Rodionov R.D.1, Temereva E.N.2 2026. Cartilage-like tissues in Bilateria: organization, development, and homology // Invert. Zool. Vol.23. No.2: 183–206 [in English].
1 Invertebrate Zoology Department, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
2 Evolutionary Biology Department, Lomonosov Moscow State University, Leninskie Gory 1-12, Moscow 119234 Russia.
Elena Temereva: temereva@mail.ru ORCID https://orcid.org/0000-0001-7791-0553
Roman Rodionov: romario17022003@gmail.com
doi: 10.15298/invertzool.23.2.01
ABSTRACT: The study of invertebrate cartilage-like tissues is always accompanied by several problems, in which the greatest one is the absence of a single definition that encompasses full diversity of these structures. Attempts to establish a universal definition of cartilaginous tissue across all Bilateria often lead to the exclusion of certain tissues that could be classified as cartilaginous with only minor adjustments to the criteria. This not only limits their inclusion in specialized reviews but also results in their omission from subsequent studies. Historically, this issue has contributed to the recognition of cartilaginous tissues exclusively within Chordata. Today, this approach has led to hypotheses suggesting independent origins of cartilaginous tissues in different groups of Bilateria. The aim of this review is to propose a new framework for studying cartilage‑like tissues in invertebrate Bilateria by introducing a broad, flexible definition that better accounts for their diversity, and to provide an analytical review based on this definition. This review examines cartilage‑like tissues in various groups of invertebrate Bilateria (Annelida, Mollusca, Arthropoda, Lophophorata, Hemichordata) and, unlike previous studies, this approach emphasizes not only the histological structure of these tissues but also their topological position during ontogeny relative to the overall body plan of the animals. Additionally, it takes into account existing data on structural and regulatory genes involved in the formation of these tissues. A detailed analysis of the structure, development and topology of cartilaginous tissues in invertebrate animals made it possible to formulate the following definition for it: this is a tissue localized in the blastocele and provides a support function due to the mechanical properties of its cellular component or extracellular matrix, consisting of fibrillary protein and amorphous substance. This perspective enables a more comprehensive analysis of the evolutionary origins and development of cartilage-like tissues. In particular, this approach supports the hypothesis that, at least in Spiralia, structures composed of cartilage‑like tissues may be homologous at the level of the tissue primordium, since they are topologically associated with the central nervous system in addition to previously recognized shared characteristics, such as histological structure and matrix components. The literature review found no evidence contradicting this hypothesis; at the same time, several arguments supporting it were identified. A common origin of cartilage‑like structures across all Bilateria is also discussed in this review, which systematizes existing knowledge and advances hypotheses on the evolution of cartilage‑like tissues in Bilateria.
KEY WORDS: cartilage, cartilage-like tissues, invertebrate cartilage, extracellular matrix, non-cellular cartilage, mucopolysaccharides.