–D) FEG/ESEM micrographs of resin- embedded samples. The dissolution of resin allows the direct visualization of a fractured JLC process (arrow), showing (B) the internal granules, (C) granules inside the cell membrane (arrow), and (D) the variation in JLC granule size.
Figure 6. Histology of CDLs in different functional states.(A–C) Semi-thin longitudinal sections and (D–F) FEG/ESEM micrographs showing CDLs in the compliant state (A, D), standard state (B, E) and stiff state (C, F). Asterisks, cells; ce, coelomic epithelium; cm, collagen fibers; my, myoepithelium.
Figure 7. Arrangement of collagen fibrils.(A–C) FEG/ESEM and (D–F) TEM micrographs. (A, D) Compliant state. (B, E) Standard state. (C, F) Stiff state.
Figure 9. Microfibrils.TEM micrographs showing the distribution of microfibrils in CDLs in standard (A) and stiff (B) states.
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