Deciphering regeneration through non-model animals: A century of experiments on cephalopod mollusks and an outlook at the future

Abstract

The advent of marine stations in the last quarter of the 19th Century has given biologists the possibility of observing and experimenting upon myriad marine organisms. Among them, cephalopod mollusks have attracted great attention from the onset, thanks to their remarkable adaptability to captivity and a great number of biologically unique features including a sophisticate behavioral repertoire, remarkable body patterning capacities under direct neural control and the complexity of nervous system rivalling vertebrates. Surprisingly, the capacity to regenerate tissues and complex structures, such as appendages, albeit been known for centuries, has been understudied over the decades. Here, we will first review the limited in number, but fundamental studies on the subject published between 1920 and 1970 and discuss what they added to our knowledge of regeneration as a biological phenomenon. We will also speculate on how these relate to their epistemic and disciplinary context, setting the base for the study of regeneration in the taxon. We will then frame the peripherality of cephalopods in regeneration studies in relation with their experimental accessibility, and in comparison, with established models, either simpler (such as planarians), or more promising in terms of translation (urodeles). Last, we will explore the potential and growing relevance of cephalopods as prospective models of regeneration today, in the light of the novel opportunities provided by technological and methodological advances, to reconsider old problems and explore new ones. The recent development of cutting-edge technologies made available for cephalopods, like genome editing, is allowing for a number of important findings and opening the way toward new promising avenues. The contribution offered by cephalopods will increase our knowledge on regenerative mechanisms through cross-species comparison and will lead to a better understanding of the complex cellular and molecular machinery involved, shedding a light on the common pathways but also on the novel strategies different taxa evolved to promote regeneration of tissues and organs. Through the dialogue between biological/experimental and historical/contextual perspectives, this article will stimulate a discussion around the changing relations between availability of animal models and their specificity, technical and methodological developments and scientific trends in contemporary biology and medicine.

Keywords: arm; cellular and molecular pathways; hectocotylus; history of science; invertebrates; octopus; pallial nerve; regeneration.

FIGURE 1

Hectocotyli of cephalopods. 1-5. Octopus vulgaris; 6-11. Argonauta argo; 12-14. Tremoctopus violaceus (Vérany, 1851, table 41. Out of copyright).

FIGURE 2

(A) Schematic drawing of Octopus vulgaris morphology. General anatomy is shown in (A) while (B) shows main structures of the nervous system with the brain (CNS) located in the head of the octopus, two pallial nerves arising (in red) from its posterior part and eight nerve cords (in red) from the anterior part innervating the arms. (C) highlights main structures in the arm (transverse section) and (D) highlights neural components of the pallial nerve and stellate ganglion, together with main connections. Particularly, pallial nerves are a paired neural structure composed of fibers covered in connective tissue, whose cell soma are mainly located in the subesophageal mass of the brain. Some of these fibers make synapsis (D) in the stellate ganglion for the control of the breathing muscles, while other axons travel directly to the skin to innervate chromatophores in the mantle (Young, 1971; Budelmann and Young, 1985). While complete transection of both nerves leads to animal death due to paralysis of respiratory muscles (Fredericq, 1878), the lesion of just one of them is easily managed by the animals, even though camouflage and breathing are impaired on the ipsilateral side of the injury (Fredericq, 1878; Sereni, 1929b; Imperadore et al., 2017). AC Amacrine cells, ANC axial nerve cord, BA brachial artery, BG brachial ganglion, Ch chromatophores, CBT cerebro-brachial tracts, CL cellular layer, CNS central nervous system, Cp centripetal cell, GS ganglion of sucker, INC. intramuscular nerve cords, LR lateral roots, Mn motoneurons, Mu muscular tissue, Nb neurobiotin, Np neuropil, OL optic lobe, PN pallial nerve, S sucker, SEM supra-esophageal mass, SF sensory fibers, Sk skin, SN stellar nerve, StG stellate ganglion, SUB sub-esophageal mass, v blood vessels, VR ventral roots. Adapted by permission from Springer Nature: Springer -Verlag GmbH Germany, Invertebrate Neuroscience: Neural pathways in the pallial nerve and arm nerve cord revealed by neurobiotin backfilling in the cephalopod mollusk Octopus vulgaris, Imperadore et al., Copyright ^© 2019.

FIGURE 3

Diagrammatic drawings of pallial nerve. (A) Intact nerve. CNS, central nervous system; m. c., pallial nerve (mantel connective, in the old terminology); i n., intercalary neuron; st. g., stellate ganglion; m. n., motor neuron; mus., mantel muscles; n. mus., nerves to mantle; st. n., stellar nerve; n. cr., nerves to chromatophores; s. n., sensory neuron. (B) Sectioned nerve in the process of regenerating. m. c. centre, central stump; m. c. per., peripheral stump. Other lettering as in A. (Sereni and Young, 1932. Figure 1, p. 176, and 21, p. 195, respectively. ^© Stazione Zoologica Anton Dohrn. Reproduced by permission).