Regulation of Meiotic Prophase One in Mammalian Oocytes

In female mammals, meiotic prophase one begins during fetal development. Oocytes transition through the prophase one substages consisting of leptotene, zygotene, and pachytene, and are finally arrested at the diplotene substage, for months in mice and years in humans. After puberty, luteinizing hormone induces ovulation and meiotic resumption in a cohort of oocytes, driving the progression from meiotic prophase one to metaphase two. If fertilization occurs, the oocyte completes meiosis two followed by fusion with the sperm nucleus and preparation for zygotic divisions; otherwise, it is passed into the uterus and degenerates. Specifically in the mouse, oocytes enter meiosis at 13.5 days post coitum. As meiotic prophase one proceeds, chromosomes find their homologous partner, synapse, exchange genetic material between homologs and then begin to separate, remaining connected at recombination sites. At postnatal day 5, most of the oocytes have reached the late diplotene (or dictyate) substage of prophase one where they remain arrested until ovulation. This review focuses on events and mechanisms controlling the progression through meiotic prophase one, which include recombination, synapsis and control by signaling pathways. These events are prerequisites for proper chromosome segregation in meiotic divisions; and if they go awry, chromosomes mis-segregate resulting in aneuploidy. Therefore, elucidating the mechanisms regulating meiotic progression is important to provide a foundation for developing improved treatments of female infertility.

Chiasmata and recombination nodules in Lilium longiflorum

We determined the frequency of chiasmata at late diplotene in microsporocytes of Lilium longiflorum (lily). Because there are long, intimate associations of homologous chromosomes in addition to short associations that appear to be single chiasmata, the number of chiasmata counted depends on how the long associations are interpreted. Using a defined method, we determined that there was an average of 54.8 ± 6.0 chiasmata per complete set of diplotene bivalents. Recombination nodules are 100-nm ellipsoids that are found on the central element of synaptonemal complexes. There is correlative evidence that strongly indicates recombination nodules are located at the sites of crossing-over in late pachytene. Using spreads of synaptonemal complexes stained with uranyl acetate – lead citrate, we determined that the frequency of recombination nodules was 1/57.2 μm of synaptonemal complex. Using separate silver-stained spreads of synaptonemal complexes from lily microsporocytes, we determined that the average length of a complete set of pachytene synaptonemal complexes was 3149 ± 668 μm. Therefore, an average set of synaptonemal complexes would have 55.1 (3149 ÷ 57.2) recombination nodules, a number that closely matches the average number of chiasmata in a set of late diplotene bivalents.Key words: chiasmata, recombination nodules, synaptonemal complex, Lilium longiflorum.

Meiosis in Coprinus

Meiotic synchrony in the genus Coprinus has permitted the sequential study of spindle pole body (SPB) behaviour through the meiotic process. The SPBs are monoglobular in the young basidia immediately after the last premeiotic mitosis. From 10 to 15 h before karyogamy until pachytene, spindle pole bodies are not found. They become conspicuous in diplotene and persist until the completion of meiosis. During diplotene, metaphase, anaphase and telophase stages the spindle pole bodies are monoglobular but at late diplotene they duplicate and become diglobular with an isthmus connecting the 2 globular elements. The spindle pole bodies remain in a diglobular state until diakinesis when the isthmus breaks separating the 2 daughter spindle pole bodies. The diglobular SPBs in late diplotene and prophase II are believed to represent the duplicated form of the monoglobular state. The spindle pole bodies in Coprinus contain no centrioles. In thin sections the SPBs appear to be fibrillar amorphous structures with a dense inner core surrounded by a less-dense outer zone.

Meiosis in Coprinus. III. Timing of meiotic events in C. lagopus (sensu Buller)

The close synchrony of meiotic events within the basidiocarp of Coprinus lagopus permits the time sequence study of division stages. It takes about 16 h from the beginning of karyogamy to the completion of meiosis (zygotene to tetrad formation). Nuclear fusion and chromosome pairing occupy about 4 h, pachytene 5 h, diplotene 4 h, and all the other stages including the second meiotic division appear to be very transitory and together occupy less than 3 h. The centrosome divides at late diplotene. The entire second meiotic division takes about 1 hour and different stages overlap a great deal. The four sterigmata are formed after the completion of second division. The basidiospores mature in 8–10 h after the tetrad formation.