A, C, E: Paraffin sagittal sections through the subcommissural organ (SCO) of a PN2 rat after a single injection of AFRU into a lateral ventricle (inset 1C, labeled Ab). A: Immunostaining using AFRU (raised in rabbits) as primary antibody. The SCO is immunoreactive. Arrow points to RF-material. Insert: Detailed magnification of RF fibrils immunoreactive with AFRU (arrows). C: Adjacent section to that shown in A, immunostained using anti-rabbit IgG as primary antibody to reveal the antibody (AFRU) administered in vivo. The latter appears exclusively bound to the newly released secretory material aggregated on the surface of the SCO to form the pre-RF (square) and to aggregated RF-material lying in the ventricle (arrow). E: Detailed magnification of area framed in C showing the location of the antigen-antibody complexes formed in vivo at the pre-RF layer on the SCO surface and at the aggregated RF-material. B, D, F: Paraffin sagittal sections through the SCO of a PN2 rat after a single injection of anti-P15 into a lateral ventricle. B: Sagittal section through the SCO immunostained with anti-P15. The ependymal cells of the SCO appear immunostained (arrow). Lower insert: detailed magnification of the secretory ependyma (E) showing the location of the immunoreactive material in the supranuclear cytoplasm. n: nuclei. Upper insert: detailed magnification of RF fibrils immunoreactive with anti-P15 (arrows). D: Section immunostained with anti-rabbit IgG as primary antibody to reveal the location of the antibody administered in vivo at pre-RF (arrow). F: Detailed magnification of area framed in previous figure showing the location of the antigen-antibody complexes formed in vivo at the pre-RF (arrows). IIIV: third ventricle. Most sections counterstained with hematoxilin.