Vertebrate Head Metamerism: The Current State of an Old Problem

Abstract

The problem of vertebrate head metamerism was first posed more than 200 years ago. This issue gave rise to two approaches: segmentalism, which relies on the model of the typical head segment, and antisegmentalism, which rejects the existence of cephalic segments altogether. The truth, probably, lies somewhere in the middle. Data on the development of the head mesoderm, cranial neural crest, and cranial nerves lead to the conclusion that there are five metameres in the vertebrate head: premandibular, mandibular, hyoid, 1st metahyoid, and 2nd metahyoid. According to the hypothesis of F. Edgeworth, this number of head metameres is primary; the head spreading back occurred as a result of the polymerization of visceral arches inside the nervus vagus metamere which had originally included only the 2nd metahyoid arch. According to the hypothesis of C. Gegenbaur, initially, the visceral arches were more numerous in this area and the branchial nerves merged into a single nervus vagus system only secondarily. The Cambrian paleontology may assist in resolving this controversy. In any case, vertebrate head metamerism is primarily heteronomous (as interpreted by P.P. Ivanov): the premandibular and mandibular metameres are morphologically atypical and lie beyond the Hox gene expression zone. Developmental biology shows that the bodies of other metameric animals are arranged on the basis of similar principles.