Anatomy and physiology of cervical ripening.

The uterine cervix is a unique organ composed predominately of the extracellular matrix proteins, collagen, elastin, and glycosaminoglycans. During pregnancy and labor, this organ is metabolically active, which is rare in adult tissue. The metabolism is under reproductive hormonal control and is more complex than previously appreciated. Smooth muscle cells, which comprise 10-15% of cervical tissue, undergo programmed cell death and play a role in cervical softening. Apoptosis is a genetically timed event and could explain the species-specific length of gestation. Further research in the next several years will reveal more completely the exciting process of cervical ripening. Only when this phenomenon is understood will rational therapy for preterm labor and post-term pregnancy with an unripe cervix be available. Specific defects in cervical ripening will then be diagnosed and treated. For example, if apoptosis is shown to play an important role in the process of cervical ripening, it could be inhibited. Conversely, it could be induced in the unripe cervix. If we would look for it, we would find that it is probably occurring today in the clinical use of cervical ripening agents. The most important contributor to cervical softening, however, is a rearrangement and realignment of the collagen, elastin, and smooth muscle cells, which occurs due to mechanical forces and to a rearrangement of the collagen that occurs as the content of glycosaminoglycans varies in the cervix with time. One form of dermatan sulfate, decorin, may help to separate the collagen fibrils and then open them up. This rearrangement also involves fiber shortening below the critical length for tensile strength, allowing for extensibility of the cervix. Because of its orientation in the cervix, elastin contributes to the ratchet-like mechanism of dilatation. Finally, the cervix undergoes change in two phases--softening, which involves collagen realignment, and dilatation. The proteolytic enzymes in the cervix degrade cross-linked, newly synthesized collagen, and they help activate other enzymes in a cascade. However, the predominate anatomic and physiologic change in ripening is the rearrangement of collagen.