The Enigmatic Reissner's Fiber and the Origin of Chordates.

Francisco Aboitiz,Juan F Montiel

doi:10.3389/fnana.2021.703835

Francisco Aboitiz, Juan F Montiel

Open Access

https://doi.org/10.3389/fnana.2021.703835

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Abstract

Reissner’s fiber (RF) is a secreted filament that floats in the neural canal of chordates. Since its discovery in 1860, there has been no agreement on its primary function, and its strong conservation across chordate species has remained a mystery for comparative neuroanatomists. Several findings, including the chemical composition and the phylogenetic history of RF, clinical observations associating RF with the development of the neural canal, and more recent studies suggesting that RF is needed to develop a straight vertebral column, may shed light on the functions of this structure across chordates. In this article, we will briefly review the evidence mentioned above to suggest a role of RF in the origin of fundamental innovations of the chordate body plan, especially the elongation of the neural tube and maintenance of the body axis. We will also mention the relevance of RF for medical conditions like hydrocephalus, scoliosis of the vertebral spine and possibly regeneration of the spinal cord.

Highlights

The phylum Chordata, including lancelets, sea squirts, and vertebrates, is partly characterized by the presence of a segmented musculature flanking a fibrous notochord that defines a semi-rigid anteroposterior axis, and a muscular postanal tail used for swimming
Another typical feature of chordates is that the central nervous system is shaped as a hollow neural tube that runs from head to tail, filled with cerebrospinal fluid (CSF) that circulates along the neural canal (NCa) (Figure 1; Wicht and Lacalli, 2005; Aboitiz and Montiel, 2007; Glover and Fritzsch, 2009; Striedter and Northcutt, 2020)
We propose the additional participation of Reissner’s fiber (RF), another highly conserved but much less conspicuous feature of chordates, which probably collaborated with the notochord in the origin of this animal group

Summary

INTRODUCTION

The phylum Chordata, including lancelets (cephalochordates -Amphioxus), sea squirts (urochordates), and vertebrates, is partly characterized by the presence of a segmented musculature flanking a fibrous notochord that defines a semi-rigid anteroposterior axis, and a muscular postanal tail used for swimming. Another typical feature of chordates is that the central nervous system is shaped as a hollow neural tube that runs from head to tail, filled with cerebrospinal fluid (CSF) that circulates along the neural canal (NCa) (Figure 1; Wicht and Lacalli, 2005; Aboitiz and Montiel, 2007; Glover and Fritzsch, 2009; Striedter and Northcutt, 2020). Since it was first described by Reissner (1860) in the lamprey, it has remained an enigmatic component of the chordate neural tube, to which many functions have been ascribed

Composition and Comparative Structure

Sites of Production

Roles in Later Neuronal Development and Function

Development of a Straight Body Axis

ROLE OF RF IN EARLY CHORDATE EVOLUTION