- The synaptonemal complex is a highly ordered protein structure that forms between homologous chromosomes during meiotic prophase I. It provides the scaffold that enables chromosomes to align, pair, and undergo recombination with remarkable precision. In its mature form, the synaptonemal complex consists of two parallel lateral elements connected by a central element, creating a zipper‑like architecture that stabilises homologous pairing. Proteins such as SYCP1, SYCP2, and SYCP3 form the core structural components of this complex, while cohesins and other meiotic regulators contribute to its assembly and maintenance. The formation of the synaptonemal complex marks a critical transition in meiosis, ensuring that homologous chromosomes are correctly synapsed before crossover formation begins.
- The assembly of the synaptonemal complex begins with the organisation of chromosome axes, where SYCP3 and SYCP2 form the lateral elements. SYCP1 then bridges these elements by forming transverse filaments that connect homologous chromosomes across the central region. This structure supports the formation of recombination nodules, where crossover events are initiated and processed. The stability provided by the synaptonemal complex ensures that recombination intermediates are properly resolved, preventing chromosomal mis‑segregation. Disruption of any component of the complex, including mutations in SYCP3, SYCP2, or SYCP1, can lead to defective synapsis, meiotic arrest, aneuploidy, or infertility.
- The synaptonemal complex is essential for fertility in both males and females. In males, defective synapsis leads to spermatocyte apoptosis and conditions such as azoospermia or oligozoospermia. In females, abnormalities in synaptonemal complex formation can impair oocyte development, reduce ovarian reserve, or contribute to meiotic nondisjunction, increasing the risk of aneuploid conditions. Because of its central role in meiotic chromosome behaviour, the synaptonemal complex is a major focus in reproductive genetics and clinical andrology.
- Beyond reproduction, the synaptonemal complex has gained attention in cancer biology and genome stability research. Misexpression of meiotic proteins in somatic cells, including components of the synaptonemal complex, has been observed in certain tumours. Such aberrant expression may interfere with DNA repair pathways, contributing to genomic instability and tumour progression. The involvement of synaptonemal complex proteins in DNA damage responses highlights their broader biological significance.
- The synaptonemal complex is evolutionarily conserved across eukaryotes, from yeast to mammals. Despite differences in protein composition among species, the overall architecture and function remain remarkably similar. This conservation underscores its essential role in ensuring accurate meiotic chromosome segregation and maintaining genetic diversity across generations. Comparative studies in organisms such as mice, zebrafish, and plants continue to reveal insights into the molecular evolution of meiotic machinery.
Further reading
- Grey C. & de Massy B., 2022. Coupling crossover and synaptonemal complex in meiosis. Genes Dev. 36(1-2), 4-6. doi: 10.1101/gad.349286.121. DOI: 10.1101/gad.349286.121, PMID: 35022326, PMCID: PMC8763052 (Download PDF), Genesdev (Download PDF)
- Adams I.R., & Davies O.R. 2023. Meiotic Chromosome Structure, the Synaptonemal Complex, and Infertility. Annu Rev Genomics Hum Genet. 24, 35-61. DOI: 10.1146/annurev-genom-110122-090239, PMID: 37159901, Annualreviews (Download PDF)
