Reissner's Substance Expressed As a Transient Pattern in Vertebrate Floor Plate

The function of the floor plate in dorso-ventral patterning of the developing nervous system and in the guidance of commissural axons is well established. However, several morphological aspects concerning the exact localization of its rostral and caudal end and the regional and temporal specialization are still controversial. We present new insights revealed by the expression of Reissner's substance in the floor plate during early neurogenesis of zebrafish, Xenopus, chick and rat. We used a polyclonal antiserum raised against Reissner's substance, which is a secretory product of radial glia in the roof plate of the adult vertebrate brain. In early embryonic stages the rostral boundary of floor plate immunoreaction vary in the different vertebrates. Immunoreactive cells are not only present in the epichordal region (rat) but also in prechordal areas of the midbrain (chick) and forebrain (zebrafish and Xenopus). During further development, Reissner's substance expression disappears first in the most rostral areas and later also in the spinal cord. However, immunopositive labelling in the isthmus region at the mes-metencephalic boundary, described originally as the flexural organ, is most extensive and detectable during a long period of embryonic development. It is proposed that the gradual restriction of Reissner's substance expression to the isthmus reflects the complex differentiation processes in this region also in later embryonic development. Furthermore, the expression pattern in zebrafish indicates that Reissner's substance could play a role in axonal decussation.