Functional morphology of the compass-rotular ligament of Echinus esculentus (Echinodermata: Echinoida): a non-mutable collagenous component of Aristotle’s lantern

Abstract

This paper provides the first detailed account of the histological and ultrastructural organisation of an echinoderm ligament that is non-mutable. Each of the five compass ossicles on the aboral side of the lantern of regular echinoids is linked to an underlying rotular ossicle by a compass-rotular ligament (CRL). The structure and anatomical relations of the CRL of Echinus esculentus L. were examined by light microscopy and scanning and transmission electron microscopy, and its responsiveness to neuroactive agents was observed in stress relaxation tests. The CRL consists of (1) thick collagen fibres that form an outer crossed fibre lattice and an internal system of parallel suspensory fibres and (2) regions composed mainly of bundles of ca. 12 nm microfibrils. The CRL is sparsely cellular, most cell bodies belonging to heterogeneous vesicle-containing cells that undergo apoptosis-like cytoplasmic fragmentation and produce linear aggregations of cell fragments. Coelothelia investing the CRL show intense secretory activity reminiscent of lamellar body production by mammalian mesothelia and are the source of bundles of cell processes that penetrate deeply into the CRL, branch sparingly and terminate between the thick fibres. Isolated CRL preparations are unresponsive to seawater containing 1 mM acetylcholine or 100 mM K + , but show a transient increase in stress relaxation rate when treated with 0.56 M KCl. The effects of solutions of other alkali metal chlorides suggest that the latter response is due to the direct action of KCl on extracellular components of the CRL and is not nervously mediated.

Functional Morphology of the Arm Spine Joint and Adjacent Structures of the Brittlestar Ophiocomina nigra (Echinodermata: Ophiuroidea)

“…Such cell processes are a prominent component of every echinoderm mutable collagenous structure that has been investigated [32] and are absent from the small number of echinoderm collagenous structures that have been shown to be non-mutable [62–64]. The functional significance of those in the spine ligament is discussed below under ‘Juxtaligamental components’.…”

“…[71]). Non-mutable echinoderm collagenous structures are either unresponsive or weakly responsive (due to non-neurally mediated physicochemical effects) [62,64]. …”

Diverse and Productive Source of Biopolymer Inspiration: Marine Collagens

“…Most of the data on aquatic organisms relate to fish and very few to marine invertebrates. Marine invertebrates possess collagens that are similar to those of higher vertebrates, namely homotypic and heterotypic fibrillar collagens, , basement membranes, and microfibrils. , However, in contrast to mammalian type I and V collagens, which are always composed of heterotrimeric α1/α2 chains, collagen type I and V in marine organisms can be either homotrimeric or composed of α1/α2/α3 chains. , Invertebrate fibrillar collagens are also characterized by the presence of specific imperfections in the collagenous domain . Moreover, examples of collagens with unique features have been described (Table ).…”

“…In both mutable and nonmutable collagenous structures of echinoderms, collagen fibril bundles are delimited by loose networks of hollow microfibrils 10–14 nm in diameter . These can be aggregated into fibers or sheets but most often form loose sheaths around bundles of collagen fibrils.…”

“…90,91 In both mutable and nonmutable collagenous structures of echinoderms, collagen fibril bundles are delimited by loose networks of hollow microfibrils 10−14 nm in diameter. 41 These can be aggregated into fibers or sheets but most often form loose sheaths around bundles of collagen fibrils. They resemble the fibrillin-containing microfibrils of mammalian connective tissue and may confer elasticity on MCTs, 40,92 thus promoting the recovery of the tissue to its original restingcondition dimensions following deformation.…”