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

Abstract

The skeletal morphology of the arm spine joint of the brittlestar Ophiocomina nigra was examined by scanning electron microscopy and the associated epidermis, connective tissue structures, juxtaligamental system and muscle by optical and transmission electron microscopy. The behaviour of spines in living animals was observed and two experiments were conducted to establish if the spine ligament is mutable collagenous tissue: these determined (1) if animals could detach spines to which plastic tags had been attached and (2) if the extension under constant load of isolated joint preparations was affected by high potassium stimulation. The articulation normally operates as a flexible joint in which the articular surfaces are separated by compliant connective tissue. The articular surfaces comprise a reniform apposition and peg-in-socket mechanical stop, and function primarily to stabilise spines in the erect position. Erect spines can be completely immobilised, which depends on the ligament having mutable tensile properties, as was inferred from the ability of animals to detach tagged spines and the responsiveness of isolated joint preparations to high potassium. The epidermis surrounding the joint has circumferential constrictions that facilitate compression folding and unfolding when the spine is inclined. The interarticular connective tissue is an acellular meshwork of collagen fibril bundles and may serve to reduce frictional forces between the articular surfaces. The ligament consists of parallel bundles of collagen fibrils and 7–14 nm microfibrils. Its passive elastic recoil contributes to the re-erection of inclined spines. The ligament is permeated by cell processes containing large dense-core vesicles, which belong to two types of juxtaligamental cells, one of which is probably peptidergic. The spine muscle consists of obliquely striated myocytes that are linked to the skeleton by extensions of their basement membranes. Muscle contraction may serve mainly to complete the process of spine erection by ensuring close contact between the articular surfaces.

Neural anatomy of echinoid early juveniles and comparison of nervous system organization in echinoderms

“…Most notably, in both classes, we found the presence of spine nerves spreading all along the shaft of the spines and that are innervated by lateral nerves emanating from the radial nerve cords. In ophiuroids, spine nerves are also present, and they are similarly innervated by peripheral nerves protruding from the radial nerve cords (Wilkie, 2016; Zueva et al, 2018).…”

From spinodal decomposition to alternating layered structure within single crystals of biogenic magnesium calcite

“…The brittle star is an echinoderm that possesses several skeletal parts 17 , including lenses, spicules, teeth, and arm vertebrae, which, respectively, serve its visual system 5 , locomotor system, oral system, and act as joints 18 . A study of the lens nanostructure showed that each lens comprises a single crystal composed of two coherent Mg-calcite phases: a phase comprising nanoparticles of high-Mg calcite (~5 nm diameter, ~40 mol% MgCO 3 ), coherently incorporated in a phase consisting of a poorer Mg-calcite matrix (~13 mol% MgCO 3 ) 6 .…”

Regeneration of the digestive system in the crinoid Himerometra robustipinna occurs by transdifferentiation of neurosecretory-like cells

“…This study paid particular attention to the structure of these cells and their transformation during the regeneration process. Until now it was believed that JLCs are a specific type of neurosecretory-like cells characteristic of echinoderms, the function of which is to alter the mechanical properties of connective tissue [ 43 , 44 , 61 – 66 ]. A characteristic feature of JLCs is the presence in their cytoplasm of electron-dense granules whose largest dimension varies between species but is within the range 0.1–0.85 μm.…”

“…In addition, a neuropeptide, referred to as somatostatin, has been detected in them [ 48 ]. In this regard, they are now considered to be a special type of neurosecretory neuron [ 66 ] and, accordingly, may have an ectodermal origin. Thus, the transdifferentiation of ectodermal (nerve) cells into endodermal ones occurs during regeneration of the visceral mass in H .…”