Visualization of DNA in pachytene by monoclonal antibodies against BrdU reveals synaptonemal complex-like structures

Most of the techniques used to visualize structures of the synaptonemal complex (SC) are based on specific staining properties or immunocytochemical detection of proteinaceous SC components. The SC is therefore considered to be mostly protein. We have now accomplished visualization of the DNA within the SC by use of the BrdU antibody technique, following BrdU substitution during the last premeiotic S phase. Preparations of mouse meiotic chromosomes were obtained by spreading on a water surface. The DNA content in the SCs, which appeared as light threads, was clearly lower than the DNA content in the surrounding chromatin. At higher magnification, dark, longitudinal structures appeared in these threads. These structures are made up of DNA, which forms the inner part of the lateral elements of the SCs. Within the SCs, DNA is confined mostly to these threads. Thus, DNA staining reveals the same structures already known from protein staining of SCs. The DNA mass surrounding the SCs is often nonhomogeneously distributed along the chromosome axis. The more dense parts appear to be chromomeres. The DNA staining technique described in this paper may therefore be a useful complement to standard protein staining techniques for pachytene chromosomes.