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.

Transcriptome-wide selection and validation of a solid set of reference genes for gene expression studies in the cephalopod mollusk Octopus vulgaris

“…In addition, Ov-slc25a40 and Ov-Ppm1b were identified as shared best reference genes in the Nervous, Allex, and Adult groups of tissues ( Table 2 ). Notably, the arm tips showed the highest variation in gene expression among the analyzed anatomical structures, likely due to the biological peculiarities of the octopus’ arm that maintains the ability of regeneration and indeterminate growth throughout adult ontogeny ( Fossati et al, 2013 , 2015 ; Nödl et al, 2015 ; Zullo et al, 2017 ; Tarazona et al, 2019 ; e.g., Zullo et al, 2019 ; van Giesen et al, 2020 ; see also De Sio and Imperadore, 2023 ).…”

The salamander blastema within the broader context of metazoan regeneration

“…Amongst Spiralia, several annelid and nemertean lineages can regenerate large portions of the anterior-posterior axis, while other lineages appear to have lost this ability completely ( Bely et al, 2015 ). Mollusk regeneration is relatively unexplored, and research has primarily focused on the regeneration of neurons and neural structures in a limited number of groups ( Moffett, 1995 ; 2000 ; Matsuo and Ito, 2011 ; Bely et al, 2015 ; Imperadore et al, 2017 ; De Sio and Imperadore, 2022 ). Cephalopods can regenerate limbs, but the underlying molecular biology of this process remains uninvestigated ( Zullo et al, 2017 ; De Sio and Imperadore, 2022 ; Imperadore et al, 2022 ).…”

“…Mollusk regeneration is relatively unexplored, and research has primarily focused on the regeneration of neurons and neural structures in a limited number of groups ( Moffett, 1995 ; 2000 ; Matsuo and Ito, 2011 ; Bely et al, 2015 ; Imperadore et al, 2017 ; De Sio and Imperadore, 2022 ). Cephalopods can regenerate limbs, but the underlying molecular biology of this process remains uninvestigated ( Zullo et al, 2017 ; De Sio and Imperadore, 2022 ; Imperadore et al, 2022 ). When observed, regeneration in arthropods, annelids, and mollusks employs an epimorphic blastema, though the relatedness of these structures remains unresolved even within these clades.…”

Lampreys and spinal cord regeneration: “a very special claim on the interest of zoologists,” 1830s-present

“…Young completed his education at Oxford in 1928, where he read zoology, neurophysiology, and some comparative anatomy ( Young, 1996 ). In 1929, Young began a fellowship at the Stazione under the Italian physiologist Enrico Sereni, commencing investigations of the autonomic nervous systems of fishes and degeneration and regeneration in octopus’ pallial nerves ( Sereni and Young, 1932 ; De Leo, 2008 ; Imperadore and Fiorito, 2018 ; De Sio and Imperadore, 2023 ). Young soon also grew interested in the lateral line nerves of fishes, understood today to be sensory systems involved in vibration and motion detection.…”

“…19th-century biologists had examined the robust structural and functional regeneration that takes place in the CNS of invertebrates and the mammalian PNS (Stahnisch, 2003(Stahnisch, , 2016(Stahnisch, , 2022. Investigators so far in the 20th century had focused on optic nerve regeneration in frogs and toads, amphibian tail and spinal cord regeneration, chemical factors inducing nerve growth in chick embryos, spinal cord regeneration in goldfish, and nervous system regeneration in crustaceans and cephalopods (Sperry, 1943(Sperry, , 1945Cohen and Levi-Montalcini, 1956;Bernstein, 1964;Clemente, 1964;Hoy et al, 1967;Larner et al, 1995;Meyer, 1998;Allen, 2004;Imperadore and Fiorito, 2018;De Sio and Imperadore, 2023). Santiago Ramón y Cajal also had examined what he called the "plastic" capacities of the mammalian CNS from the 1890s through the 1930s, concluding that while some sprouting of damaged axons was possible, this regrowth had uncertain functional relevance (Ramón y Cajal, 1928;Stahnisch and Nitsch, 2002;Stahnisch, 2003).…”