Seminars on the development of bone and teeth

Abstract

Professor Ann Huysseune, Research Group Evolutionary Developmental Biology, Biology Department, Ghent University, Belgium:

Zebrafish, like other members of the carp family, have lost teeth on the oral jaw margins, but retained teeth on the last (seventh) pharyngeal arch (so-called pharyngeal teeth). After a brief introduction into the anatomy of this pharyngeal dentition, differences and similarities with mammalian teeth will be highlighted. As in most tooth-bearing vertebrates, teeth in zebrafish are replaced continuously throughout life, a capacity lost in mammals. Thus, a number of studies in our lab have focused on the mechanism possibly underlying this capacity for continuous renewal, including the role of Wnt signalling, stem cells and vascularisation. Ongoing research focuses on the evolutionary origin of teeth in jawed vertebrates, again taking advantage of zebrafish. It is now generally accepted that teeth share a common ancestry with skin denticles, that are part of the dermal skeleton. Skin denticles and teeth start their development by the interaction between an epithelium and the underlying mesenchyme. An unanswered question is how external epithelia could have transferred odontogenic (tooth-forming) competence to internal epithelia (i.e., in the oropharynx). We have recently demonstrated that pharyngeal teeth in zebrafish derive from endodermal epithelium, yet only after two conditions are met: (1) the endodermal epithelium (which makes the enamel organ of the teeth) must first be covered by periderm-like cells, and (2) the endodermal pouch anterior to the seventh pharyngeal arch (and in which the posteriormost gill slit will form) must have made contact with the skin, establishing a physical link between external (ectoderm) and internal (endoderm) epithelium. It is known that excess retinoic acid (RA) can expand the zebrafish dentition over the more anterior arches. We are currently testing hypotheses on the possible relationship between retinoic acid, nitric oxide, and the signalling molecule sonic hedgehog, for regulating tooth formation throughout the extent of the oropharynx.

Abstract

Professor Paul Eckhard Witten, Research Group Evolutionary Developmental Biology, Biology Department, Ghent University, Belgium:

The current use of zebrafish as a model in biomedical research has generated additional tasks for a lab specialized in research about the evolution and development of fish skeletal tissues. Vertebrate skeletal development is largely conserved. Still, the long evolution in the aquatic environment has also generated differences between the zebrafish (teleost) and the human (mammalian) skeleton. The first part of the seminar will address key differences concerning skeletal design, skeletal cells, skeletal metabolism and skeletal development.

The second part focuses on the development of the vertebral column. Our understanding about the function of the notochord in patterning the vertebral column has been revolutionised. In zebrafish (teleosts) the notochord, not the somites, are responsible for the formation of vertebral centra anlagen; only the neural and hemal arches are under somitic control, a clear example of uncoupling of two developmental processes. Uncoupling of bone formation and bone mineralisation is another example for which we have collected experimental evidence. In Atlantic salmon and in Zebrafish, low dietary phosphorus (P) intake does not stop regular bone growth. Rather, bone matrix formation is increased. The new bone lacks minerals but can mineralise nevertheless once P levels in the diet are increased. In the Chihuahua zebrafish, a model for osteogenesis imperfecta (OI, brittle bone disease), the OI phenotype can be partly rescued with a low phosphorus diet. Ongoing studies address the function of osteocytes in this process and the question if the arrest of bone matrix mineralisation under low dietary phosphorus conditions is an active or passive process.

Host: Steen Vang Petersen