General

The origin and evolutionary history of the dentition

Teeth are elements of the dermal skeleton, present in a wide range of – typically - jawed vertebrates. Because of their excellent preservation in the fossil record, the important evolutionary information they contain and their paradigmal status in developmental research, many paleontological and neontological disciplines focus on teeth. Our research group focuses on questions related to the development and to the evolution of teeth.

Development

What is the embryonic origin of the teeth, how do teeth develop and how has development evolved in various lineages? Which signalling pathways are involved in tooth development, in particular, what is the mechanism that provides lifelong replacement of the teeth in most non-mammalians? Stem cells could be a key element of lifelong tooth replacement. Epithelial stem cells have been identified in integumental structures such as hairs, in continuously growing teeth of rodents, and in the gut. We propose that epithelial stem cells are involved in the process of continuous tooth replacement that characterizes non-mammalian vertebrates, as exemplified by the zebrafish (Huysseune & Thesleff 2004, Huysseune & Witten 2008). Arguments are based on morphological observations of tooth renewal in the zebrafish and on the similarities between molecular control of hair and tooth formation. Dissection of the molecular cascades underlying regulation of the epithelial stem cell niche might open perspectives for new regenerative treatment strategies in clinical dentistry.

Evolution

Where and when did teeth arise in evolutionary times? According to the classical theory teeth derive from skin odontodes that invaded the oral cavity in conjunction with the origin of jaws (the “outside in” theory). We have recently proposed a revised "outside in" hypothesis (Huysseune et al., 2009). We suggest that teeth may have arisen before the origin of jaws, as a result of competent, odontode-forming ectoderm invading the oropharyngeal cavity through the mouth as well as through the gill slits, interacting with neural-crest derived mesenchyme.

Research objects

We use – mostly – teleost fish to answer a number of these questions: teleost fish are often homodont, but some species are heterodont s.l., all teleosts are polyphyodont. Several species are easily reared as lab animals. In addition, species like the zebrafish have become model organisms well suited for genetic, molecular and developmental studies. The possibility of large scale mutagenesis and the rapidly increasing number of transgenic lines available for zebrafish adds to the advantages of working with this species.

Development, plasticity, and remodelling of skeletal tissues

A second line of research in our lab focuses on development, plasticity, and remodelling of skeletal tissues, in particular bone, cartilage and tissues intermediate between bone and cartilage, in teleost fish. The vertebrate skeleton is an organ system with a high degree of plasticity in terms of size and shape of skeletal elements, their internal microstructure and cellular composition, as well as in skeletal meristic characters. Lifelong, and beyond development, the vertebrate skeleton responds to altered intrinsic and/or environmental conditions. Once formed, skeletal tissues are not static but subjected to constant reshaping and remodelling or even replacement/regeneration (teeth and antlers). Changes in the skeleton can be achieved through plasticity of skeletogenic cells (modulation), transdifferentiation of skeletal tissues into other skeletal tissues (metaplasia) and resorption of skeletal tissues and their replacement by another skeletal tissue type (remodelling). Processes are analyzed within a comparative zoological and evolutionary context. Projects and collaborations are linked to biomedical research (using fish model organisms) and to research on skeletal deformities in farmed fish.

Development

Current biomedical research projects concern the fusion and the establishment of vertebral body identity. We attempt to contribute to the understanding of the fusion process through detailed, comparative, histo-morphological analyses and comparison of spatial and temporal patterns of expression of a number of key genes involved in skeletal development and remodelling. We investigate alternative pathways of cartilage subdivision in the pectoral fin endoskeleton of the zebrafish and we characterise bone resorption patterns and bone resorbing cells in teleost fish with acellular bone (bone without osteocytes).

Pathology

Current aquaculture-related studies concern the impact of mechanical load on the development of spinal deformities, patterns of early vertebral mineralisation in advanced marine teleosts, and the impact of inflammatory factors on the development of spinal deformities.