Transient midline raphe glial structure in the developing rat

Abstract

A major glial structure is present during development within the midline raphe of the midbrain, hindbrain, and cervical spinal cord of the rat. It is composed of great numbers of glial cell bodies lying immediately ventral to the cerebral ventricular system and the large radial processes extending from these cells toward the ventral surface of the brain roughly within the midsagittal plane. There is also a smaller group of glial cells on the dorsal surface of the aqueduct and the central canal whose processes extend to the dorsal surface of the brain. The entire structure exhibits an intensely positive immunoreactivity with the antibody to the S-100 protein, a nervous-system-specific protein found primarily in the cytoplasm of astrocytes. This immunoreactivity makes possible a clear visualization of the extent, magnitude, and continuity of this structure from at least embryonic day 15, the first age examined, until postnatal days 7-8, when it is no longer visible by this technique. This glial structure has several prominent morphological characteristics. During prenatal and early postnatal development the fibers forming the ventral aspect of the structure in the midbrain and hindbrain are formed into two parallel plates on either side of the midline with S-100-negative tissue between the plates. As development progresses, S-100-positive fibers are continually added so that the plates become thicker at the expense of the nonstaining intervening area. By postnatal day 4 only a single midline plate of fibers is visible, occupying the entire midline raphe. In the region of the pontine flexure the entire structure takes on a distinctly pleated configuration. This fact produces a curious "sine wave" appearance when the plane of section crosses these vertical pleats. At postnatal day 5 the structure begins to disappear, and it is no longer visible by 7-8 days postnatal. This glial structure does not stain with antisera to glial fibrillary acidic protein, a protein associated with fibrous astrocytes, or routine cell stains such as cresyl violet. With these techniques the raphe area appears essentially devoid of identifiable cellular elements.

Cortical Axon Guidance by the Glial Wedge during the Development of the Corpus Callosum

“…Midline glial populations have long been shown to be associated with the formation of commissures and boundaries between neuromeres (Levitt and Rakic, 1980;Van Hartesveldt et al, 1986;Mori et al, 1990;Silver et al, 1993;Marcus and Easter, 1995;Cummings et al, 1997;Fitch and Silver, 1997;Pires-Neto et al, 1998). During the formation of the anterior commissure in mammals, glia develop on either side of the tract, forming a tunnel-like structure as the fibers of the anterior commissure begin to cross the midline (Cummings et al, 1997;Pires-Neto et al, 1998).…”

“…In vertebrates, roof plate glia in the dorsal midline of the spinal cord prevent growing axons from crossing the midline (Snow et al, 1990) through the actions of bone morphogenetic proteins (Augsburger et al, 1999). In the midline raphe of the midbrain, hindbrain, and cervical spinal cord, an extensive radial glial structure exists both during development and in adulthood (Van Hartesveldt et al, 1986;Mori et al, 1990) called the midline raphe glial structure (MRGS). The glial cells that make up the MRGS label with both an antibody against S-100 protein (Van Hartesveldt et al, 1986) and with the monoclonal antibody R2D5 (Mori et al, 1990) but are not GFAP-positive (Van Hartesveldt et al, 1986).…”

“…In the midline raphe of the midbrain, hindbrain, and cervical spinal cord, an extensive radial glial structure exists both during development and in adulthood (Van Hartesveldt et al, 1986;Mori et al, 1990) called the midline raphe glial structure (MRGS). The glial cells that make up the MRGS label with both an antibody against S-100 protein (Van Hartesveldt et al, 1986) and with the monoclonal antibody R2D5 (Mori et al, 1990) but are not GFAP-positive (Van Hartesveldt et al, 1986). The MRGS forms a continuous band of radial glial fibers separating the right and left brainstem but with some interruptions to allow for the passage of decussating fibers.…”

Optic chiasm and infundibular decussation sites in the developing rat diencephalon are defined by Glial Raphes expressing p35 (Lipocortin 1, Annexin I)

“…Immunohistochemistry of the floor plate has shown that the median p35 raphe is flanked by paramedian bands of another Ca' +-binding protein, ,3100-p (van Hartesveldt et al, 1986;McKanna, unpublished). We find no evidence of paramedian SlOO-p bands at the developing chiasm.…”

Modulators of axonal growth and guidance at the brain midline with special reference to glial heparan sulfate proteoglycans

“…It has been known for some time that nonneuronal cells in the dorsal brain stem midline differ from their lateral neighbors in their ability to take up exogenous label borne by afferent axons (Barradas et al 1989, Kageyama andRobertson 1993) as well as in glycogen deposition, binding of antibodies against S-100 protein, RC1 and R2D5 anti- gens, cytoskeletal components and annexins (Van Hartesveldt et al 1986, Barradas et al 1989, Snow et al 1990a, Mori et al 1990, McCabe and Cole 1992, Hamre et al 1996, Wu et al 1995. It is interesting to notice that both the floor plate and the roof plate at both cord and midbrain (median ventricular formation) levels, together with the lamina terminalis, are part of the discrete embryonic sites containing cells which have exited the cell cycle very early during embryogenesis (Raedler et al 1981, Kahane andKalcheim 1998).…”