No Correlation between Endo- and Exoskeletal Regenerative Capacities in Teleost Species

Zebrafish Fin: Complex Molecular Interactions and Cellular Mechanisms Guiding Regeneration

The zebrafish caudal fin has become a popular model to study cellular and molecular mechanisms of regeneration due to its high regenerative capacity, accessibility for experimental manipulations, and relatively simple anatomy. The formation of a regenerative epidermis and blastema are crucial initial events and tightly regulated. Both the regenerative epidermis and the blastema are highly organized structures containing distinct domains, and several signaling pathways regulate the formation and interaction of these domains. Bone is the major tissue regenerated from the progenitor cells of the blastema. Several cellular mechanisms can provide source cells for blastemal (pre-)osteoblasts, including dedifferentiation of differentiated osteoblasts and de novo formation from other cell types, providing intriguing examples of cellular plasticity. In recent years, omics analyses and single-cell approaches have elucidated genetic and epigenetic regulation, increasing our knowledge of the surprisingly complex coordination of various mechanisms to achieve successful restoration of a seemingly simple structure.

The Fish Family Poeciliidae as a Model to Study the Evolution and Diversification of Regenerative Capacity in Vertebrates

The capacity of regenerating a new structure after losing an old one is a major challenge in the animal kingdom. Fish have emerged as an interesting model to study regeneration due to their high and diverse regenerative capacity. To date, most efforts have focused on revealing the mechanisms underlying fin regeneration, but information on why and how this capacity evolves remains incomplete. Here, we propose the livebearing fish family Poeciliidae as a promising new model system to study the evolution of fin regeneration. First, we review the current state of knowledge on the evolution of regeneration in the animal kingdom, with a special emphasis on fish fins. Second, we summarize recent advances in our understanding of the mechanisms behind fin regeneration in fish. Third, we discuss potential evolutionary pressures that may modulate the regenerative capacity of fish fins and propose three new theories for how natural and sexual selection can lead to the evolution of fin regeneration: (1) signaling-driven fin regeneration, (2) predation-driven fin regeneration, and (3) matrotrophy-suppressed fin regeneration. Finally, we argue that fish from the family Poeciliidae are an excellent model system to test these theories, because they comprise of a large variety of species in a well-defined phylogenetic framework that inhabit very different environments and display remarkable variation in reproductive traits, allowing for comparative studies of fin regeneration among closely related species, among populations within species or among individuals within populations. This new model system has the potential to shed new light on the underlying genetic and molecular mechanisms driving the evolution and diversification of regeneration in vertebrates.

Skeletal Biology and Disease Modeling in Zebrafish

Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Zebrafish can regenerate endoskeleton in larval pectoral fin but the regenerative ability declines

We show for the first time endoskeletal regeneration in the developing pectoral fin of zebrafish. The developing pectoral fin contains an aggregation plate of differentiated chondrocytes (endochondral disc; primordium for endoskeletal components, proximal radials). The endochondral disc can be regenerated after amputation in the middle of the disc. The regenerated disc sufficiently forms endoskeletal patterns. Early in the process of regenerating the endochondral disc, epithelium with apical ectodermal ridge (AER) marker expression rapidly covers the amputation plane, and mesenchymal cells start to actively proliferate. Taken together with re-expression of a blastema marker gene, msxb, and other developmental genes, it is likely that regeneration of the endochondral disc recaptures fin development as epimorphic limb regeneration does. The ability of endoskeletal regeneration declines during larval growth, and adult zebrafish eventually lose the ability to regenerate endoskeletal components such that amputated endoskeletons become enlarged. Endoskeletal regeneration in the zebrafish pectoral fin will serve as a new model system for successful appendage regeneration in mammals.