An integrated view on the regeneration of the different skeletal elements in the arm of Amphiura filiformis

Echinoderms are marine invertebrates that possess an elaborate calcite skeleton presenting a great diversity in morphology among classes. The arm skeleton of ophiuroids, for example, is jointed and metameric, each segment containing one caboral, one oral and two lateral shields, a variable number of spines, and a central “vertebra”. Representatives of all classes can efficiently regenerate body parts after trauma; however, the skeleton has a crucial role in regaining protection and functionality. In the present study Amphiura filiformis was used as experimental model to study at structural level the skeletal elements of the arm, their anatomy, and their development during regeneration. Light and electron (TEM and SEM) microscopy approaches together with gene expression pattern analyses were used to assess this subject. SEM and histological analyses on non-regenerating arms allowed to outline the anatomy of the skeletal elements in relation to the other organs. Microscopy results on regenerating arms instead showed that sclerocytes (i.e. skeletogenic cells) and developing spicules are mostly found in specific regions of the dermis and in two central rows on the aboral side of the radial water canal (RWC). These cells often presented cytoplasmic pockets containing collagen fibrils, thus suggesting a possible double function (i.e. collagen and calcite deposition). The first sclerocytes were detected at stage 2 in the dermis of the future shield regions, in agreement with [1], whereas the spicules of the “vertebral” region were visible only at later stages in the regeneration process. Vertebral primordia seem to be spatially and temporally linked to the appearance of the tube feet on both sides of the RWC and only at late stages fuse together to form a single vertebra. The RWC-related development of vertebra primordia supports the extraxial–axial theory (EAT) proposed by [2]. Preliminary results of in situ hybridisation on regenerating arms of genes known to be specifically expressed in axial and extra-axial components seem also consistent with the EAT and the ocular plate rule described by [2] in other echinoderms. Ultimately further ultrastructural and molecular investigations are needed to better understand how skeletogenic precursor cells can lead to such complex and diverse structures and to shed light on their origin. [1] Czarkwiani et al 2016 Front Zool 13(18) [2] Mooi et al 2005 Evol Dev 7(6): 542-555