Ascidians as excellent chordate models for studying the development of the nervous system during embryogenesis and metamorphosis

Abstract

The swimming larvae of the chordate ascidians possess a dorsal hollowed central nervous system (CNS), which is homologous to that of vertebrates. Despite the homology, the ascidian CNS consists of a countable number of cells. The simple nervous system of ascidians provides an excellent experimental system to study the developmental mechanisms of the chordate nervous system. The neural fate of the cells consisting of the ascidian CNS is determined in both autonomous and non-autonomous fashion during the cleavage stage. The ascidian neural plate performs the morphogenetic movement of neural tube closure that resembles that in vertebrate neural tube formation. Following neurulation, the CNS is separated into five distinct regions, whose homology with the regions of vertebrate CNS has been discussed. Following their larval stage, ascidians undergo a metamorphosis and become sessile adults. The metamorphosis is completed quickly, and therefore the metamorphosis of ascidians is a good experimental system to observe the reorganization of the CNS during metamorphosis. A recent study has shown that the major parts of the larval CNS remain after the metamorphosis to form the adult CNS. In contrast to such a conserved manner of CNS reorganization, most larval neurons disappear during metamorphosis. The larval glial cells in the CNS are the major source for the formation of the adult CNS, and some of the glial cells produce adult neurons.

Evolution of bilaterian central nervous systems: a single origin?

“…The problem is that although they do mediate A/P patterning of the CNS in bilaterians [45,46], they mediate A/P patterning of other tissues as well [47-54]. Thus, while their expression patterns have been used to infer homologies between the CNS in insects and vertebrates, it remains possible that they patterned the entire body axis of the Urbilaterian and were independently coopted into the CNSs of protostomes and deuterostomes.…”

The CNS connectome of a tadpole larva of Ciona intestinalis (L.) highlights sidedness in the brain of a chordate sibling

“…Not only does its development result from a fixed pattern of cell lineage and result in a mere ~ 2600 cells in the larva of Ciona intestinalis (Satoh, 1999), but the genome, first in Ciona intestinalis (Dehal et al., 2002) and now in nine other species (Brozovic et al, 2016), has been sequenced. Even though the events of early neural development and the nervous system’s subsequent metamorphosis have been identified, together with many of their underlying causal gene networks (Satoh, 2003; Sasakura et al, 2012), the detailed cellular organization of their product, the CNS of the swimming larva, still remains almost entirely unresolved.…”

Cdc25 and the importance of G2control

“…3B). 43 By prolonging G 2 during this event, the epidermal cells can use their cytoskeleton for cell shape changes without the competing demands of mitosis. Although microtubules are an integral part of the cytoskeleton for cell shape and spindle fiber formation, these results suggest that restricting Cdc25 activity is required specifically in cells that rely heavily on actin and myosin for morphogenetic movements.…”