Cellular, bone-like tissue in the bucklers of the thornback ray Raja clavata (Batoidea, Chondrichthyes)

Chondrichthyans have lost the cellular bone characteristic of other jawed vertebrate skeletons. However, we identify cellular bone-like tissue in the ‘bucklers’ of one group of extant batoids (rays). Bucklers are modified scales, distinctive for some rays, with enlarged bases. As placoid scales, they possess crowns of orthodentine and osteodentine, but a unique basal tissue. This consists of a cell-rich material, previously misidentified as an acellular tissue. The newly formed basal tissue grows appositionally and episodically from a cell-rich periosteum-like layer. This closely resembles cellular bone, with entombed cells situated between bundles of attachment fibres anchoring the buckler to the underlying dermal tissue and the ‘periosteum’ to the buckler surface. In histologically more mature buckler tissue, the cell spaces and attachment fibres are remodelled, forming enlarged fluid-filled spaces. The result is a unique mineralized tissue in these rays, initially sharing similarities with cellular bone, but with an unusual mature state where cell spaces are modified throughout the base, possibly through resorption. Our findings of cellular bone forming the attachment tissues in ray bucklers demonstrate the chondrichthyan capacity to deposit bone-like tissues within the odontode module, contrary to previous understandings of hard tissue evolution in vertebrates.

Methods
CT-scanning

A single buckler specimen was CT-scanned in Science Innovation Platforms, Natural History Museum (NHM), on a Zeiss Radia Versa equipped with a 10W sealed transmission source at 100kV, 10µA and an HE1 source filter. 3001 projections were generated at 7.1µm voxel size.  Scans were rendered using Avizo 3D 2021.2 (https://www.thermofisher.com/uk/), including virtual slices and rotatable models.