Spatial Organization and Mineralization of the Basal Plate of Elasmoid Scales in Osteichthyans

TLDR: The various characteristic organizations of the skeletal tissues or isopedine that constitute the basal plate of osteichthyan elasmoid scales, all are varieties of bone tissue that have undergone more or less important specialization linked to the general regression of dermal ossifications and to functional adaptations.

Abstract: In Sarcopterygii ( Latimeria, Neoceratodus, Protopterus, Leptdosiren ) and Amiidae (Amia) collagen fibrils of the basal plate are packed in bundles whereas they remain isolated in Teleostei. The basal plate looks like plywood, a system of superimposed layers of parallel fibers or fibrils the directions of which rotate with a regular angle in two successive layers. The double twisted plywood is constituted of two imbricate systems, the odd and the even, where the rotation of the fibrillar directions is right-handed in Sarcopterygii and lefthanded in Amiidae and numerous primitive Teleostei. The orthogonal plywood, with its two main orthogonal fibrillar directions, characterizes the evolved Teleostei and some more primitive ones. In most teleostean species, as in Amia and Protopterus , mineralization of the basal plate in elasmoid scales involves Mandl's corpuscles that mineralize without contact with a pre-existing calcified tissue; they grow and coalesce with the neighbouring ones and fuse to the mineralizing front. Their shape is directly influenced by the local arrangement of the collagenous fibrils. In two teleostean families (Osteoglossidae and Mormyridae) Mandl's corpuscles are completely lacking but spreading of mineralization in the basal plate has a peculiar aspect. Whatever that may be, the various characteristic organizations of the skeletal tissues or isopedine that constitute the basal plate of osteichthyan elasmoid scales, all are varieties of bone tissue that have undergone more or less important specialization linked to the general regression of dermal ossifications and to functional adaptations.