Reiterative pattern of sonic hedgehog expression in the catshark dentition reveals a phylogenetic template for jawed vertebrates

For a dentition representing the most basal extant gnathostomes, that of the shark can provide us with key insights into the evolution of vertebrate dentitions. To detail the pattern of odontogenesis, we have profiled the expression of sonic hedgehog , a key regulator of tooth induction. We find in the catshark ( Scyliorhinus canicula ) that intense shh expression first occurs in a bilaterally symmetrical pattern restricted to broad regions in each half of the dentition in the embryo jaw. As in the mouse, there follows a changing temporal pattern of shh spatial restriction corresponding to epithelial bands of left and right dental fields, but also a subfield for symphyseal teeth. Then, intense shh expression is restricted to loci coincident with a temporal series of teeth in iterative jaw positions. The developmental expression of shh reveals previously undetected timing within epithelial stages of tooth formation. Each locus at alternate, even then odd, jaw positions establishes precise sequential timing for successive replacement within each tooth family. Shh appears first in the central cusp, iteratively along the jaw, then reiteratively within each tooth for secondary cusps. This progressive, sequential restriction of shh is shared by toothed gnathostomes and conserved through 500 million years of evolution.

Cell lineage in the developing neural tube

Acquisition of cell type specific properties in the spinal cord is a process of sequential restriction in developmental potential. A multipotent stem cell of the nervous system, the neuroepithelial cell, generates central nervous system and peripheral nervous system derivatives via the generation of intermediate lineage restricted precursors that differ from each other and from neuroepithelial cells. Intermediate lineage restricted neuronal and glial precursors termed neuronal restricted precursors and glial restricted precursors, respectively, have been identified. Differentiation is influenced by extrinsic environmental signals that are stage and cell type specific. Analysis in multiple species illustrates similarities between chick, rat, mouse, and human cell differentiation. The utility of obtaining these precursor cell types for gene discovery, drug screening, and therapeutic applications is discussed.Key words: stem cells, oligodendrocytes, astrocytes, neurons, spinal cord.