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.