A critical review of the 'neugliederung' concept in relation to the development of the vertebral column.

The literature on the early embryonic development of the vertebral column in various animal species was analyzed to evaluate so many unrelated or contradictory observations. The recurring problems are described. One of the first was the lack of correspondence between the metameric boundaries of the 'primitive vertebral bodies' arising from the somites and those of the adult vertebral bodies, as presumably shown by their relationship to the vertebral processes and spinal nerves. A century ago, Remak introduced the concept of 'Neugliederung', according to which the ultimate vertebral body boundaries are determined by a shift of a half segment in comparison with the earlier segment boundaries. Another question was about the nature of the structures the primitive segments give rise to (axial skeletal tissue and/or muscular tissue and/or nervous tissue), which led to the fundamental problem which system (skeleton or musculature) retains its segmentation. The observation of what was called the intervertebral fissure led to a far-reaching modification of the 'Neugliederung' concept, making a regrouping of parts of the primitive vertebral bodies both possible and probable. The functional necessity for the alternation of muscle and skeleton primordia was also assumed. Although the difference in rostro-caudal level between the boundaries of the somites and the definitive vertebral bodies, measured for instance in relation to the position of the intersegmental vessels, does not support a resegmentation of the vertebral column, because the somite is not a precursor of the adult vertebra, few authors have rejected this view. Subsequently, the discussion focussed on the problem of a) the animal species in which and b) the time at which the intervertebral fissure occurs, and the relationship of this fissure to the primitive vertebral cavity. Under the influence of comparative embryology, a number of authors concentrated mainly on identifying certain embryonic primordia as homologues of phylogenetic vertebral elements, and also on attempting to determine the recombination by which such elements could form a definitive vertebra. According to these authors, not only the vertebral bodies but also the arches and ribs undergo a 'Neugliederung'. Another variant of the 'Neugliederung' concept envisages a gradual shift of the original boundaries of the somites with respect to the transverse level of the definitive vertebrae with or without migration of cellular material. A critical consideration of the concepts at issue, and notably of the 'Neugliederung' concept, is presented. Certain phenomena held to be pillars of the 'Neugliederung' theory could not be confirmed. The functional interpretation of the resegmentation was also reduced to its proper proportions by the analysis. A development of the axial skeleton without resegmentation is just as conceivable in functional terms as one in which resegmentation occurs. A functionally meaningful relationship between muscles and skeletal elements is possible in both cases...