Apoptosis and human placenta: expression of proteins belonging to different apoptotic pathways during pregnancy.
Apoptosis occurs during normal development and it is important for the right balance between the loss of old, non-functional cells and the formation of new cells in different organs and tissues. Apoptosis is triggered by different cell-type-specific signals which involve several pathways, such as mitochondrial and receptor-mediated pathways, resulting in caspase cascade activation. Several studies have suggested that apoptosis plays an important role in the normal development, remodelling and aging of the placenta. Moreover, it has been demonstrated that apoptosis increases as pregnancy progresses suggesting that it is a normal physiological phenomenon throughout gestation. In the last years, it has been hypothesized that the process known as syncytial fusion is directly or indirectly related to apoptotic events. In particular, it has been suggested that cytotrophoblast cells early express most important apoptotic proteins that translocate in the syncytiotrophoblast with the fusion. This suggests that apoptosis has a central role in the villous trophoblast turnover. Recently, another important involvement of apoptotic processes in human placenta has been demonstrated. In particular, the apoptosis, mainly through Fas-FasL or TRAIL-R-TRAIL signalling, may be a defence mechanism against rejection of the fetal allograft by maternal immune system. The whole data suggest that regulation of apoptotic events is important to allow a correct development, differentiation and function of the placenta throughout pregnancy and that an unbalance of this process leads to severe pathologies such as pre-eclampsia and intrauterine growth retardation. Therefore, due to its extensive proliferation and invasive properties, the placenta mimics a malignant tumor and represents an interesting model to evaluate those processes leading to carcinogenesis.