Human life begins with sperm and oocyte fusion. After fertilization, various fusion events occur during human embryogenesis and morphogenesis. For example, the fusion of trophoblastic cells constitutes a key process for normal placental development. Fusion in the placenta is facilitated by syncytin 1 and syncytin 2. These syncytins arose from retroviral sequences that entered the primate genome 25 million and more than 40 million years ago respectively. About 8% of the human genome consists of similar human endogenous retroviral (HERVs) sequences. Many are inactive because of mutations or deletions. However, the role of the few that remain transcriptionally active has not been fully elucidated. Syncytin proteins maintain cell–cell fusogenic activity based on env gene-mediated viral cell entry. In this review, we summarize how syncytins and their receptors are involved in fusion events during human reproduction. The significance of syncytins in tumorigenesis is also discussed.
Bikem Soygur and Leyla Sati
The circadian system regulates the daily temporal organization in behavior and physiology, including neuroendocrine rhythms and reproduction. Modern life, however, increasingly impacts this complex biological system. Due to limitations of working with human subjects exposed to shift work schedules, most chronoregulation research has used rodent models. Recent publications in these model systems have emphasized the negative effects of circadian rhythm disruption on both female and male reproductive systems and fertility. Additionally, there is growing concern about the long-term effects of circadian rhythm disruptions during pregnancy on human offspring and their descendants as circadian regulation during pregnancy can also alter epigenetic programing in offspring. However, to truly know if such concerns apply to humans will require retrospective and prospective human studies. Therefore, this review will highlight the latest available evidence regarding potential effects of chronodisruption on both female and male reproductive systems. Additionally, it presents a comprehensive summary of transgenerational and epigenetic effects on adult offspring that result from maternal chronodisruption.