Transient midline raphe glial structure in the developing rat.

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.