Following DNA replication in premeiotic S-phase, the chromosomes start to condense and a proteinaceous core, or axial element, forms between the sister chromatids. The chromatin itself is organized into loops with only the chromatin at the base of the loops attached to the axial element (Weith and Traut 1980). When homologous chromosomes synapse in zygonema to form a bivalent, the axial elements align and are joined by transverse filaments. Once synapsis has been initiated the axial elements are called lateral elements. Finally, a central element is formed between the two axial elements, completing the tripartite structure of the synaptonemal complex (Moses 1968; von Wettstein, Rasmussen et al. 1984). Since the behavior of the axial/lateral elements is analogous to the behavior of the meiotic chromosomes, visualization of the axes by silver staining or antibody localization is sufficient to describe chromosome synapsis and desynapsis during meiotic prophase I.
In leptotene spermatocytes short stretches of axial elements start to form between the sister chromatids. The axial elements elongate as meiosis progresses and the aligned homologous chromosomes begin to synapse in early zygonema (arrows). Synapsis continues as axial element formation is completed, however this process does not occur synchronously for all homologs within the same nucleus. Asynchrony is most obvious in late zygotene spermatocytes in which univalents (unsynapsed homologs, arrows), aligned homologs (small arrows), synapsing homologs (arrows) and homologs that have completed synapsis (arrowhead) are all present at the same time. Complete synapses of all bivalents, with the exception of the sex-chromosomes in spermatocytes, is characteristic of the pachytene stage. Pachynema lasts about seven days in spermatocytes and is divided in substages based on cytological characteristics. In early pachynema the X and Y chromosomes are synapsed along the region at the base of the XY bivalent, however, desynapsis of the XY bivalent occurs while all other, autosomal, bivalents remain synapsed in mid pachynema (arrow). The late pachytene stage in spermatocytes is characterized by the appearance of a thickening of the chromosome ends (arrows). In diplonema , the synaptonemal complex disintegrates and the bivalents begin to desynapsis. However, the homologs remain held together by chiasmata until separation in metaphase I. In females, oocytes arrest after completion of meiotic prophase I, and remain in the dictyate stage until oogenesis resumes when the female becomes sexually mature. In contrast to spermatocytes, the pachytene stage in oocytes only lasts about three days, and differs from spermatocytes by the absence of the late pachytene substage.
Back to Meiosis