The structure of endometrial microvessels.

Recent studies have provided substantial evidence to highlight abnormalities in the structure of endometrial microvessels in users of progestogen-only contraception. Structural changes alone are unlikely to lead to breakthrough bleeding, but appear to be associated with a reduction in vessel integrity, and may reflect alterations in the control and growth of endometrial microvessels in those exposed to exogenous progestogens. In users of low-dose progestogens, immunohistochemical studies have demonstrated changes in superficial endometrial vascular morphology, density and in endometrial migratory cell populations. In Norplant users the endometrial endothelial basal lamina is deficient in the initial months of exposure, when bleeding problems are most common. The basal lamina refers to the very thin structure consisting mostly of collagen IV and laminin seen underlying endothelial and epithelial cells at the electron microscope level. In addition, hysteroscopic studies have demonstrated increased fragility of superficial vessels. Changes in endometrial microvascular anatomy associated with normal menstruation have been reviewed, and are compared with those seen following contraceptive steroid exposure. Likely mechanisms of breakthrough bleeding such as increased superficial vascular fragility and the possible alterations in endometrial vascular structure leading to this are discussed.