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In ray-finned fishes, are what exactly are the fins made of? Is it modified skin, dead tissue like scale or nail on humans, or something completely different? Can the fin heal?

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In teleosts, the fin skeleton is made of structures called lepidotrichia and actinotrichia, whereas homologue structures called ceratotrichia are found in the fins of the chondrichties. These are covered by living skin. Lepidotrichia are calcified, segmented and branched bone-like rays that extend along the whole length of the fin, shaping it. Actinotrichia are mostly made of collagen and are found at the distal end of the ray, where they provide flexible support to the fin edge1.

Fins are able to regenerate, and this capability has been studied in a few different teleosts2,3,4.

References:

  1. I. Durán, M. Marí-Beffa, J.A. Santamaría, J. Becerra, L. Santos-Ruiz, Actinotrichia collagens and their role in fin formation, Developmental Biology, Volume 354, Issue 1, 2011, DOI: 10.1016/j.ydbio.2011.03.014.
  2. J. BECERRA, L. C. U. JUNQUEIRA, I. J. BECHARA, G. S. MONTES, Regeneration of Fin Rays in Teleosts: A Histochemical, Radioautographic, and Ultrastructural Study, Archives of Histology and Cytology, 1996, Volume 59, Issue 1, DOI: 10.1679/aohc.59.15
  3. Norman E. Kemp, Jae Ho Park, Regeneration of lepidotrichia and actinotrichia in the tailfin of the teleost Tilapia mossambica, Developmental Biology, Volume 22, Issue 2, 1970, DOI: 10.1016/0012-1606(70)90157-0.
  4. Uemoto, T., Abe, G. & Tamura, K. Regrowth of zebrafish caudal fin regeneration is determined by the amputated length. Sci Rep 10, 649 (2020). DOI: 10.1038/s41598-020-57533-6
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