Structural biology of the human fetal membranes.

During pregnancy the fetus is surrounded by amniotic fluid which is enclosed within a sac formed by the fetal membranes. They maintain their structural integrity and remain intact until after the onset of labour at term. The structural characteristics of the fetal membranes is not established. It is not known therefore how they remain intact dtiring pregnancy and why they rupture easily during labour at term in comparison to earlier gestations. This knowledge is essential to understand why they sometimes rupture before the onset of labour leading to serious maternal and fetal complications. Immunohistochemical, immunofluorescence, and electron microscopic techniques are used to study the histological characteristics of the non-infected fetal membranes that spontaneously rupture at term after the onset of labour and to investigate the distribution and organization of the structural integrity-maintaining factors e.g. extracellular matrix, cytoskeletal, and cell-cell and cell-matrix adhesion molecules. A semi-quantitative immunofluorescence technique for the confocal laser microscopy is developed to measure the density and content of these factors. A novel zone of altered membrane structure is detected within the rupture site. It shows altered organization, density and content of the structural integrity- maintaining factors in comparison to the membranes obtained away from the rupture site where these factors are found to be distributed and integrated in such fashion to confer strength to the membranes. These alterations indicate that this zone is structurally the weakest area of the membranes and represent overlapping features of degradation and matrix deposition. These features together with the detected increase in tenascin expression and number of the myofibroblasts are analogous to those seen in a 'wound response'. A balance between these overlapping features keeps this zone intact during pregnancy and during labour membrane rupture may be facilitated by the presence of this structurally weak zone. The rupture site is then formed by tearing and extension of the initial rupture point during labour and delivery. The similarity of the membrane structural changes to those detected in the cervix and vagina at term indicates that they are a part of a preparatory phase necessary to prepare these structure to the events of labour. If these changes become severe they may predispose the fetal membranes to their prelabour rupture.