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

The nervous system of amphioxus: structure, development, and evolutionary significance

Amphioxus neuroanatomy is important not just in its own right but also for the insights it provides regarding the evolutionary origin and basic organization of the vertebrate nervous system. This review summarizes the overall layout of the central nervous system (CNS), peripheral nerves, and nerve plexuses in amphioxus, and what is currently known of their histology and cell types, with special attention to new information on the anterior nerve cord. The intercalated region (IR) is of special functional and evolutionary interest. It extends caudally to the end of somite 4, traditionally considered the limit of the brain-like region of the amphioxus CNS, and is notable for the presence of a number of migrated cell groups. Unlike most other neurons in the cord, these migrated cells detach from the ventricular lumen and move into the adjacent neuropile, much as developing neurons do in vertebrates. The larval nervous system is also considered, as there is a wealth of new data on the organization and cell types of the anterior nerve cord in young larvae, based on detailed electron microscopical analyses and nerve tracing studies, and an emerging consensus regarding how this region relates to the vertebrate brain. Much less is known about the intervening period of the life history, i.e., the period between the young larva and the adult, but a great deal of neural development must occur during this time to generate a fully mature nervous system. It is especially interesting that the vertebrate counterparts of at least some postembryonic events of amphioxus neurogenesis occur, in vertebrates, in the embryo. The implication is that the whole of the postembryonic phase of neural development in amphioxus needs to be considered when making phylogenetic comparisons. Yet this is a period about which almost nothing is known. Considering this, plus the number of new molecular and immunocytochemical techniques now available to researchers, there is no shortage of worthwhile research topics using amphioxus, of whatever stage, as a subject.

Anatomy of the Hesse photoreceptor cell axonal system in the central nervous system of amphioxus

The present study reports the organization of the Hesse cell axonal system in the central nervous system of the amphioxus, with the use of a polyclonal antiserum raised against lamprey gonadotropin-releasing hormone-I (GnRH-I). In the spinal cord, the rhabdomeric photoreceptor cells of the bicellular organs were well labeled with this antibody. These cells sent smooth, straight, lateral processes that bent and became beaded as they passed ventrally and crossed to the contralateral side of the cord. There, the processes of several cells aggregated to give rise to a longitudinal fiber bundle. Beaded collaterals of these processes were directed to ventral neuropil and did not appear to contact giant Rohde cell axons. The crossed projections of the Hesse photoreceptors are compared with those of vertebrate retinal ganglion cells. Other antisera raised against GnRH weakly labeled rhabdomeric photoreceptors located dorsally in the brain, the Joseph cells. The finding that GnRH antibodies label amphioxus photoreceptor cells and axons is not definitive proof that the photoreceptors contain GnRH. Regardless of whether the antibody recognizes amphioxus GnRH, which has not yet been identified by structure, the antibody has revealed the processes of the Hesse photoreceptor cells.

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

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.

Distribution of neuropeptide Y immunoreactivity in the central and peripheral nervous systems of amphioxus (Branchiostoma lanceolatum Pallas)

Immunocytochemistry techniques were employed to investigate the distribution of neuropeptide Y-like-immunoreactive (NPY-ir) cells and fibers in the central and peripheral nervous systems of adult amphioxus. NPY-ir neurons of the commissural type were abundant in the brain and present but more scarce in the spinal cord. These neurons gave rise to conspicuous NPY-ir tracts that coursed along the entire length of the nerve cord. Some fibers exhibited conspicuous Herring body-like swellings. In the peripheral nervous system, small NPY-ir neurons and a large number of thin, beaded NPY-ir fibers were observed in the atrial region, indicating the involvement of this substance in visceral regulation. A few NPY-ir fibers, possibly afferent to the spinal cord, coursed in the ventral branches of the spinal nerves of this region, whereas no NPY-ir fibers coursed in the preoral or velar nerves or in the dorsal branches of the other spinal nerves. These results indicate that NPY is widely used as a neuroregulator/neurotransmitter in the central and peripheral nervous systems of this primitive chordate. In addition, this study demonstrates the presence of tall, thin NPY-ir cells in the putative adenohypophyseal homologue, the Hatschek's pit organ, which is located in the roof of the preoral cavity (vestibule).