Somatomotor system of the adult amphioxus (Branchiostoma lanceolatum) revealed by an anticalretinin antiserum: An immunocytochemical study

Abstract

In a classic study with silver staining methods, the somatomotor system of the amphioxus spinal cord was described as consisting of three different types of neuron segmentally arranged in two opposite fan-shaped group types (Bone [1960] J Comp Neurol 115:27-64). The present study reports the presence of calretinin-like immunoreactivity in the somatomotor system of the amphioxus, which allows us to reevaluate old descriptions of amphioxus motoneurons. In the spinal cord, two types of calretinin-like immunoreactive (CR-ir) motoneurons, large and small, sent processes toward the ventrolateral region of the cord, where they branched and gave rise to processes coursing longitudinally in the somatomotor bundles. These processes produced a number of long and thin collaterals directed to several neuropil regions. Short collaterals were directed to the region of the neuromuscular contacts at the ventrolateral surface of the cord. The groups of CR-ir motoneurons exhibited a segmental organization and were localized only facing the myomeres, i.e., opposite to the entrance of the dorsal nerve roots, which is at variance with the above-mentioned classical report. CR-ir motoneurons were also observed in the brain between a level just rostral to the nerve III entry and nerve VI. The CR-ir somatomotor bundle ascended to the region of the neuromuscular junction of myomere 1. Additional faintly CR-ir neurons were observed in the region of the lamellate body of the brain. Our results reveal for the first time that calretinin immunoreactivity in the central nervous system of amphioxus was limited to a few types of neuron and that calretinin was not expressed in the peripheral nervous system, unlike vertebrates.

Neuronal organization of the brain in the adult amphioxus (Branchiostoma lanceolatum): A study with acetylated tubulin immunohistochemistry

“…The Journal of Comparative Neurology | Research in Systems Neuroscience family (Castro et al, 2004). These cells form alternate groups along the spinal cord, unlike the pattern described by Bone (1960).…”

“…Somatic motoneurons in the brain of both larval and adult amphioxus are nontranslumenal cells that send processes only ipsilaterally (Lacalli and Kelly, 2003a;Castro et al, 2004;present results), as with most vertebrate motoneurons. In adults they were present in intermediate and caudal brain regions that are adjacent to myomeres (Castro et al, 2004; present results), but not rostral to them, in line with observations in embryos and early larvae (Lacalli and Kelly, 2003a;Candiani et al, 2008a).…”

“…The specificity of the immunostaining in amphioxus was tested by preincubation of the primary antiserum with human CR recombinant protein (0.73 lM; SWant). These controls completely abolished the immunoreaction in the amphioxus nervous system (Castro et al, 2004).…”

“…GA1159, RRID: AB_2314445; dilution 1/1,000; for details see Anad on et al, 1998), NPY (Castro et al, 2003), and CR (Castro et al, 2004) were used for comparison. We also used paraffin series of transverse sections of formalin-fixed amphioxus that were stained with a silver impregnation method (Holmes, 1953).…”

“…The calretinin antibody has been used in amphioxus (Castro et al, 2004) and characterized by Western blot in various groups of fishes (see Graña et al, 2013). The specificity of the immunostaining in amphioxus was tested by preincubation of the primary antiserum with human CR recombinant protein (0.73 lM; SWant).…”