Floor plate, glia and other support cells in the anterior nerve cord of amphioxus larvae

Abstract

 AbstractLacalli, T.C. and Kelly, S.J. 2002. Floor plate, glia and other support cells in the anterior nerve cord of amphioxus larvae. -Acta Zoologica (Stockholm) 83 : 87-98 Serial electron micrograph reconstructions and interval series were used to examine the support cells, including glia, of the anterior nerve cord in 6-, 8and 12.5-day larvae of Branchiostoma floridae and one newly metamorphosed juvenile. The floor plate begins immediately behind the infundibular cells. It consists for the most part of a single file of midline cells, but adjacent lateral floor plate cells occur in some places. The floor plate is interrupted at one point, in the posterior part of the cerebral vesicle above the tegmental neuropile. A class of early developing axons crosses the midline at this point, which suggests that the floor plate may have a developmental role in axon guidance. The structural integrity of the cord is maintained by ependymal and ependymoglial cells that attach to its sides. Two other glial cell types were found in larvae. Both appear to originate adjacent to the floor plate and hence are referred to here as midline glia. Those in somites 1 and 2 remain connected to the central canal; they appear to be a mixed population that may include precursors of midline support cells which are present later in the juvenile. Those caudal to somite 3 detach early from the central canal and develop an extensive network of axial processes; they are referred to here as axial glia and treated as a subcategory of midline glia. Based on their site of origin and the absence of glial filaments, their closest counterpart among vertebrate glia may well be the oligodendrocyte. To our knowledge, this is the first report of a possible amphioxus homologue of this important vertebrate cell type.

Distribution of tyrosine hydroxylase, dopamine, and serotonin in the central nervous system of amphioxus (Branchiostoma lanceolatum): Implications for the evolution of catecholamine systems in vertebrates

“…Nevertheless, the caudal limit of Otx expression still separates the “anterior brain” (corresponding to the vertebrate forebrain and part of the midbrain), from the “posterior brain” (corresponding to the vertebrate hindbrain; Holland and Holland, 1999; Holland and Chen, 2001). In addition, the morphology and distribution of various neuronal and glial cells observed in larval and adult amphioxus supported the hypothesis of the cephalochordate anterior brain as broadly homologous to vertebrate forebrain and midbrain (Lacalli et al, 1994; Lacalli, 1996, 2002a, b).…”

The evolutionary origins of glia

“…The neural tube, peripheral nerves, and epidermis are bordered by a continuous basal lamina that separates them as a single tissue from the rest of the body (Ruppert, 1997). Several types of glia are described by Lacalli and Kelly (2002) in the anterior neural tissue of amphioxus larvae. The ependymoglia are characterized by prominent filament bundles, similar to vertebrate radial glia.…”

A Notch-regulated proliferative stem cell zone in the developing spinal cord is an ancestral vertebrate trait

“…It presents a transient anterior swelling called the cerebral vesicle (Wicht and Lacalli, 2005). In larvae, the CNS posterior to the cerebral vesicle seems to be made of a single layer of cells, surrounded by axon tracts (Lacalli and Kelly, 2002). Electron microscopy shows it may be a little more complicated in adults, though not much (Bone, 1960).…”

“…In amphioxus scattered neurons of different types, some of which express marker genes found in specific subpopulations of vertebrate spinal cord cells, are found in the nerve cord posterior to the anterior swelling called the cerebral vesicle. In larvae the neural tube appears to be one cell thick (Lacalli and Kelly, 2002), though may develop more cells by the time adulthood is reached. Tunicates like Ciona have just 4 cells in the posterior neural tube, and no neurons, though this is inferred to be a secondary loss of complexity.…”