The Reissner's Fiber Termination in Some Lower Chordates

Abstract

The caudal end of the neural tube of the tunicate Oikopleura, the cephalochordate Branchiostoma and newly hatched fry of the clupeiform teleosts Clupea, Engraulis and Sardinops was studied by means of the electron microscope. In Oikopleura and the teleost larvae either Reissner's fiber or an amorphous mass of fiber substance leaks out of the neural tube into the surrounding tissue spaces. In Branchiostoma the disintegrated fiber material is apparently engulfed by the caudal ependymal cells. A relationship seems to exist between the degree of fiber disintegration within the neural tube and the degree of specialization of the caudal neural tube ependymal cells, the two extremes being represented by Branchiostoma with a “closed” Reissner's fiber system with highly specialized caudal ependymal cells and a teleost fry with the intact fiber leaving the neural tube between almost undifferentiated ependymal cells. These observations on lower chordates are in accordance with the hypothesis that Reissner's fiber acts as a detoxicator for the neural tube fluid.

Ultrastructural Study of the Filum Terminate and Caudal Ampulla of the Spinal Cord of Amphioxus (Branchiostoma lanceolatum Pallas)

“…Neither neural pores nor free passage of R F material through the walls of the CA towards the connective tissue were found in this study, in agreement with light microscope observations by Olsson (1955) and Hofer (1959), and consistent with the presence of junctions between ependymal cells in amphioxus. The existence of large lysosome-like bodies in ependymal cells of the amphioxus CA has previously been noted by Obermuller-Wilen & Olsson (1974). These dense bodies very probably correspond with the Gomori (+) droplets of the CA walls, also observed by Olsson (1955) and Hofer (1959) but are thought to be situated intercellularly.…”

Amphioxus neuroglia: Molecular characterization and evidence for early compartmentalization of the developing nerve cord

“…scospondin (sspo) is a phylogenetically ancient glycoprotein that, in most metazoans, is secreted by specialized cells of the nervous system (Table 2) (Gobron et al, 1999; Helm et al, 2017; Holmberg & Olsson, 1984; Mashanov et al, 2009; Olsson, 1956, 1972; Olsson et al, 1994). In vertebrates, sspo is secreted into the lumen of the central nervous system and aggregates to form a thread‐like structure, called Reissner's fiber, which extends caudally to the terminal ampulla of the spinal cord, where it is disassembled (Obermüller‐Wilén & Olsson, 1974; Troutwine et al, 2020). During development, sspo is first produced by the floor plate, which expands laterally at the midbrain‐hindbrain boundary, forming the flexural organ (Meiniel et al, 2008; Olsson, 1956).…”

The Enigmatic Reissner’s Fiber and the Origin of Chordates

“…In addition, it is of interest to recall that the posterior spinal cord innervating the chordate tail is produced by secondary neurulation, i.e., the formation of a secondary neural tube derived from tail bud mesenchyme, that later coalesces with the main neural tube ( Handrigan, 2003 ; Beaster-Jones et al, 2008 ; Henrique et al, 2015 ). In cephalochordates, urochordates and vertebrates the tail’s NCa contains a RF ( Obermüller-Wilén and Olsson, 1974 ), and a RF is also present in the regenerating tail of lepidosaurians and other vertebrates ( Meiniel et al, 1996 ; Alibardi, 2021 ). Therefore, it would be highly interesting to investigate whether RF contributed to the elongation and maintenance of the secondary neural canal in the tailbud of early chordates.…”