Human meiosis VIII. Chromosome pairing and formation of the synaptonemal complex in oocytes

Abstract

Human oocytes from three foetal ovaries have been investigated by serial sectioning and three dimensional reconstruction of nuclei from electron micrographs. Analysis of three leptotene-early zygotene, five mid zygotene and ten late zygotene oocytes including complete reconstruction of five zygotene nuclei have led to the following observations and conclusions. 1) Formation of the synaptonemal complex involves, as in other organisms, the organization of a lateral component along each chromosome, attachment of the telomere regions to the inner membrane of the nuclear envelope at leptotene, redistribution of the attached telomeres during the formation of a chromosome bouquet at early zygotene and thereafter progressive chromosome pairing with the synaptonemal complex. 2) Chromosome and bivalent interlockings as well as breaks were common at mid zygotene, but absent when pairing and synaptonemal complex formation were almost complete. Most of the breaks had occurred in the vicinity of presumptive interlockings and appear to be instrumental in the resolution of interlockings. 3) Nonhomologous pairing involving short regions of the complement were observed in two of the mid zygotene nuclei, thus resulting in associations of three or more chromosomes. 4) Twelve of the 23 bivalents can be identified by relative lengths, centromere indices and the structural markers on chromosomes 1, 6, 9, 13–15 and 21–22. 5) The length of the lateral component complement exceeds that of the spermatocytes by a factor of two. 6) On an average 58 recombination nodules was observed per nucleus at mid zygotene (61% of the chromosome complement paired) and 70 at late zygotene (98% of the chromosome complement paired). Thus in spite of the twofold longer synaptonemal complexes in the oocytes, the number of nodules is the same in both sexes.