EXPRESSION OF A LACZ TRANSGENE REVEALS FLOOR PLATE CELL MORPHOLOGY AND MACROMOLECULAR TRANSFER TO COMMISSURAL AXONS

The floor plate is situated at the ventral midline of the neural tube and is an important intermediate target for commissural axons. During elongation, these axons converge bilaterally on the ventral midline ne ural tube and after crossing the floor plate make an abrupt rostral tu rn. Ample evidence indicates that the initial projection of commissura l axons to the floor plate is guided by a chemotropic factor secreted by floor plate cells. However, the way in which the subsequent interac tion of these axons with the floor plate leads them to make further tr ajectory changes remains undefined. In an effort to gain further under standing of the structure and function of floor plate cells, we have t aken advantage of a line of transgenic mice in which these cells expre ss beta-galactosidase and thus can be stained by histochemical means. In this line, a genomic imprinting mechanism restricts the expression of the lacZ transgene to only a proportion of the floor plate cells, a llowing their morphology to be appreciated with particular clarity. Ou r analysis revealed that the basal processes of floor plate cells are flattened in their rostrocaudal dimension and possess fine lateral bra nches which are aligned with commissural axons. Unexpectedly, beta-gal actosidase activity was also detected within longer transverse linear profiles traversing the floor plate whose ultrastructural appearance w as not that of floor plate cells but instead corresponded to that pf c ommissural axons. Enzyme activity was not detected in more proximal ax onal segments or in the neuronal cell bodies from which these axons or iginated. Therefore, we propose that the transgene product, and potent ially other proteins synthesized by floor plate cells, can be transfer red to decussating axons.