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

Abstract

Amphioxus (Cephalochordata) belongs to the most basal extant chordates, and knowledge of their brain organization appears to be key to deciphering the early stages of evolution of vertebrate brains. Most comprehensive studies of the organization of the central nervous system of adult amphioxus have investigated the spinal cord. Some brain populations have been characterized via neurochemistry and electron microscopy, and the overall cytoarchitecture of the brain was studied by Ekhart et al. (2003; J. Comp. Neurol. 466:319-330) with general staining methods and retrograde transport from the spinal cord. Here, the cytoarchitecture of the brain of adult amphioxus Branchiostoma lanceolatum was reinvestigated by using acetylated tubulin immunohistochemistry, which specifically stains neurons and fibers, in combination with some ancillary methods. This method allowed reproducible staining and mapping of types of neuron, mostly in brain regions caudal to the entrance level of nerve 2, and its comparison with spinal cord populations. The brain populations studied and discussed in detail were the Retzius bipolar cells, lamellate cells, Joseph cells, various types of translumenal cells, somatic motoneurons, Rohde nucleus cells, small ventral multipolar neurons, and Edinger cells. These observations expand our knowledge of the distribution of cell types and provide additional data on the number of cells and the axonal tracts and commissural regions of the adult amphioxus brain. The results of this comprehensive study provide a framework for comparison of complex adult populations with the early brain neuronal populations revealed in developmental studies of the amphioxus.

An anterior signaling center patterns and sizes the anterior neuroectoderm of the sea urchin embryo

“…Although there are significant differences in the nervous systems created from this early territory among the deuterostomes, the ANE initially arises from a relatively simple, flat neuroepithelium during the early stages of development and many of the cell types that this territory produces are remarkably conserved (Burke et al, 2014;Castro et al, 2015;Cavodeassi, 2013;Garner et al, 2015;Pani et al, 2012;Range, 2014). Based on the spatial and temporal expression of ANE regulatory genes in several species, it appears that the ANE gene regulatory network (GRN) hierarchy is also remarkably conserved among many deuterostome embryos, suggesting that these could be homologous territories (Range, 2014).…”

Amphioxus photoreceptors - insights into the evolution of vertebrate opsins, vision and circadian rhythmicity

“…It seems that the LB processes might repress the startle reaction and contribute to the hovering of larvae during swimming. Later in development compact LB disaggregates (possibly due to forward expansion of JCs), since only scattered lamellate cells were detected in adults (Castro et al, 2015;Meves, 1973). As described earlier, amphioxus larvae display a typical circadian rhythm guided behavior (diurnal migration), while adults live mostly borrowed in the sand during both day and night.…”

“…In larvae, JCs are probably located posterior to the LB, while in adults the JCs form a cap above the scarce lamellate cells. About 400-450 JCs are present in the adult amphioxus (Castro et al, 2015). Studies of the ultrastructure of JCs confirmed their rhabdomeric character.…”

Organization of alpha‐transducin immunoreactive system in the brain and retina of larval and young adult Sea Lamprey (Petromyzon marinus), and their relationship with other neural systems

“…The existence of photoreceptors in the central nervous system appears to be an ancestral feature in chordates, because the brain and spinal cord of amphioxus contains at least four different populations of photoreceptors (Koyanagi, Kubokawa, Tsukamoto, Shichida, & Terakita, ; Castro, Becerra, Manso, Sherwood, & Anadón, ; Castro, Becerra, Manso, & Anadón, ; Vopalensky et al, ). Non‐mammalian vertebrates possess a wide range of intrinsically photoreceptive sites expressing members of the non‐visual opsin family (see Hankins, Davies, & Foster, ).…”