The Trabecula cranii: development and homology of an enigmatic vertebrate head structure

Abstract

Abstract The vertebrate cranium consists of three parts: neuro-, viscero- and dermatocranium, which differ in both developmental and phylogenetic origin. Traditionally, developmental origin has been used as a criterion for homology, but this becomes problematic when skull elements such as the parietal bone are now shown, by modern fate-mapping studies, to have different developmental origins in different groups of tetrapods. This indicates a flexibility of developmental programmes and regulatory pathways which has probably been very important in cranial evolution. The trabecula cranii is an intriguing cranial element in the anterior cranial base in vertebrates. It forms a viscerocranial part of the neurocranium and is believed to be neural crest-derived in gnathostomes, but a similarly named structure in lampreys has been shown to have a mesodermal origin. Topographically, this trabecula seems to be homologous to the gnathostome trabecula cranii, and might also have the same function: to form a border between adjacent morphogenetic domains, to constrain and redirect growth of both brain and stomodeum and thus to refine developmental schedules of both. We suggest that such a border zone can recruit cells from either the mesoderm (as in the lamprey) or from the neural crest (as in the gnathostomes investigated), and still retain its homology. In our view, the trabecula is an interface element that integrates the respective divergent morphogenetics programs of the preotic head into a balanced unit; we suggest that such a definition can be used to define "the sameness" of this element throughout vertebrates.