Changes in proliferative potential, apoptosis and Bcl-2 protein expression in cytotrophoblasts and syncytiotrophoblast in human placenta over the course of pregnancy

In order to evaluate placental trophoblast proliferation and apoptosis during pregnancy, we investigated proliferating cell nuclear antigen (PCNA) expression, apoptosis and Bcl-2 protein expression in the human placenta using avidin/biotin immunoperoxidase method to examine PCNA and Bcl-2 protein expression, and TUNEL method to assess apoptosis. The appearance of apoptotic cells in very early term placental trophoblasts was also examined by transmission electron microscopy. PCNA was immunolocalized in the nuclei of cytotrophoblasts (C-cells). Determination of the mean percentage of PCNA-positive nuclei of C-cells revealed that PCNA expression in C-cells was highest in very early term (4th to 5th wk) placentas and significantly decreased with the advance of pregnancy. Bcl-2 protein was immunolocalized in the cytoplasm of syncytiotrophoblast (S-cell), being least abundant in very early term placentas, less abundant in early term and midterm placentas, and most abundant in term placentas. On the basis of TUNEL method, apoptosis was apparent in the nuclei of both C-cells and S-cell. The apoptosis positive rate of C-cell nuclei was highest in very early term 4th to 5th wk placentas, and significantly decreased in early term 7th to 9th wk and midterm placentas, but somewhat increased in term placentas compared to that in midterm placentas. On the other hand, apoptosis positive rate of S-cell nuclei was remarkably higher only in very early term 4th to 5th wk placentas compared to that in early term, midterm and term placentas. Transmission electron microscopy revealed the appearance of apoptotic nucleus in very early term placental trophoblasts. These results demonstrate for the first time that apoptosis in the human normal placenta predominates in both C-cells and S-cell in very early term 4th to 5th wk pregnancy and drastically diminished after 7th wk of pregnancy. An apparent increase in apoptosis in C-cells in term placentas compared to that in midterm placentas may reflect aging of the placenta or parturition-associated biological change. The abundant expression of Bcl-2 protein in S-cell in term placentas may be responsible for the diminished occurrence of apoptosis in S-cell in term placentas.