Human meiosis IX. Crossing over and chiasma formation in oocytes

Abstract

Human oocytes from foetal ovaries have been analyzed by serial sectioning and three dimensional reconstruction from electron micrographs. Analysis of five late zygotene, twenty pachytene, four early and three mid diplotene nuclei allowed the following observations and conclusions: 1) Nuclei at pachytene can by morphological criteria be allocated into three substages. 2) The synaptonemal complex complement is twice as long in the oocytes as in the spermatocytes. 3) The total number of nodules decreases from a mean of 68 at late zygotene to a mean of 30 at early diplotene. During pachytene, an increasing number of nodules transforms into bars. A mean of 6 bars was observed in late zygotene nuclei compared to a mean of 17 at late pachytene. 4) Nodules and bars are distributed nonrandomly among the bivalents and bivalent arms. The observed frequency of bivalents devoid of recombination structures is only between one- and two-thirds of the frequency expected if nodules were distributed at random. 5) The distribution of recombination structures along the bivalent arms is not uniform. More nodules and bars than expected from a random distribution are located in the vicinity of telomeres. Comparison of neighbour nodule/bars distances at different substages of pachytene indicated that the elimination of recombination structures primarily affects nodules and bars close to each other. 6) The distribution of recombination structures along the individual bivalents was uneven, regions with high frequency of recombination nodules and bars alternating with regions of low frequency. This pattern is very similar to that reported for human spermatocytes, which indicates the same distribution of crossovers in the two sexes. 7) Elimination of the synaptonemal complex starts at early diplotene at a limited number of sites until about 68 intact segments are created. 8) Chiasmata are not morphologically recognizable after the elimination of the synaptonemal complex is completed at mid diplotene. © 1985 Carlsberg Laboratory.