Morphogenesis, pattern formation and function of the dentition ofHeterodontus (Selachii)

Summary

As in all skarks, the ectoderm ofHeterodontus folds in behind the jaw cartilage during embryogenesis. The anterior part of this ectodermal fold becomes organized into the Inner Enamel Epithelium which cooperates with the mesenchyme. Already during the infolding, both tissues begin to form teeth. This process begins with a spontaneous division of the fold along its long axis into tooth-forming and non tooth-forming tissue sections. In this way the tooth formula of the “first dentition” is established. The Inner Enamel Epithelium and the mesenchyme only gradually attain competence for tooth formation, so that the first formed tooth germs become incomplete tooth shards (Fig. 8). Shortly before the end of the embryonic phase, as soon as the infolding stops, the tooth transport mechanism begins to work.Heterodontus hatches with a characteristic “first dentition”, which has a typical dental formula of 17to19/13to15.

Because of allometric growth, the number of tooth families increases strongly in the course of a lifetime, but to different degrees in the various species. The “first dentition” is composed of teeth having numerous needle-like cusps; it is only slightly heterodont and serves in feeding on soft-bodied benthonic animals. In the course of ontogeny, a highly heterodont dentition becomes differentiated. The anterior part of the adult dentition is composed of teeth with 1–3 cusps, while the distal part bears molariform teeth. On the basis of these distal teeth, two groups of species may be distinguished: a primitive group (Francisci-type) distributed in the Indopacific and having slender, keeled crushing teeth (probably specialized for a diet of echinoderms), and a group of species derived from it (Portusjacksoni-type) found only in the western Pacific and having broad, powerful crushing teeth capable of cracking hardshelled molluscs.

It is characteristic that the most distal tooth family of the first dentition becomes the main crushing tooth within the molariform group in the course of ontogeny. This tooth family lies at the point where the most force can be brought to bear by the jaw apparatus, and remains there life-long; that is, it does not change position relative to the jaw.

Since the infolded ectoderm lies on the inner side of the jaw, its growth is controlled by the growth of the jaw. The jaw on the Portusjacksoni-type grows forward sharply to form a beak so that the infolded ectoderm must grow forward with it and thus can form extra tooth families. The infolded ectoderm expands outwards in the course of ontogeny toward the distal ends of the rami. As soon as a gap appears between tooth primordia in this fold, it is filled by a new tooth germ whether the gap results from forward migration of an already-formed tooth or from elongation of the folded ectoderm during growth.

In theHeterodontus dentition there are two tooth-form gradients: On one hand, tooth-form in each family changes in the course of time; on the other, the dentition is heterodont at each ontogenetic stage. Thus, within each dentition there is a tooth form and tooth size gradient. Tooth form and size, therefore, are determined by two coordinate parameters:

• 1. the age of the animal and

• 2. the position in the dentition.

The observations on the dentition ofHeterodontus lead to the interpretation that the insertion of new teeth and tooth families, as well as their form and size, is regulated by a complex system of reference points which transmit positional information to the tooth-forming cells.