Concordia discors: duality in the origin of the vertebrate tail

Abstract

The vertebrate tail is an extension of the main body axis caudal to the anus. The developmental origin of this structure has been a source of debate amongst embryologists for the past century. Some view tail development as a continuation of the morphogenetic processes that shape the head and trunk (i.e. gastrulation). The alternative view, secondary development, holds that the tail forms in a manner similar to limb development, i.e. by secondary induction. Previous developmental studies have provided support for both views. Here I revisit these studies, describing caudal morphogenesis in select vertebrates, the associated genes and developmental defects, and, as a relevant aside, consider the developmental and evolutionary relationships of primary and secondary neurulation.I conclude that caudal development enlists both gastrulation and secondary induction, and that the application of recent high-resolution cell labelling technology may clarify how these discordant programmes interact in building the vertebrate tail.

Early Palaeozoic dentine and patterned scales in the embryonic catshark tail

“…These observations indicate that chondrichthyan scales on different regions of the body can be considered as independent developmental modules. We suggest that the 'patterned tail scale' module represents the more ancient, relative to the scattered body scales, which may be related to suggestions that the tail is itself an independent developmental unit relative to the rest of the body (reviewed in Handrigan 2003). Data on pattern regulation and histology of tail scales in other taxa, and the histology of bilaterally axially patterned scales on the body during early ontogeny, for example, in holocephalans (Didier 2004) and other elasmobranchs (Miyake et al 1999;Freitas & Cohn 2004) would test these hypotheses.…”

Mechanisms driving neural crest induction and migration in the zebrafish andXenopus laevis

“…More posterior neural crest form later and perhaps are subject to different inductive and migratory signals, in analogy to the mechanisms involved in the origin of trunk and tailbud dermomyotome. 33,34 Following their initial specification, a common ancestor ( Fig. 1), it is not surprising that there have been substantial changes in the molecular and cellular underpinnings of what appears, superficially, to be the same process-that is, neural crest formation, migration and differentiation.…”

Oscillatory lunatic fringe activity is crucial for segmentation of the anterior but not posterior skeleton

“…Later in development (~10.0 dpc) the tailbud forms, and further mesodermal cells arise from this structure (Gossler and Tam, 2002). Although this distinction between primary body formation (giving rise to cervical, thoracic and lumbar vertebrae) and secondary body formation (giving rise to post-anal structures) was originally proposed in 1925 (Holmdahl, 1925), it remains unclear how, and to what extent, the genetic regulation of somitogenesis between these two processes may vary (reviewed by Handrigan, 2003).…”