Visceral graft and regeneration in the crinoid Antedon mediterranea

Physiological and immunocytochemical evidence that glutamatergic neurotransmission is involved in the activation of arm autotomy in the featherstarAntedon mediterranea(Echinodermata: Crinoidea)

“…Although aligned longitudinally, as are the juxtaligamental cell processes, these granules were much sparser than the intracellular granules of juxtaligamental processes (e.g. Heinzeller and Welsch, 1994;Ferreri et al, 2003) and might therefore represent glutamatergic terminals of neuronal axons directly innervating the juxtaligamental processes. However, glutamate storage vesicles are typically clear and small (diameter 30-40nm) (Crivellato et al, 2005) and, although possibly present in the vicinity of juxtaligamental cell bodies (Welsch et al, 1995), cell processes containing such vesicles have not been reported as occurring within the ligaments.…”

“…The simplest explanation for this invariable relationship between the sites of stimulation and fracture is that the autotomy response is a reflex that involves only a local nervous pathway connecting the site of damage to the nearest proximal syzygy. This was confirmed by the incidental observation that mechanical stimulation of the epidermis resulting from the handling of isolated arm pieces sometimes invoked syzygial fracture even before the preparations were installed in the test apparatus, which implies that such arm pieces contain the complete neural apparatus required for the autotomy response, including a sensory pathway from the epidermis to the brachial nerve, and a motor pathway from the brachial nerve to the juxtaligamental cells whose processes permeate the syzygial ligament and are likely to be directly responsible for its destabilisation at autotomy (Holland and Grimmer, 1981;Heinzeller and Welsch, 1994;Ferreri et al, 2003;Wilkie, 2005).…”

“…Syzygial fracture results from the drastic loss of tensile strength of the syzygial ligament (Holland and Grimmer, 1981;Wilkie et al, 1999;Ferreri et al, 2003). In the set-up used for these experiments, in which the initial resistance of preparations to a bending force acting in the oral direction could have been due to only the diarthrial ligaments and not the brachial muscles [as these are located on the oral side of the interbrachial hinge joint (Fig.1B)], diarthrial flexion theoretically could have resulted from either: (i) a reduction in the mechanical resistance of the diarthrial ligaments that was sufficient to allow the joint to be flexed by the 10g bending load alone or with the assistance of muscle contraction, or (ii) a contraction of the brachial muscles that was strong enough to overcome the (unaltered) resistance of the ligaments, thereby extending the aboral ligament and/or compressing the oral ligaments.…”

“…All investigated examples of echinoderm autotomy have been shown to depend on the rapid destabilisation of collagenous structures, such as ligaments, tendons and dermis, that cross the autotomy plane (Wilkie, 2001). For instance, the arms of comatulid crinoids ('featherstars') are autotomised at a specialised type of joint -the syzygy -as a result of the sudden loss of tensile strength of the short ligament fibres that link opposing skeletal ossicles at the joint (Holland and Grimmer, 1981;Wilkie et al, 1999;Ferreri et al, 2003).…”