Why do membranes rupture at term? Evidence of increased cellular apoptosis in the supracervical fetal membranes

OBJECTIVE: Reduced tensile strength of the human fetal membranes overlying the cervix has previously been identified. We used transcervical application of Bonney's blue dye, before the onset of term labor to identify the supracervical membranes for analysis after elective cesarean section delivery. We hypothesized that pro- and antiapoptotic proteins, which are representative of both the extrinsic and intrinsic pathways, would be expressed differentially in the supracervical membranes compared with membranes taken from distal sites. Membrane apoptosis would provide a mechanism for the reduced tensile strength that presumably precedes spontaneous intrapartum rupture of the membranes. STUDY DESIGN: Bonney's blue dye was applied transcervically to the chorion-facing fetal membrane before elective cesarean delivery at term. After delivery, samples of fetal membranes were obtained from the supracervical site, where the membrane was marked by the dye ( approximately 8-cm diameter) and compared with samples from a distal site ( 2-cm from the placental edge). Samples from the supracervical and distal sites were fixed and paraffin embedded for immunohistochemical analyses and histologic review and stored at -80 degrees C for Western blotting analysis. RESULTS: The supracervical area of fetal membranes exhibited increased markers of apoptosis that included M30 immunohistochemical staining, cleaved-caspsase-3, cleaved-caspase-9, and decreased immunoreactive Bcl- 2. Histologic sections that were stained with hematoxylin and eosin demonstrated features of degenerative changes and apoptosis that occurred predominantly at the supracervical site. CONCLUSION: There is evidence of increased cellular apoptosis at the supracervical site in fetal membranes at term. Both morphologic and biochemical changes that were observed at the supracervical site suggest that the intrinsic apoptotic pathway plays an important role in spontaneous membrane rupture at term.