Meeting Abstract
Tesserae are minute, mineralized tiles covering the cartilaginous skeletons of elasmobranch fishes, linked to one another via flexible joints. Despite tesserae being a defining feature of these fishes, little is known about their development and 3-dimensional, ultrastructural organization, partly from the lack of standardization in sectioning plane, skeletal elements and species investigated. Our approach integrates histology, electron microscopy and synchrotron and laboratory µCT scans of ontogenetic series of round stingray Urobatis halleri skeletons, showing that the tessellated pattern commonly recognized in adult animals results from isolated mineralized platelets that appear early in development and grow toward one another until they collide to form a continuous tessellation. Backscatter electron microscopy reveals striking, formerly unrecognized patterns of mineral heterogeneity in tesserae, features of oscillating mineral density that likely contribute to structural reinforcement of the tissue. High-res CT data indicate that the joints between tesserae are far more structurally complex than previously believed, involving complicated arrangements of mineralized and fibrous materials, but no structural interdigitations. Inter-species comparisons reveal that despite large variation in tesseral shape and size, there are commonalities in tesseral features, especially in mineral density and cell distribution, suggesting universal principles of tesseral growth and form across elasmobranchs. We provide new insights into the development and function of this ancient tissue type, proposing a system of accretive growth and local inhibition to explain the tessellated pattern distinct to these fishes.
