Synaptonemal complexes at premeiotic interphase in the mouse spermatocyte.

Male mice were injected intraperitoneally with 125 microCi (1 Ci = 3.7 X 10(10) becquerels) of [3H]thymidine at 1-hr intervals and killed 1 hr after the second injection. Testes were prepared for bright-field and electron microscopic autoradiography. Primary spermatocytes, identified by light microscopy to be at the premeiotic interphase stage, were found to be heavily labeled. Electron microscopic examination disclosed the coincidental occurrence of synaptonemal complexes and label within the nuclei of premeiotic interphase spermatocytes, indicating synapses of homologues had begun during the S phase. The significance of this finding for the traditional view of meiosis is discussed.