The Concept of Bone Tissue in Osteichthyes

Abstract

The purpose of this paper is to highlight the difficulties encountered when attempting to give a histological definition of bone tissue in Osteichthyes. Although the three basic components of bone tissue can be present (i.e. osteocytes, an organic matrix, and a mineral phase), it has long been known that bony tissues in Osteichthyes can lack trapped cells and/or mineral. This phenomenon has blurred the classical distinction between the generally adopted categories of connective tissues in a way that the osteichthyan skeleton should be described rather in terms of a continuum of structures. This paper illustrates this by discussing the evolutionary trends, within the Osteichthyes, of acellularization (i.e. the acquisition of acellular bone within the various osteichthyan lineages) and of loss of capacity of mineralization (e.g. in the case of isopedine of the basal plate of elasmoid scales). A further example of the difficulty of classifying skeletal tissues within bony fishes is provided by chondroid bone, a tissue with characteristics intermediate between cartilage and bone and found mostly in articular areas in the head of Teleostei. Each of the bone and bone-derived tissues of the aforementioned continuum represents the outcome of developmental and functional constraints, which appear to be more diverse in Osteichthyes than in Tetrapoda.