The metencephalic floor plate of chick embryos expresses two secretory glycoproteins homologous with the two glycoproteins secreted by the subcommissural organ

The nature and the function of the compounds secreted by the floor plate (F P) of the metencephalon are little known. The FP cells of the hindbrain rea ct with antibodies (AFRU) against the glycoproteins secreted by the subcomm issural organ (SCO). One of the these proteins, RF-Gly I, is a 540-kDa core glycosylated protein. The aims of the present investigation were to identi fy by immunoblot the AFRU-immunoreactive compound secreted by the FP of chi ck embryos, to establish temporal and regional patterns of this secretory a ctivity, and to obtain information about the fate of these compounds. It wa s established that the SCO and FP of chick embryos secrete two AFRU-immunor eactive compounds of 540 and 230 kDa. The two compounds secreted by the FP have been designated as FP-Gly I and FP-Gly II. The expression of these pro teins was circumscribed to the metencephalic FP, and occurred from HH 29 to HH 36. Within the FP cells, FP-Gly I and FP-Gly II were confined to the su pranuclear and apical regions, which under the electron microscope displaye d numerous cisternae of the rough endoplasmic reticulum and granules. Aggre gates of AFRU-immunoreactive material appeared on the free surface of the F P. The possibility that FP-Gly I and FP-Gly II are released into the ventri cle to reach distant targets is discussed.