Abstract:

One of the hallmarks of tetrapod bone is the presence of numerous cells (osteocytes) within the matrix. Osteocytes are
vital components of tetrapod bone, orchestrating the processes of bone building, reshaping and repairing (modeling and
remodeling), and probably also participating in calcium-phosphorus homeostasis via both the local process of
osteocytic osteolysis, and systemic effecton the kidneys. Given these critical roles of osteocytes, it is thought-provoking
that the entire skeleton of many fishes consists of bone material that does not contain osteocytes. This raises the
intriguing question of how the skeleton of these animals accomplishes the various essential functions attributed to
osteocytes in other vertebrates, and raises the possibility that in acellular bone some of these functions are either
accomplished by non-osteocytic routes or not necessary at all. In this review,we outline evidence for and against the fact
that primary functions normally ascribed to osteocytes, such as mechanosensation, regulation of osteoblast/clast
activity and mineral metabolism, also occur in fish bone devoid of these cells, and therefore must be carried out through
alternative and perhaps ancient pathways. To enable meaningful comparisons with mammalian bone, we suggest
thorough, phylogenetic examinations of regulatory pathways, studies of structure and mechanical properties and
surveys of the presence/absence of bone cells in fishes. Insights gained into the micro-/nanolevel structure and
architecture of fish bone, its mechanical properties and its physiology in health and disease will contribute to the
discipline of fish skeletal biology, but may also help answer questions of basic bone biology.