Meeting Abstract

S2-1.1  Thursday, Jan. 3  A Brief History of Theories Regarding Vertebrate Head Organization NORTHCUTT, R. Glenn; University of California, San Diego rgnorthcutt@ucsd.edu

In vertebrate heads, morphology is extremely complex and comprises numerous iterative structures arising from all embryonic germ layers. The search for a fundamental plan uniting these serial structures dates back 200 years. The first theory, termed the vertebral theory of skull organization, was proposed by W. Goethe and L. Oken in the early 19th Century. They believed that the rostral trunk vertebrae moved forward into the head to form the skull bones. This theory was subsequently elaborated by R. Owen as part of his archetype, but it was rejected by T.H. Huxley in the 1858 Croonian Lecture, in which he demonstrated that skull development is radically different from vertebral development. In 1878, Francis Balfour claimed that head mesoderm in sharks is divided into somites and hypomeric subdivisions (branchiomeres) that are serially homologous to trunk mesodermal divisions. This theory was elaborated by Edwin Goodrich in 1918 and 1930 and had a remarkably long life until Drew Noden demonstrated in 1983 that branchiomeric muscles arise from cephalic paraxial mesoderm, not lateral plate mesoderm. In fact, it is now unclear whether cephalic paraxial mesoderm (somitomeres) is even segmented. Although details of the development of cephalic paraxial mesoderm remain unclear, it is now known that the brain and spinal cord are divided into a series of rostrocaudal segments (neuromeres) and that generally there are two neuromeres aligned to a single branchial arch and its associated muscles. The situation becomes even more complex when additional iterative series, the pharyngeal pouches and ectodermally derived neurogenic placodes, are added to the equation. At present, it is unlikely that all cephalic iterative structures can be accommodated in any segmental theory.